Multi-layer system from thin layers for temporary bonding

Abstract

The invention relates to a method for providing a multi-layer system, to a multi-layer system and to a method for bonding and debonding.

Description

[0001] Description

[0002] Multilayer system of thin layers for temporary bonding

[0003] The present invention relates to a method for providing a multilayer system, a substrate stack and a method for bonding and debonding with a multilayer system.

[0004] Several methods for debonding two temporarily bonded substrates are known in the prior art. The substrates are, in particular, a product substrate and a carrier substrate, with the carrier substrate enabling handling, further processing, and transport of the product substrate. After processing, the carrier substrate is separated from the product substrate.

[0005] Bonding adhesives are widely used to enable a temporary, relatively easily removable bond between two substrates. This temporary, adhesive coating serves primarily as an intermediate layer in a substrate stack. Bonding adhesives are usually polymers, particularly thermoplastics. The debonding of both substrates occurs, for example, through a shearing process at elevated temperatures. Debonding can also be achieved through additional mechanical action or chemical treatment of the bonding adhesive.

[0006] One of the newest and most important processes for separating substrate stacks is laser debonding. In laser debonding, laser light is coupled through a substrate that is as transparent as possible and absorbed by the adjacent coating (release layer) on the back. The laser light is preferably coupled through a largely transparent carrier substrate. The transparency of the carrier substrate for a specific electromagnetic radiation allows largely unhindered access of the photons to the release layer.

[0007] One method for separating two substrates consists in applying a special release layer in combination with a bonding adhesive to a carrier substrate, particularly a transparent one. The transparency of the carrier substrate for a specific electromagnetic radiation allows unhindered access of the photons to the release layer. The release layer is modified accordingly by the photons and reduces the adhesive force to the bonding adhesive. US Pat. No. 10,468,286B2 describes such a process. Depending on where the bonding adhesive was applied—i.e., directly to the carrier substrate or after the release layer—the bonding adhesive must also be largely transparent to the selected electromagnetic radiation.

[0008] In laser debonding, polymers, particularly polyimide-based polymers, can be used as a release layer because they can be selectively removed using a UV laser beam source. Separation occurs at the carrier substrate-bonding adhesive interface. The UV laser beam source used for this requires glass carrier substrates that have the necessary transparency for the specific electromagnetic radiation in the UV range. US9,827,740B2 shows a system consisting of a bonding adhesive and a polyimide release layer applied directly to the glass carrier substrate. In US 10,703,945B2, the bonding adhesive contains a light-absorbing material, whereby only one polymer layer is used for simultaneous bonding and debonding. The release layer can, in particular, also be a metal layer. In WO201 1 / 159456A2, for example, an adhesive layer with a metal layer is used for laser debonding.Due to the strong absorption of laser radiation by the metal coating, separation of the product substrate and the carrier substrate is possible. However, bonding two substrates without a bonding adhesive is not possible in WO2011 / 159456A2. US Pat. No. 9,269,561B2 also shows a release layer consisting of a bonding adhesive and a metal coating between a Si carrier substrate and a product substrate.

[0009] US Pat. No. 10,112,377B2 discloses various materials that can be used for a single-layer release liner for laser debonding. Here, too, a bonding adhesive is required in addition to the release liner to temporarily bond the substrates.

[0010] The use of polymeric bonding adhesives has the disadvantage that after UV laser debonding, surface cleaning is necessary to remove bonding adhesive residues. Furthermore, the requirements of 3D stacking and CMO S-compatible processes require high-quality silicon carrier substrates, which are not transparent in the UV range. Furthermore, polymer-based bonding adhesives are not heat-resistant at higher temperatures.

[0011] If a metal layer is applied to the product substrate and / or the carrier substrate and used as a bonding layer, the prior art requires additional layers to enable gentle and largely non-destructive laser debonding, since the surface of the coating is destructively removed. This at least one additional layer serves as protection for the product substrate and is, in particular, an anti-reflection coating (AR coating). Additional protective layers include relaxation layers, for example. WO2015 / 014265A1 discloses such an AR coating and a relaxation layer in addition to the metal layer used as a release layer.

[0012] A problem with the prior art is that exposure to laser beams can destroy the substrates, especially expensive functional components. This requires additional layers, particularly polymer-based adhesive layers, in addition to the release layer. Furthermore, the UV-curable bonding adhesives used for laser debonding are not compatible with silicon carrier substrates. Additional layers are therefore necessary in the prior art and serve either to protect the substrates and / or as an adhesive layer for bonding the substrates.

[0013] Many products from the semiconductor industry, such as electronic and optoelectronic components, consist partly of layer sequences of dissimilar materials. Many of these multilayer systems are used in bonding processes, but are also required for many manufacturing processes that involve bonding and debonding. In particular, laser debonding requires lasers of certain wavelengths to operate efficiently.

[0014] It is therefore the object of the invention to at least partially eliminate, and in particular completely eliminate, the disadvantages cited in the prior art. In particular, it is an object of the invention to demonstrate an improved method for providing and using multilayer systems for bonding and debonding.

[0015] The present object is achieved by the features of the independent claims. Advantageous developments of the invention are specified in the dependent claims. The scope of the invention also includes all combinations of at least two features specified in the description, the claims, and / or the drawings. For specified value ranges, values ​​within the specified limits are also considered disclosed limit values ​​and can be claimed in any combination.

[0016] Accordingly, the invention relates to a method for providing a multilayer system comprising at least two layers, in particular for temporarily bonding substrates to form a substrate stack, comprising the following steps in the following order: i) providing a multilayer system, ii) determining an absorption curve of the multilayer system for laser radiation of a specific wavelength, iii) varying at least one parameter of the multilayer system, iv) determining the absorption curve of the multilayer system for the laser radiation of the specific wavelength using the at least one parameter varied according to step iii), v) repeating steps i) to iv) until the degree of absorption is greatest, wherein in step i) the multilayer system with the greater absorption curve is provided in each case.

[0017] A multilayer system consists of at least two layers. The layers preferably have a uniform thickness and are arranged one above the other in a planar manner, although the layer can also be applied in a structured manner instead of a planar pattern. The same material is present within the layers of the multilayer system. The layers are so-called thin films, particularly preferably with a layer thickness in the nanometer range. Known multilayer systems can be advantageously used with regard to the structure and arrangement of the layers.

[0018] The provision in step i) also includes the provision of material data for the multilayer system, so that a computer-aided calculation or simulation for the respective parameter can also be carried out during the determination. In other words, the parameter of the multilayer system is determined in relation to an absorption coefficient in different combinations, and the highest is selected in each case. When repeating the process, technically reasonable values ​​are selected depending on the parameter for the further variation or adjustment of the multilayer system. The wavelength of the laser radiation, on the basis of which the respective absorption coefficient or adsorption coefficient comparison is carried out, remains constant.

[0019] Parameters can include, for example, the sequence or structure of the layers of the multilayer system, as well as the layer thicknesses. When determining the absorption coefficient, this can be measured or calculated for each specific case. Preferably, a simulation of the multilayer system is performed with respect to the respective parameter.

[0020] In the search for solutions to the disadvantages described in the prior art, it has surprisingly been found that, given certain parameters, the absorption behavior of the multilayer systems provided can be significantly improved, particularly in established multilayer systems. In this way, multilayer systems or material combinations can be used in other areas of application. Furthermore, thinner layers can be used for debonding, and multilayer systems without additional polymer adhesive / adhesive layers and without anti-reflective layers can be used for bonding and simultaneous debonding. Furthermore, a higher degree of absorption minimizes the energy input and thus the heat input in substrates arranged behind the multilayer system. This advantageously prevents destruction during laser debonding.

[0021] In a preferred embodiment of the method for providing a multilayer system, it is provided that the at least one parameter of the multilayer system is a layer thickness of a layer of the multilayer system. In other words, the layer thickness of a specific layer of the multilayer system is varied, i.e. increased or decreased, in order to achieve the greatest possible degree of absorption. Surprisingly, it has been found that by changing the layer thickness in a multilayer system made up of thin layers, a greater degree of absorption can be achieved through interference effects. Thus, the method can advantageously significantly increase the degree of absorption by changing the thickness of a layer. Due to the systematic variation, otherwise unnoticed effects on the absorption behavior remain undetected.The method thus advantageously determines a layer structure for a multilayer system that is particularly interference-optimized. The wavelength of the laser radiation, for which the absorption coefficient is to be maximum, remains the same.

[0022] Laser debonding is limited to laser radiation of certain wavelengths, as only these can be generated efficiently. In this respect, it has also been surprisingly discovered that, for certain wavelengths, a greater degree of absorption can be achieved by varying the layer thickness. By systematically changing the layer thickness of the individual coating layers in a multilayer system, greater absorption can surprisingly be achieved, allowing a multilayer system to be used not only as a bonding layer, but also simultaneously as a release layer in laser debonding. The multilayer system can advantageously be provided without changing or exchanging materials, thus enabling the use of existing systems (i.e., coatings or multilayer systems known to those skilled in the art in the semiconductor industry).The existing multilayer systems or materials can also be partially modified in terms of the arrangement of the materials. In particular, the layer thicknesses are adapted with regard to the absorption behavior, especially the absorptivity and reflectivity of the entire multilayer system, since it has surprisingly been found that the same or greater absorption levels can be achieved with the same or lower overall thickness of the multilayer system. In this way, a layer-thickness-optimized multilayer system can be advantageously used not only for bonding but also for debonding. This advantageously minimizes the energy input into other materials. Furthermore, material can be saved and the thickness of the multilayer system reduced.By deliberately reducing the holding forces of the multilayer system of the substrate stack to be debonded by irradiating it with laser radiation of a specific wavelength, the multilayer system can also be advantageously used as a debonding layer.

[0023] In a preferred embodiment of the method for providing a multilayer system, it is provided that the at least one parameter of the multilayer system is a layer thickness of a further layer of the multilayer system. Thus, in addition to the layer thickness of one layer, the layer thickness of a further layer of the multilayer system is also varied simultaneously. This advantageously enables efficient and rapid provision of a multilayer system with the greatest possible absorption coefficient, with respect to laser radiation of the specific wavelength and layer structure, by changing the thickness. If the multilayer system has three layers, one layer thickness is preferably kept constant and the simulation or test series is determined for two adjacent layers.

[0024] In a preferred embodiment of the method for providing a multilayer system, the wavelength during the determination in steps ii) and iv) is between 1,100 nm and 10,000 nm, preferably between 1,100 nm and 5,000 nm, and more preferably between 1,500 nm and 2,500 nm. Research into multilayer systems has also shown that, for laser debonding with multilayer systems consisting of thin, particularly polymer-free, layers, the absorption behavior can be influenced, particularly in certain wavelength ranges, by varying parameters. Laser debonding of the multilayer systems according to the invention is preferably carried out in the infrared range.

[0025] The invention further relates to a substrate stack comprising at least one multilayer system provided according to the method for providing a multilayer system, having at least two layers made of different materials. The multilayer system is preferably designed as an intermediate layer and connects two substrates to form the substrate stack. The multilayer system has a layer structure optimized in terms of layer thickness, wherein the layer thicknesses are selected such that the multilayer system has the greatest degree of absorption for a specific wavelength, while at the same time the layers can be kept as thin as possible. The multilayer system is therefore optimally adapted with regard to the layer thickness or another parameter for the highest possible absorption of electromagnetic radiation of a specific wavelength. The multilayer system can therefore advantageously be used as a bonding layer and as a debonding layer in the substrate stack.Thus, the substrate stack can be separated non-destructively, efficiently, and easily using laser radiation, or in particular, a product substrate can be removed. The multilayer system is preferably produced on one substrate and then bonded to another substrate, so that the multilayer system can be used as a bonding layer and simultaneously as a debonding layer.

[0026] In a preferred embodiment of the substrate stack, the multilayer system has a total thickness between 1 nm and 10 pm, more preferably between 5 nm and 2 pm, most preferably between 10 nm and 1 pm, and most preferably between 10 nm and 500 nm. This makes the substrate stack stable and small. Furthermore, debonding along or in the region of the multilayer system can be carried out advantageously simply and efficiently.

[0027] In a preferred embodiment of the substrate stack, the respective layers of the multilayer system each have a layer thickness between 1 nm and 1 pm, preferably between 1 nm and 500 nm, even more preferably between 1 nm and 250 nm. It has surprisingly been found that even with very thin layers, a high degree of absorption can be achieved by optimizing the layer thicknesses. The interference can therefore be generated particularly well in multilayer systems for thin layers with layer thicknesses in the sub-wavelength range with respect to the laser radiation. In particular, the combination of the layer thicknesses can advantageously achieve a high degree of constructive interference of the laser radiation in the multilayer system.

[0028] In a preferred embodiment of the substrate stack, the multilayer system comprises at least one layer with a layer thickness between 10 nm and 100 nm, preferably between 20 nm and 100 nm, more preferably between 25 nm and 75 nm, most preferably between 35 and 65 nm. In the course of developing the method for providing a multilayer system and the substrate stack comprising the multilayer system, it has been found that a particularly high increase in the degree of absorption can be achieved if at least one layer has the corresponding layer thickness.

[0029] In a preferred embodiment of the substrate stack, at least one layer of the multilayer system comprises, preferably consists of, titanium (Ti), aluminum (Al), aluminum nitride (AlN), tantalum nitride (TaN), (germanium) Ge, (tin) TiN, or copper (Cu). The layer thickness is particularly preferably between 25 and 75 nm.

[0030] In a preferred embodiment of the substrate stack, at least one layer of the multilayer system consists of amorphous silicon dioxide (SiO2). The layer thickness of this layer of the multilayer system is preferably greater than the other layers or the other layer. The layer thickness is preferably more than 500 nm, more preferably more than 200 nm.

[0031] In a preferred embodiment of the substrate stack, the substrate stack comprises at least one carrier substrate and one product substrate, wherein the carrier substrate is connected to the product substrate by the multilayer system. The multilayer system is thus arranged as an intermediate layer and simultaneously as a bonding layer between the carrier substrate and the product substrate. In this way, debonding with the substrate stack can be carried out particularly quickly and efficiently. In a preferred embodiment of the substrate stack, the multilayer system, preferably the substrate stack, does not comprise a polymer-based bonding adhesive. In other words, an additional bonding layer or auxiliary layer can be dispensed with due to the high absorption coefficient of the multilayer system. The substrate stack is particularly preferably free of polymer-based materials, so that the substrate stack can be processed at particularly high temperatures.In addition, the adhesive layer and thus the subsequent and time-consuming removal of residues can be dispensed with.

[0032] In a preferred embodiment of the substrate stack, the multilayer system, preferably the substrate stack, does not have an anti-reflection layer. The anti-reflection layer, which is usually arranged on the side of the multilayer system or the bonding layer facing away from the laser beam, can be omitted during laser debonding due to the high absorption coefficient of the optimally constructed multilayer system. Furthermore, destruction can be advantageously prevented by the multilayer system even without an anti-reflection layer.

[0033] In a preferred embodiment of the substrate stack, at least one substrate arranged on the multilayer system, in particular a carrier substrate, is made of silicon. In this way, the multilayer system can advantageously be irradiated through the substrate with laser radiation having a wavelength greater than 1300 nm. Laser debonding can thus advantageously be performed from the back of the substrate stack.

[0034] In a preferred embodiment of the substrate stack, the absorption coefficient of the multilayer system with respect to laser radiation of a specific wavelength is greater than 0.5, preferably greater than 0.65, more preferably greater than 0.75, even more preferably greater than 0.85, and most preferably greater than 0.9. This ensures that destruction of the other substrate arranged behind the multilayer system, in particular the product substrate, is prevented during debonding of the substrate stack.

[0035] In a preferred embodiment of the substrate stack, the multilayer system comprises exactly three layers, two of the three layers being made of the same material and separated from each other by a remaining layer. The layers of the multilayer system adjacent to the substrates are thus made of the same material and preferably include a smaller layer, which is preferably a metal layer.

[0036] In a preferred embodiment of the substrate stack, it is provided that the substrate stack is debondable by irradiating the multilayer system with laser radiation of a specific wavelength.

[0037] Furthermore, the invention relates to a method for bonding substrates to a substrate stack according to the invention, comprising the following steps,

[0038] 1 ) Providing a first substrate, in particular a carrier substrate,

[0039] 2) Bonding a second substrate, in particular a product substrate, to the first substrate.

[0040] The substrate provided in step 1) functions, in particular, as a bonding layer. Bonding is particularly simple and efficient with the multilayer system. The layers of the multilayer system can be arranged on the first substrate and / or the second substrate.

[0041] Furthermore, the invention relates to a method for debonding a substrate stack with the following steps, a) providing a substrate stack according to at least one of claims 5 to 13, b) irradiating the multilayer system through at least one substrate of the substrate stack with laser radiation of a specific wavelength and then, c) separating the substrate stack in the region of the multilayer system.

[0042] Debonding or laser debonding is particularly easy, safe and fast with a substrate stack or a substrate stack with a thickness-optimized multilayer system.

[0043] Since the arrangement of the layers is often determined by the process or application, as well as by the respective substrates, layer thickness optimization with regard to the degree of absorption is an unexpected and extremely useful effect. This is especially true because it provides a previously unrecognized opportunity to adapt the absorption behavior of thin layers for laser bonding. The order of the materials in the multilayer system is usually maintained for practical reasons.

[0044] In an exemplary embodiment of the method for providing a multilayer system, an optimization of the layer thicknesses is first carried out for each individual layer LI to Ln of a given multilayer system consisting of the layers LI to Ln, preferably with three (LI to L3), particularly preferably with two layers (LI, L2), wherein the absorption of the entire multilayer system is determined numerically and also measured experimentally.

[0045] Parameters such as the carrier substrate (preferably Si), wavelength (preferably in the IR range to match the Si carrier substrate), and the laser entrance angle (for example, in the main beam as 0° i.e. perpendicular to the surface) are constant. The layer thicknesses can be varied simultaneously in the simulation while maintaining a constant laser wavelength. The simulation thus determines a thickness distribution with the maximum absorptivity of the multilayer system. In the test, the substrate stack with the multilayer system is tested at defined layer thicknesses with regard to residual bond strength, ablation shape, homogeneity, and stability of the manufacturing and processing parameters.

[0046] In a preferred embodiment of the method, it is further provided that the material layers of the multilayer system are first confirmed in their arrangement and then optimized in their layer thickness to achieve maximum light absorption and minimize reflection losses. This allows the substrate stack created by bonding and optimized for laser debonding with the multilayer system (particularly as an intermediate layer) to be separated again in a later process step by laser debonding.

[0047] The separation of the substrates takes place by debonding or delamination along the interface using laser irradiation. In a preferred embodiment, debonding involves laser irradiation through the carrier substrate with light of a selected wavelength, intensity, and pulse duration (ΔT in the ps to fs range). The pulses are particularly preferably in the picosecond range. In the process for debonding a substrate stack, the product substrate is detached from the carrier substrate by focusing laser radiation of a specific wavelength through the carrier substrate onto the multilayer system optimized via interference or thickness. As a result, at least one layer of the multilayer system is destroyed or its adhesion properties are severely reduced by melting, evaporation, and / or sublimation with photo- or thermochemical conversion of the multilayer temporary bonding layer.

[0048] An important aspect of the process for providing a multilayer system is the determination and provision of a multilayer system with the maximum possible absorption coefficient, preferably with the same or lower total thickness. The absorption coefficient of a multilayer system featuring a layer structure with an interference-optimized layer thickness distribution is thus as high as possible, or close to 1 (100%). By adjusting the layer thicknesses of the individual layers in a multilayer system, a higher absorption can be achieved, which makes it possible to use an existing multilayer system as a bonding layer and simultaneously as a release layer during laser debonding.

[0049] The arrangement of the individual layers is usually determined by the bonding process and the bonded substrate stacks commonly used in the semiconductor industry and known to those skilled in the art. The optimization of the multilayer system is therefore preferably carried out without changing materials, allowing the use of existing systems. The layer thickness of the existing materials is optimized with regard to the absorptivity and reflectivity of the entire multilayer system. If, for example, an optimal layer thickness is exceeded or not met, the interference changes and thus the absorption of the multilayer system is reduced. The layer thickness of the individual layers is in the nm range and thus enables a high level of interaction with the electromagnetic waves. Furthermore, a layer structure optimized using interference enables simplified laser debonding, since the product substrate does not need to be protected with an additional anti-reflection coating (AR).Since the multilayer system is used for bonding and laser debonding, no additional bonding adhesive is required for bonding.

[0050] The thicknesses of the individual bonding and laser debonding layers depend on the method used (CVD, PVD, MBE, surface oxidation, etc.). These are typically between 10 nm and 500 nm, preferably between 20 nm and 100 nm.

[0051] The optimization of multilayer systems is primarily a graphical optimization, preferably with two parameters being optimized. Multidimensional optimization (i.e., more than two parameters) is possible, but less preferred. The layer thicknesses are determined primarily through simulations during optimization. In the test, further criteria for laser debonding are examined with the layer thicknesses selected from the simulation, in particular process efficiency and stability, as well as residual bond strength, ablation shape, and homogeneity.

[0052] If the layers, the substrate and the carrier substrate from a given substrate stack are known, the layer thickness d of the individual layers of the multilayer system is easiest to control and change. The layer thickness d of the individual layers of the multilayer system is therefore primarily changed or varied. The laser wavelength and the laser angle (angle of incidence) remain unchanged. If a multilayer system consists of two layers, both layer thicknesses dl and d2 can be varied simultaneously. The multilayer systems consist of several layers LI to Ln, preferably LI to L3 layers. The selected parameters, in particular for example two layer thicknesses dl and d2 are varied and the absorption coefficient in the debond structure is calculated and displayed graphically. The absorption coefficient in the debond structure should be as high as possible.Preferably, up to a maximum of three layers are used to maximize absorption. The high-absorption region in the graph shown must be large enough to not be too sensitive to changes. The layer thicknesses d1 and d2 from the high-absorption region are selected for the thickness of layers L1 and L2.

[0053] The method for providing a multi-layer system, the multi-layer system and the bonding and debonding method are particularly advantageous because:

[0054] Laser debonding is only possible for some bond layers through an optimized multi-layer system,

[0055] The combination of several very thin layers enables high absorption to be achieved through interference between the layers of the multilayer system,

[0056] By optimising the individual layer thicknesses of a multi-layer system, the layer thicknesses (nm range) are reduced, meaning less material has to be applied,

[0057] The available pulse energy is orders of magnitude lower for shorter pulses (in the range of J for high-power lasers compared to pJ for the “ultrafast” picosecond and femtosecond lasers), thus reducing the total energy input into the material being processed, which leads to a smaller heat-damaged zone due to the shorter exposure time and the resulting lower heat diffusion.

[0058] Higher separation efficiency by debonding or delamination along the interface using laser irradiation with ultra-short laser pulses. No additional layers are required to protect the product substrate (anti-reflection (AR) layer) and no bonding adhesive is required.

[0059] The production of the substrate stack with a multilayer system is therefore not only suitable for bonding but also for laser debonding of a substrate stack. In particular, existing materials of a multilayer bonding layer (multilayer system) between the carrier substrate and the product substrate are used. The transparent carrier substrate for substrate-side irradiation with laser radiation is then selected. Silicon as a carrier substrate, for example, is transparent at a wavelength X > 1300 nm or at X > 1900 nm, thus lasers in the near infrared (NIR) and mid-infrared (MIR) are selected here. Therefore, silicon is the preferred carrier substrate in this case, and laser debonding in the infrared range is possible. The carrier substrate (Si) and the laser source with laser wavelength (selection when using Si carrier substrates: e.g., 1940 pm, 1960 pm, or 2030 pm) are thus determined.

[0060] Then, the optimal layer thickness(es) of the individual layers of the multilayer system, which should not be exceeded or undercut, is determined. The material layers are optimized in their layer thickness, particularly in a simulation, so that maximum light absorption (absorptivity) is achieved and reflection losses are minimized. Several layer thicknesses, preferably two, are varied simultaneously. A substrate stack with the layer thickness-optimized multilayer system can then be laser debonded by laser irradiation with a selected wavelength, intensity, and pulse duration (ΔT in the range of 1 ps to 1 ps). The complete detachment or separation of the product substrate by debonding or delamination along the interface using the laser irradiation takes place in the region of the multilayer system. An exemplary method for providing a multilayer system, in particular for temporary bonding of substrates, comprises:

[0061] -a first layer of a first material with a first layer thickness and

[0062] -a second layer made of a second material with a second layer thickness, the method comprising at least the following steps in the following order: a) determining the absorption coefficient of the multilayer system for laser radiation of a specific wavelength for various first layer thicknesses, the second layer thickness being constant, b) selecting the first layer thickness such that the absorption coefficient of the multilayer system is maximum, c) determining the absorption coefficient of the multilayer system for laser radiation of the specific wavelength for various second layer thicknesses, the first layer thickness being constant as the first layer thickness selected in step b), d) selecting the second layer thickness such that the absorption coefficient of the multilayer system is maximum, e) providing the multilayer system with a first layer thickness according to step b) and a second layer thickness according to step d).

[0063] In this respect, this exemplary method describes how the respective optimal layer thicknesses can be determined to achieve the highest possible absorption coefficient of the multilayer system. Surprisingly, it has been found that the thin layers of the multilayer system have high absorption coefficients despite their low thickness, because the layer thicknesses are optimally arranged and designed for laser debonding. In particular, the occurrence of interference in the multilayer systems with thin layers with certain layer thickness distributions is responsible for the higher absorption coefficient. If, for example, an optimal layer thickness is exceeded or not reached, the interference changes and the absorption of the multilayer system decreases. The layer thickness of the individual layers is in the nm range and thus enables a high level of interaction with the electromagnetic waves.Furthermore, an interference-optimized layer structure enables simplified laser debonding, as the product substrate does not need to be protected with an anti-reflective (AR) coating. Since the multilayer system is used for bonding and laser debonding, no additional bonding adhesive is required for bonding.

[0064] The (temporary) bonding layer consists of a multilayer system. The multilayer system serves simultaneously as a bonding layer and as a release layer during laser debonding. The temporary bonding layer preferably comprises several layers that are used for the bonding and debonding process. The materials of the multilayer system are known to those skilled in the art. The temporary bonding layer consists of several layers whose thickness is optimized so that the multilayer system results in maximum absorption of the laser radiation. The interference-optimized layer structure enables improved and simpler laser debonding, whereby no additional layers are required to protect the substrates or to bond the substrates, such as an anti-reflection (AR) protective layer and / or a relaxation layer and / or a bonding adhesive.The individual layers can serve, for example, as selective absorber layers or as phase shifters. The product substrate is separated from the carrier substrate via the optimized multilayer system during laser debonding, whereby damage to the product substrate and / or the carrier substrate is largely minimized or, if possible, eliminated. A key prerequisite is the strong absorption of laser light by the interference-optimized multilayer system. Heat conduction during ablation is minimized or largely negligible through the use of ultrashort laser pulses. The distribution of the absorbed laser energy is determined by the absorption in the multilayer material system, which is triggered by linear and nonlinear processes when the material system is irradiated with ultrashort laser pulses, preferably in the ps range.Due to the high photon densities that can be generated when using very short pulses, the material is removed quickly, so that no or only a small amount of heat is transferred into the remaining adjacent substrate.

[0065] Separation by debonding or delamination along the interface using laser irradiation requires maximum radiation absorption by a release layer consisting of a multilayer system through linear and / or nonlinear processes. Debonding occurs primarily thermally, particularly through the formation of gases, but also partially chemically. The middle layer is usually the absorptive layer and absorbs the energy of the laser radiation. The auxiliary layer(s) reacts / interacts with the absorptive layer.

[0066] The transparency of the carrier substrate for specific electromagnetic radiation allows largely unhindered access of photons to the multilayer system. Examples of carrier materials include silicon (Si), glass, sapphire, and silicon carbide. While the use of glass carrier substrates enables the use of UV lasers, it has several disadvantages, such as poor thermal conductivity and incompatibility with certain semiconductor processes and semiconductor processing equipment. Therefore, carrier substrates made of silicon (Si) are preferred. Since Si substrates are not transparent to the UV spectrum, lasers in the infrared (IR) range, preferably in the mid- and near-infrared (MIR and NIR), are used, since the silicon carrier wafers are transparent to selected wavelengths in the mid- and near-IR. Highly efficient and cost-effective lasers are currently only available at specific wavelengths.Furthermore, due to other material properties, the accessible wavelength range is significantly limited. The laser source and laser wavelength are therefore constant.

[0067] An exemplary method for temporarily bonding a product substrate to a carrier substrate made of silicon (Si) comprising at least the following steps:

[0068] Production of a multilayer system as a bonding and release layer for temporary bonding to the carrier substrate and / or the product substrate, bonding the product substrate to the carrier substrate.

[0069] No bonding adhesives are required for temporary bonding. The connection is created primarily using direct bonding processes or other well-known bonding techniques such as metal diffusion bonding or anodic bonding.

[0070] Furthermore, a substrate stack, in particular produced by a method comprising a product substrate and a carrier substrate, wherein the product substrate and the carrier substrate are connected by a multilayer system as a temporary bonding layer, can be separated in a simplified manner by laser debonding by means of laser irradiation of the multilayer system. The substrate stack preferably comprises the following components:

[0071] Product substrate,

[0072] Multilayer system with an interference-optimized layer structure for temporary bonding and laser debonding in the IR range,

[0073] Silicon carrier wafer, transparent for selected mid- and near-IR wavelengths.

[0074] In a preferred embodiment, the layers of the multilayer system are produced over the entire surface. In a less preferred embodiment, at least one of the layers is applied in a structured manner.

[0075] An exemplary method for laser debonding a product substrate from a carrier substrate made of silicon, wherein the product substrate and the carrier substrate are connected via a multilayer system and form a substrate stack, comprises in particular at least the following steps:

[0076] Holding and fixing the substrate stack on a substrate holder,

[0077] Focusing a debonding radiation, in particular a laser beam from a laser source, through the carrier substrate onto the interference-optimised multilayer system and thereby melting, evaporating and / or sublimating the multilayer temporary bonding layer,

[0078] Separation of the product substrate from the carrier substrate.

[0079] This eliminates the need for an anti-reflective coating as a protective layer on the product substrate. Through targeted energy input and energy conversion within the multilayer bonding system, the stress on the substrates, particularly their functional components, is minimized, particularly thermal and / or photothermal.

[0080] Furthermore, an exemplary method for manufacturing and processing a substrate stack may comprise the following steps:

[0081] Providing a carrier substrate which is largely transparent to light of a predetermined wavelength, in particular a silicon carrier wafer,

[0082] Production of an optimized multi-layer system as a bonding and release layer for temporary bonding to the carrier substrate and / or the product substrate,

[0083] Bonding the product substrate with the carrier substrate,

[0084] Processing the product substrate,

[0085] Detachment of the product substrate from the carrier substrate by focusing a debonding radiation, in particular a laser beam from a laser source, through the carrier substrate onto the interference-optimized multilayer system and thereby melting, evaporating and / or sublimating the multilayer temporary bonding layer.

[0086] According to a preferred embodiment, the laser source is a pulsed laser source, in particular an ultra-short pulsed laser source.

[0087] According to another preferred embodiment, the ultrashort pulsed laser source is a femtosecond laser source. According to another preferred embodiment, the system is additionally equipped with a scanner for scanning the pulsed laser beam.

[0088] Depending on the effect of the laser radiation upon absorption by the multilayer system, the multilayer system is separated from the substrate by delamination / lift-off and / or ablation. Debonding preferably occurs along the interface between the carrier substrate and the multilayer system (delamination).

[0089] In a more preferred embodiment, the release means are the substrate holders on which the product substrate and the carrier substrate are or can be fixed. Separation occurs, for example, by parallel displacement of the substrate and carrier substrate relative to each other or by lifting the substrate or carrier substrate. Both are known to those skilled in the art and will not be described in detail. Other mechanical, physical, and / or chemical aids can also be used for separation.

[0090] The laser acts on the multilayer system and reduces the bond strength between the Si carrier substrate and the multilayer system. The bond strength is reduced by more than 50%, preferably by more than 75%, and even more preferably by more than 90%.

[0091] Substrates and carrier substrate

[0092] The substrates and carrier substrates can have any shape, but are preferably circular. The diameter of the substrates is, in particular, industrially standardized. For wafers, the industry-standard diameters are 1 inch, 2 inches, 3 inches, 4 inches, 5 inches, 6 inches, 8 inches, 12 inches, and 18 inches. The carrier substrates are adapted in size and shape to the size and shape of the product substrates to ensure the handling technology used is as simple as possible. It is also conceivable to fix, process, and detach non-circular substrates, such as panels, from the carrier substrate.

[0093] The carrier substrate consists predominantly, preferably entirely, of one or more of the following materials: glass, mineral (especially sapphire), semiconductor material (especially silicon), polymer, composite material (SiC). Glass carrier substrates are often preferred for laser debonding, as they allow the use of electromagnetic radiation in the UV-VIS wavelength range in combination with a UV-VIS transparent bonding adhesive to minimize heating.

[0094] If silicon carrier substrates are preferred, electromagnetic radiation in the infrared (IR) wavelength range, especially in the near and mid-IR, is required depending on the transparency of the Si carrier substrates.

[0095] In a particularly preferred embodiment, the carrier substrate is made of silicon. Si carrier substrates are compatible with CMOS processes or front-end processes.

[0096] The transparency of the carrier substrate for electromagnetic radiation is described by the transmittance, which indicates the ratio of transmitted and incident radiation. However, the transmittance depends on the thickness of the irradiated body and is therefore specified relative to a unit length of 1 cm. With respect to the selected thickness of 1 cm and for the respective selected wavelength, the carrier substrate has, in particular, a transmittance greater than 60%, preferably greater than 70%, even more preferably greater than 80%, most preferably greater than 90%, and most preferably greater than 95%. The transparency is particularly preferably related to the wavelength of the debonding laser radiation.

[0097] The thermal conductivity of the carrier substrate is preferably between 0.1 W / (m*K) and 5000 W / (m*K), more preferably between 0.5 W / (m*K) and 2500 W / (m*K), even more preferably between 1 W / (m*K) and 1000 W / (m*K).

[0098] The thickness of the carrier substrate can vary depending on the diameter and the requirements for structural stability.

[0099] Laser radiation

[0100] In particular, the laser radiation is selected so that the interface to be separated is reached through the substrate and is strongly absorbed there by the multilayer coating.

[0101] The laser energy is delivered in the form of very short light pulses. In a preferred embodiment, this is ultra-short pulsed laser radiation.

[0102] According to a preferred embodiment, the separation results from multi-photon excitation caused by laser radiation, in particular a femtosecond laser or a picosecond laser. Laser radiation with picosecond (ps) pulses has proven to be the optimal parameter combination for processing thin metallic layers through silicon.

[0103] The separation of the multilayer coating from the substrate is achieved by irradiation of the carrier substrate with light, in particular laser radiation, which is strongly absorbed by the multilayer coating at or near the interface between the materials to be separated.

[0104] The preferred silicon substrate is opaque below a wavelength of 1.3 pm. Particularly preferred lasers and their wavelengths suitable for irradiation through Si substrates:

[0105] Nd:YAG (1,064 pm; 1,320 pm; 1,444 pm)

[0106] Ho:YLF (2.05 pm)

[0107] Ho:YAG (2.09 pm)

[0108] C ZnSe, Cr:ZnS (MIR)

[0109] In a particularly preferred embodiment, a pulsed solid-state laser, preferably an Nd:YAG laser or a Ho:YAG laser, is used. Pulsed solid-state lasers operating in the infrared range above 1.3 pm are doped with ions of Er3+ (1.55 pm), Tm3+ (1.9 pm), Ho3+ (2.09 pm), or Cr3+ (2.4 pm).

[0110] Other preferred laser wavelengths for use with Si carrier substrates are, for example, 1940 pm, 1960 pm, or 2030 pm. The power of the laser providing the laser radiation, measured as light output, in particular radiant power that can be continuously emitted at the substrate, is at least 2 W.

[0111] The preferred wavelength range of the laser is between > 1 100 nm and 10 000 nm, preferably between > 1 100 nm and 5 000 nm, more preferably between 1 500 nm and 2 500 nm.

[0112] Laser beams with at least two wavelengths can also be used. Layer thickness optimization is then performed for both wavelengths for a multilayer system.

[0113] The total energy of the laser radiation per substrate is set in particular between 1 mJ and 500 kJ, preferably between 100 mJ and 200 kJ, particularly preferably between 500 mJ and 100 kJ.

[0114] The laser beam can be operated in continuous mode or, preferably, pulsed. The pulse frequency is set in particular between 0.1 Hz and 300 MHz, preferably between 100 Hz and 500 kHz, particularly preferably between 1 kHz and 400 kHz, and most preferably between 1 kHz and 100 kHz.

[0115] The energy which hits the substrate stack per pulse of irradiation is set in particular between 0.1 nJ and 1 J, preferably between 1 nJ and 900 pj, particularly preferably between 1 nJ and 10 pj.

[0116] A beam spot size is in particular between 1 pm2 and 10 mm2, preferably between 5 pm2 and 1 mm2, particularly preferably between 400 pm2 and 1502 pm2 (measured at l / e2 of the irradiance distribution of the laser spot on the substrate).

[0117] The spatial spacing of the laser pulses on the substrate (pitch) is in particular between 0.1 pm and 1000 pm, preferably between 1 pm and 500 pm, particularly preferably between 10 pm and 200 pm, most preferably between 20 and 100 pm.

[0118] Depending on the total energy required, the number of pulses per substrate stack is in particular between 10 million pulses and 10 billion pulses, preferably between 10 million pulses and 1000 million pulses, particularly preferably between 20 million pulses and 100 million pulses.

[0119] The total energy of the laser radiation per substrate is set in particular between 1 mJ and 500 kJ, preferably between 100 mJ and 200 kJ, particularly preferably between 500 mJ and 100 kJ.

[0120] The pulses have a length in the microsecond to femtosecond range (ps-fs), preferably in the nanosecond to femtosecond range (ns-fs), in particular between 100 ns and 100 fs, preferably between 10 ps and 1 ps.

[0121] With short pulses, very high power peaks can be achieved without increasing the average laser power. Since the available pulse energy is orders of magnitude lower for shorter pulses at different pulse durations (in the range of J for high-power lasers compared to pJ for "ultrafast" picosecond and femtosecond lasers), the total energy input into the material being processed is reduced. This generally leads to a smaller heat-damaged zone due to the shorter exposure time and the resulting lower heat diffusion. High power density makes it possible to heat the material within a very short time to such an extent that ablation or sublimation is achieved. Thus, the short exposure times lead to a lower thermal energy input into the underlying material and thus to minimal damage to the non-processed area.

[0122] For pulse durations below a few picoseconds, direct ablation by the laser radiation is assumed for most materials, whereas for longer pulse durations, additional effects resulting from the interaction of laser, laser-induced plasma and matter in the various aggregate states favor thermally induced ablation.

[0123] At high radiation intensities, plasma glow sets in during material ablation with laser radiation. After the onset of plasma glow, avalanche ionization and thermal ionization occur to such an extent that material damage is no longer limited to the laser focus. It is known from the state of the art that the energy threshold for the formation of luminous plasmas decreases sharply with decreasing pulse duration.

[0124] Overall, ultrashort pulses in the ps range are preferred, so that linear and nonlinear absorption occurs in the multilayer system. Starting at a laser intensity of 10 12 W / cm2, the interaction between photons and atoms occurs not only through single-photon absorption but also through multiphoton absorption. Depending on the intensity, a linear or nonlinear process can be the dominant component of the absorption. At intensities between 10 12 up to 10 14 W / cm2, which are achieved with ultrashort pulses, multiphoton effects play a dominant role. Pulses with high intensities and a pulse duration of less than 100 ps can initiate plasma glow. Plasma glow advantageously leads to greatly increased local absorption at the multilayer system through the interaction of free electrons and ions with the residual electromagnetic field.

[0125] Preferably, the pulse energy and / or the pulse duration and / or the length of a pulse train is temporally modulated by a control unit of a laser beam source generating the pulsed laser beam, with the modulation preferably being controlled via an external signal generator. Preferably, the energy coupled into the process zone by the laser beam is temporally modulated by modulating the pulse duration of the laser pulses, with the pulse duration preferably being modulated between 0.1 ps and 20 ps.

[0126] Synonyms for the irradiation area are known to the expert as spot size or laser spot size.

[0127] The shape of the irradiation area is in particular circular, in other preferred embodiments elliptical or rectangular.

[0128] During laser debonding, laser light is coupled through a substrate that is as transparent as possible and absorbed in the adjacent release layer on the back. The laser light is preferably coupled through a largely transparent silicon carrier substrate. Si carrier substrates with typical thicknesses between 725 and 775 pm are increasingly transparent for wavelengths above 1,100 nm. Ultrashort pulses in the ps range are used, so wavelengths greater than 1,300 nm are preferred. Even more preferred are wavelengths greater than 1,900 nm due to absorption by nonlinear interactions of silicon in the range below 1,300 nm. Shorter pulse durations require longer wavelengths for extensive transparency of the Si carrier substrates.

[0129] Optical and physical processes play a role in the interaction of the laser beam with the material. These include, for example, the numerical aperture (NA) of the lens when focusing the laser beam into the material, and the energy of the laser beam or the laser power density.

[0130] The following parameters lead to different interactions of ultrashort pulse lasers with matter:

[0131] Pulse energy

[0132] Numerical aperture NA

[0133] Pulse duration

[0134] Pulse repetition rate

[0135] Laser wavelength

[0136] Beam profile

[0137] Pulse shape.

[0138] The following criteria are evaluated depending on the parameter settings for a multi-layer system:

[0139] Ablation area per shot

[0140] Debonding area or delamination area per shot

[0141] Pulse energy for ablation threshold per shot

[0142] Pulse energy for delamination threshold per shot

[0143] For example, the following parameters of the multi-layer system can be determined: Thickness of the individual layers by simulation and in testing,

[0144] If necessary, arrangement / sequence of the individual layers, materials of the additional layer(s) if further layers are necessary.

[0145] The multilayer systems are familiar to those skilled in the art, so no material optimization takes place. The materials of the multilayer systems are coatings familiar to those skilled in the art, which are used in bonding, and whose layer thicknesses are optimized for interference and the highest possible absorption of laser radiation. In many cases, the selection of the layer thicknesses makes laser debonding possible in the first place.

[0146] Multilayer system and multilayer design optimization

[0147] The idea underlying the patent is to provide a multilayer, interference-optimized layer structure for bonding and laser debonding of substrates.

[0148] A multitude of factors play a role in the absorption of radiation. The interaction is influenced by both the properties of the laser light and those of the material. For laser light, the most important factors are the wavelength, polarization, angle of incidence, and the spatial and temporal properties of the radiation, whereas for matter, the chemical composition and the microscopic or macroscopic properties are the most important factors.

[0149] The state of the art uses the effects of scattering, reflection, and absorption empirically for different coatings. Optimization of individual parameters is also common, but adjusting layer thicknesses to optimally increase absorption while minimizing losses due to reflection or transmission, and matching the pulse duration to the layer thickness, has not yet been established in the state of the art. Factors such as laser wavelength, angle of incidence, and layer material are kept constant.

[0150] The multilayer design optimization uses existing materials and coatings known to those skilled in the semiconductor industry, which are optimized, in particular, in their layer thickness, so that maximum absorption is achieved through interference of the electromagnetic radiation in the multilayer system.

[0151] The layer thicknesses are in the subwavelength range, so that for an incident wave, the multilayer system exhibits a different wave impedance than that of the individual materials used in the individual layers. This greatly improves the absorption of the multilayer system.

[0152] If electromagnetic radiation can be absorbed in a material, the strength of the absorption is described by a material parameter, the absorption coefficient, which is generally dependent on a variety of parameters (temperature, wavelength, etc.). The absorption or absorption degree is specified between 0 and 1. Of the radiation striking the surface of a body, a portion is usually reflected, a portion is transmitted through the body, and the remainder is absorbed. The absorbed energy increases the body's internal energy. The absorption coefficient (also absorption coefficient or spectral absorption coefficient SAC) indicates which fraction of the incident radiation is absorbed. It can assume values ​​between 0 and 1. The absorption degree can depend on the direction of incidence and the frequency of the incident radiation.

[0153] If the absorption for different wavelengths and different layer thicknesses of a selected layer from a multilayer system is plotted in a graph, it is possible to depict regions of varying absorption. In general, absorption is primarily a lossy interaction of the electromagnetic field in matter, which can (usually) be described by the electrical susceptibility and thus by the complex-valued refractive index n+iK. Even nonlinearities, which play a role particularly with short pulses, e.g., when the response to an increase in the electric field increases proportionally to a higher power, can be represented in this way.In addition, simulations are used to show that by changing the thin layer thicknesses in the nm range, an increase in the absorption of the entire multilayer system can be achieved by creating multiple interferences at the boundaries between the individual layers of the multilayer system.

[0154] Preferably, the laser wavelength is constant, and two parameters, in particular the layer thicknesses d1 and d2 of two layers in the multilayer system, are varied simultaneously, and the absorption is calculated. By optimizing the layer thicknesses in the multilayer system, higher absorption and a reduction in losses due to scattering or reflection, and thus higher laser debonding efficiency, can be achieved. Scattering or diffraction effects can also be exploited to change the propagation direction of the light and thus increase the interaction duration. Scattering or diffraction effects can also be exploited to protect the next, underlying layer or the underlying product substrate.The layer thicknesses are optimized for each individual layer LI to Ln of a multilayer system consisting of layers LI to Ln, preferably LI to L3. The absorption of the entire multilayer system is determined numerically and also measured experimentally. The individual influences of the layers are investigated and optimized with regard to efficiency and stability of the effect.

[0155] The separation of the multilayer system from the substrate is achieved by irradiating the substrate with light, particularly laser radiation, which is strongly absorbed by the multilayer coating at or near the interface between the materials to be separated. The following effects on the adjacent layers are exploited, for example: constructive interference, scattering, diffraction, and phase shifts.

[0156] The layers of the multilayer system can be applied by chemical or physical vapor deposition, sputtering, evaporation, epitaxy and / or spin coating, as well as combinations thereof or other suitable techniques.

[0157] Due to the increased, optimized local absorption of the coating, an antireflective coating (AR) is not necessary to significantly reduce Fresnel reflection.

[0158] Advantageously, an additional bonding layer, in particular a bonding adhesive, is also unnecessary, since the multilayer system, which contains a photothermal multilayer conversion layer, in particular a metallic or metal-containing one, simultaneously also serves as a bonding layer. An additional sacrificial layer is also not required. The energy absorbed in this process induces decomposition of the multilayer coating, resulting in separation at the interface between the substrate and the coating. Decomposition mechanisms can be, for example, sublimation or chemical reactions. The decomposition can be initiated both thermally and photochemically. Separation is particularly supported if gaseous products are formed during the decomposition.

[0159] Preferably, the at least one layer of the multi-layer system consists of the following compounds or elements, individually or in combination:

[0160] Metals, for example Ti, Au, Ag, Cu, Fe, Ni, Al, Cr, Pt, Sn

[0161] Alloys

[0162] Semiconductors (e.g. Ge)

[0163] Compounds, in particular nitride compounds, in particular TiN, TaN, AIN, GaN, InN, SiN, Si3N4

[0164] Compounds, especially oxide compounds, especially SiO2, TiO2

[0165] Compounds, especially dielectrics

[0166] Ceramic material, especially silicon carbide (SiC) and aluminum oxide (A12O3)

[0167] Highly absorbent non-metals, especially polymers with nanoparticles (polymers with Al or C particles)

[0168] The individual layers of the multilayer system can consist of a single material or a combination of materials from one of the main groups 3 (boron group), 4 (carbon group), and 5 (nitrogen group) of the Periodic Table of Elements. In a less preferred embodiment, the material is not applied over the entire surface, but rather to individual layers of the multilayer system as 2D structures, such as graphene, or 3D structures.

[0169] The multilayer system is applied as a sequence of layers of different compounds or elements to the product substrate and / or the carrier substrate. Any number n of coatings can be formed as a multilayer system (LI to Ln). Preferably, up to three layers are used in the multilayer system (LI to L3).

[0170] In a further embodiment, at least one compound or element is applied alternately several times.

[0171] The individual layers can serve, for example, as selective absorber layers or as phase shifters. Examples of absorbers are metals such as aluminum (Al) or gold (Au). Silicon dioxide (SiO2), for example, can be used as an auxiliary layer and / or phase shifter to position the field maximum of the wavelength within the selective absorber. The layer thicknesses are in the lower nm range. Thicker (metal) coatings can serve as mirrors if necessary.

[0172] Preferably, additional layers such as a sacrificial layer and / or an anti-reflection layer and / or a relaxation layer and / or a bonding adhesive are not necessary and are omitted.

[0173] The individual layers of the multilayer coating have thicknesses between 1 nm and 10 pm, preferably between 1 nm and 1 pm, and even more preferably between 5 nm and 500 nm. The very thin layer sequence enables high interaction with electromagnetic radiation. This high interaction with very thin layers is used for simplified laser debonding. By optimizing the individual layer thicknesses of a multilayer system, the layer thicknesses (nm range) are reduced, which advantageously reduces the amount of material that needs to be applied.

[0174] Metals are strong absorbers and can stop laser radiation even at a layer thickness of < 100 nm. In comparison, organic absorbers usually require a layer thickness of > 3 pm to absorb 67% of the incident light.

[0175] The thickness of the multilayer system is preferably between 1 nm and 10 pm, more preferably between 5 nm and 1 pm, most preferably between 10 nm and 1 pm.

[0176] Laser debonding optimization process for a substrate stack with a multilayer system and a silicon carrier substrate

[0177] An optimization process for separation by debonding or delamination along the interface using laser irradiation includes, for example, the following steps:

[0178] Laser selection. Silicon as a carrier substrate is predominantly transparent at wavelengths X > 1300 nm and X > 1900 nm, respectively. Therefore, lasers in the near-infrared (NIR) and mid-infrared (MIR) are selected, which exhibit low linear and nonlinear absorption in the Si carrier substrate.

[0179] Existing materials of a multilayer bonding layer between a Si carrier substrate and a product substrate are used; the multilayer system enables interference-controlled optimization of the absorption required for laser debonding. Depending on their thickness, the individual layers of the multilayer system can serve, for example, as selective absorber layers, phase shifters, or mirrors, thus maximizing the overall absorption of the multilayer system. Examples of absorbers include metals such as aluminum (Al) or gold (Au).

[0180] Silicon dioxide (SiO2) and aluminum nitride (AIN), for example, can be used as phase-shifter layers to position the field maximum of the wavelength within the selective absorber. The layer thicknesses are in the lower nm range. Thicker coatings can serve as mirrors if necessary. A metal layer, for example, can be used as a mirror layer (layer thickness > 100 nm) or as a selective absorber layer (layer thickness < 10 pm), depending on the layer thickness.

[0181] Laser throughput and laser quality optimization. In a preferred embodiment, this involves ultrashort pulsed laser radiation. The laser source and laser wavelength are fixed parameters. The following laser parameters are optimized, for example: pulse duration, pulse repetition frequency, energy, shape of the irradiated area per pulse, and multi-spot laser.

[0182] Optimization of the material layers. The thickness of the material layers is optimized to achieve maximum light absorption through interference and minimize reflection losses. The increase in absorption through optimized layer thicknesses is spatially localized and amplified within the multilayer system. Exceeding or falling below the optimal layer thicknesses of the individual layers of the multilayer system would lead to a significant reduction in absorption.

[0183] The optimization of layer thicknesses is primarily achieved through simulation and / or laser debonding tests on the substrate stack using the layer thicknesses selected from the simulation. The tests examine the bond force, ablation shape, and homogeneity remaining during laser debonding. The manufactured system is also examined for stability of the manufacturing and processing parameters.

[0184] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. These schematically show:

[0185] Figure la: a cross-sectional view of a substrate stack consisting of carrier substrate, multilayer system with three layers and product substrate with functional units.

[0186] Figure 1b : a cross-sectional view of a substrate stack consisting of carrier substrate, multilayer system with three layers and product substrate with structuring.

[0187] Figure 1 c : a cross-sectional view of a substrate stack consisting of carrier substrate, multilayer system with two layers and product substrate.

[0188] Figure 2: a cross-sectional view of a product substrate-carrier substrate stack with a schematic representation of optical components for irradiating the multilayer system with laser radiation.

[0189] Figure 3a: Schematic representation of the absorption spectrum A of a multilayer system. The representation shows the absorption of a multilayer system consisting of three layers LI, L2, and L3, where the thickness dl of layer LI and the wavelength are changed, while the thicknesses of layers L2 and L3 remain unchanged. Figure 3b: Schematic representation of the absorption spectrum A of a multilayer system. The representation shows the absorption of a multilayer system consisting of two layers LI and L2, where the thickness dl of layer LI and the thickness d2 of layer L2 are changed, while the laser wavelength remains unchanged.

[0190] In the figures, identical components or components with the same function are marked with the same reference numerals.

[0191] According to Figure 1a, for example, three layers LI (5), L2 (6) and L3 (7) have been applied over the entire surface of the product substrate 2 and / or the carrier substrate 3. The structures 8 are located in and / or on the product substrate 2. The layer thicknesses d1, d2 and d3 of the respective coatings LI (5), L2 (6) and L3 (7) are optimized. The multilayer system 4 thus consists of several layers 5, 6, 7, which are selected such that the multilayer system 4 leads to maximum absorption of the laser radiation during a laser debonding process. The layer structure 4 optimized via interference enables improved and simpler laser debonding, whereby no additional layers for protecting the substrates or for bonding the substrates, such as an anti-reflective coating and / or a relaxation layer and / or a bonding adhesive, are required.

[0192] The individual layers 5, 6, 7 of the multilayer system 4 have thicknesses between 1 nm and 1 pm, preferably between 1 nm and 500 nm, more preferably between 1 nm and 250 nm. The very thin layer sequence enables a high level of interaction with the electromagnetic wave of the laser irradiation. The thickness of the multilayer system 4 is preferably between 1 nm and 10 pm, more preferably between 5 nm and 2 pm, most preferably between 10 nm and 1 pm, and most preferably between 10 nm and 500 nm.

[0193] After coating the multilayer system 4 on the product substrate 2 and / or the carrier substrate 3, the product substrate 2 is bonded to the carrier substrate 3 in a (temporary) bonding process by aligning, contacting, and bonding, as shown in Figure 1 a. The (temporary) bonding technologies are known to those skilled in the art.

[0194] In Figures 1a and 1b, three coatings LI to L3 are shown (5, 5', 6, 6', 7, 7'), but any other number n of coatings can also be formed. Figure 1c, for example, shows an embodiment of the multilayer system with two coatings 5", 6". An optimization of the layer thicknesses is carried out for each individual layer LI to Ln of a multilayer system consisting of the layers LI to Ln, wherein the absorption of the entire multilayer system is measured. Preferably, for example, two layer thicknesses dl and d2 are first varied simultaneously in a simulation at a constant wavelength and the resulting absorption is determined according to Figure 3b. The layer thicknesses dl max and d2max of the coatings 5", 6" that lead to maximum, efficient and stable absorption are selected. Other variable laser parameters are optimized in particular by analysis during laser debonding of the substrate stack in the test.

[0195] Figure 1b shows a further embodiment of a substrate stack 1' consisting of a carrier substrate 3', a multilayer system 4' with three layers LI to L3 (5', 6', 7'), and a product substrate 2' with structuring. Figure 1c shows another embodiment of a substrate stack 1" consisting of a carrier substrate 3", a multilayer system 4" with two layers LI (5") and L2 (6"), and a product substrate 2".

[0196] In the following section, several non-limiting examples of multilayer systems (e.g., layers LI - L2 - L3 or LI - L2) are given based on the multilayer systems shown in Figures 1a to 1c. Multilayer systems known to those skilled in the art and used in the semiconductor industry, particularly for CMOS-compatible or front-end-compatible processes, consist, for example, of:

[0197] SiO2 - metal - SiO2 (LI - L2 - L3),

[0198] SiO2 - Metal 1 (LI - L2)

[0199] Metal 1 (layer thickness dl) - oxide or nitride compound (e.g. SiO2) - Metal 1 (layer thickness d2) (LI - L2 - L3),

[0200] SiO2 - nitride compound - SiO2 (LI - L2 - L3),

[0201] Nitride compound - SiO2 (LI - L2)

[0202] Oxide or nitride compound (e.g. SiO2) - Metal 1 - Metal 2 (LI - L2 - L3),

[0203] Metal 1 - Metal 2 - Metal 3 (LI - L2 - L3),

[0204] Metal 1 - Metal 2 (LI - L2),

[0205] Metal 1 - Metal 2 - Metal 1 (LI - L2 - L3).

[0206] In particular, the following multilayer systems for laser debonding are specified, which are applied on a 300 mm silicon carrier substrate (with a thickness of 775 pm with surfaces polished on both sides):

[0207] The TEOS layer is a layer of amorphous silicon dioxide (SiO2) and is preferably finely polished by chemo-mechanical polishing (CMP).

[0208] In an alternative embodiment, the 300 mm silicon carrier substrate also has a thickness of 725 pm.

[0209] Layer L2 is followed by the bonded product substrate (also made of silicon). During debonding, the laser first penetrates the 775 pm silicon carrier substrate, then layers L1 and L2.

[0210] The laser wavelength is determined by the choice of the carrier substrate and remains constant. The laser angle of incidence also remains constant.

[0211] Further concrete examples of layer systems: SiN - SiO2 (LI - L2)

[0212] TEOS (50-250 nm) - TiN (20- 100 nm) - TEOS (50-400 nm) (L 1 -L2-L3)

[0213] TiN (50 nm) - TEOS (400 nm) (L 1 -L2)

[0214] SiO2 (thermal, 50-100 nm) - TiN (50 nm) - TEOS (400 nm) (L 1 -L2-L3)

[0215] Preferably, the at least one layer of the multi-layer system consists of the following compounds or elements, individually or in combination:

[0216] Metals, for example Ti, Au, Ag, Cu, Fe, Ni, Al, Cr, Pt, Sn

[0217] Alloys

[0218] Semiconductors (e.g. Ge)

[0219] Compounds, in particular nitride compounds, in particular TiN, TaN, AIN, GaN, InN, SiN, Si3N4

[0220] Compounds, especially oxide compounds, especially SiO2, TiO2

[0221] Ceramic material, especially silicon carbide (SiC) and aluminum oxide (A12O3)

[0222] Highly absorbent non-metals, especially polymers with nanoparticles (e.g. polymers with Al or C particles).

[0223] Depending on their thickness and material, the individual layers of the multilayer system can serve as selective absorber layers, auxiliary layers, and / or phase-shifter layers, or as mirror layers, thus maximizing the overall absorption of the multilayer system. A metal layer, for example, can be used as a mirror layer (layer thickness > 100 nm) or as a selective absorber layer (layer thickness < 10 pm), depending on its thickness. Silicon dioxide (SiO2) and aluminum nitride (AIN), for example, can be used as phase-shifter layers. With three layers, the absorptive layer is usually the middle layer. With two layers, the absorptive layer is usually the first layer. The absorptive layer absorbs the energy of the laser radiation.

[0224] In one example, the absorbent layer consists of SiN and the auxiliary layer of SiO2. The interaction between the SiN and SiO2 layers produces NOx gases, which lead to layer cleavage and thus debonding.

[0225] In a preferred embodiment, the layer thickness of the absorptive layer is between 10 nm and 200 nm and the thickness of the auxiliary layer(s) is between 1 and 1000 nm.

[0226] Figure 2 shows a cross-sectional view of a product substrate-carrier substrate stack 1 during laser debonding by irradiating the multilayer system 4 with laser radiation 11. A suitable light source is, for example, a light source that emits ultrashort light pulses with a duration of 10 ps to 50 ps and a repetition frequency of 1000 Hz.

[0227] The ultrashort pulsed laser beam 11 is focused into the process zone 12 via an optics system 9. A relative movement between the substrate stack 1 and the laser beam 11 takes place using a substrate stack and / or beam positioning system (not shown). Other optical elements include, for example, beam-shaping elements, scanners, modulators, etc., and are known to those skilled in the art.

[0228] The relevant wavelength range for Si as a carrier substrate is 1940 to 2140 nm, because Si exhibits very strong nonlinearity and nonlinear absorption / refraction extends beyond 1700 nm, leading to self-focusing. The energy and power density required for ablation are also important here. For other carrier materials (e.g., sapphire), the wavelength and laser selection are often different.

[0229] Figure 3a describes a process flow for optimizing an exemplary multilayer system 4 consisting of three layers L1, L2 and L3 (5, 6, 7) according to Figure 1a, which is to be used for the temporary bonding and laser debonding of product substrate 2 and carrier substrate 3. It is conceivable that the product substrate 2, 2', 2" has no topography, either because no structures 8 are present or because the structures 8 were manufactured directly in the product substrate 2, 2', 2". Alternatively, the structures can be, for example, chips or structured coatings and form a topography.

[0230] According to Figure 3a, the thickness dl of the first layer LI is varied between 0 and 100 nm in order to determine the maximum absorption of the multilayer system at different wavelengths. Region 1 in Figure 3a shows the maximum absorption. Regions in Figure 3a with increasing number show a decreasing absorption of the multilayer system. The thicknesses d2 and d3 of the other two layers L2 and L3 are kept constant. The individual thicknesses of the layers influence the interference pattern and thus the absorption of the multilayer system. By determining the optimal layer thicknesses dl, d2 and d3, the maximum absorption of the multilayer system is determined for improved and simplified laser debonding. Representations according to Figure 3a are shown using simulations and determined using series of measurements. In this case, optimization orThis eliminates the need to change materials for individual layers, and in existing multilayer systems, optimized layer thicknesses allow for simplified laser debonding through maximum absorption. Absorption can be increased from < 10% to > 90%. Absorptivity is primarily calculated using well-known solution algorithms using linear estimation via Fresnel equations for multilayer systems, based on layer thicknesses and (linear, but complex-valued) refractive indices. Furthermore, the nonlinear properties can be used in more complex simulations that also consider field strength distributions.

[0231] In an alternative embodiment to Figure 3a, for example, for a system consisting of two layers L1 and L2 at a selected laser wavelength, the absorption can be shown as a function of two layer thicknesses dl and d2 according to Figure 3b. If the layers, the substrate and the carrier substrate from a given substrate stack are known, the layer thickness d of the individual layers of the multilayer system is the easiest to control and change. The layer thickness d of the individual layers of the multilayer system is thus primarily optimized. The laser wavelength and the laser angle (angle of incidence) remain unchanged. If a multilayer system consists of two layers, both layer thicknesses dl and d2 can be varied simultaneously according to Figure 3b. The selected parameters, in particular, for example, two layer thicknesses dl and d2 are varied and the degree of absorption in the debond structure is calculated.The absorption coefficient in the debond structure must be as high as possible. Preferably, a maximum of three layers are used to maximize absorption. Analogous to Figure 3a, region 1 in Figure 3b shows maximum absorption. Regions with increasing numbers indicate decreasing absorption of the multilayer system. Region 1 with high absorption in the graph shown must be large enough not to be too sensitive to changes. List of reference symbols e.

[0232] Substrate stack

[0233] Product wafers

[0234] carrier wafer

[0235] Multi-layer system

[0236] Shift LI

[0237] Layer L2

[0238] Layer L3

[0239] structure

[0240] optics

[0241] Optical element

[0242] laser beam

[0243] Process zone

Claims

Patent claims 1. Method for providing a multilayer system (4) of at least two layers (5, 6, 7), in particular for temporarily bonding substrates to a substrate stack (1), comprising the following steps in the following order: i) provision of a multilayer system (4), ii) determination of an absorption coefficient of the multilayer system (4) for laser radiation (11) of a specific wavelength, iii) variation of at least one parameter of the multilayer system (4), iv) determination of the absorption coefficient of the multilayer system (4) for the laser radiation (11) of the specific wavelength with the at least one parameter varied according to step iii), v) repetition of steps i) to iv) until the absorption coefficient is greatest, wherein in step i) the multilayer system (4) with the greater absorption coefficient is provided each time.

2. Method according to claim 1, wherein the at least one parameter of the multilayer system (4) is a layer thickness of a layer (5, 6, 7) of the multilayer system (4).

3. Method according to claim 2, wherein the at least one parameter of the multilayer system (4) is additionally a layer thickness of a further layer (5, 6, 7) of the multilayer system (4).

4. Method according to at least one of the preceding claims, wherein the wavelength during the determination in steps ii) and iv) is between 1100 nm and 10000 nm, preferably between 1100 nm and 5000 nm, more preferably between 1500 nm and 2500 nm.

5. Substrate stack ( 1 ) comprising at least one multilayer system (4) provided according to at least one of claims 1 to 4, with at least two layers (5 , 6, 7) of different materials.

6. Substrate stack ( 1 ) according to at least one of the preceding claims, wherein the multilayer system (4) has a total thickness between 1 nm and 10 pm, more preferably between 5 nm and 2 pm, most preferably between 10 nm and 1 pm, and most preferably between 10 nm and 500 nm.

7. Substrate stack ( 1 ) according to at least one of the preceding claims, wherein the respective layers (5, 6, 7) of the multilayer system (4) each have a layer thickness between 1 nm and 1 pm, preferably between 1 nm and 500 nm, more preferably between 1 nm and 250 nm.

8. Substrate stack ( 1 ) according to at least one of the preceding claims, wherein the multilayer system (4) comprises at least one layer (5 , 6, 7) with a layer thickness between 25 nm and 75 nm.

9. Substrate stack ( 1 ) according to at least one of the preceding claims, wherein at least one layer (5, 6, 7) of the multilayer system (4) preferably comprises titanium (Ti), aluminum (Al), aluminum nitride (AlN), tantalum nitride (TaN), (germanium) Ge, (tin) TiN or copper (Cu).

10. Substrate stack (1 ) according to at least one of the preceding claims, wherein at least one layer (5, 6, 7) of the multilayer system (4) consists of amorphous silicon dioxide (SiO2). 1 1. Substrate stack (1 ) according to at least one of the preceding claims, wherein the substrate stack (1 ) comprises at least a support substrate (3 ) and a product substrate (2), wherein the support substrate (3) is connected to the product substrate (2) by the multilayer system (4).

12. Substrate stack (1 ) according to at least one of the preceding claims, wherein the multilayer system (4), preferably the substrate stack ( 1 ), does not include a polymer-based bonding adhesive.

13. Substrate stack (1 ) according to at least one of the preceding claims, wherein the multilayer system (4), preferably the substrate stack ( 1 ), does not have an antireflection layer.

14. Method for bonding substrates to form a substrate stack ( 1 ) according to at least one of claims 5- 13 comprising the following steps, 1) Provision of a first substrate, in particular a carrier substrate (3), 2) Bonding a second substrate, in particular product substrate (2), to the first substrate.

15. Method for debonding a substrate stack (1) comprising the following steps: a) providing a substrate stack (1) according to at least one of claims 5 to 13; b) irradiating the multilayer system (4) through at least one substrate of the substrate stack (1) with laser radiation (11) of a specific wavelength; and thereafter, c) separating the substrate stack (1) in the region of the multilayer system (4).

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