Method for surface treatment of a substrate and method for bonding such a substrate to a further substrate, and a device for carrying out such methods

Abstract

The invention relates to a method for surface treatment of a substrate (B), in particular as a preparatory step for a method for bonding the substrate to a further substrate, comprising: – providing the substrate (B) having a bonding surface provided for the bonding, – treating (100) the substrate (B) at least in a portion of the bonding surface to form an amorphous layer (C) having a first thickness (D1), a primary particle radiation preferably being used for forming the amorphous layer (C), and – reducing (200) the layer thickness of the amorphous layer (C), preferably to a second thickness (D2).

Description

[0001] Method for surface treatment of a substrate and method for bonding such a substrate to another substrate and an apparatus for carrying out such methods

[0002] The present invention relates to a method for surface treatment of a substrate and a method for bonding the substrate to another substrate and an apparatus for carrying out such methods.

[0003] In the semiconductor industry, various bonding technologies have been used for several years to connect substrates. This connection process is called bonding. A distinction is made between temporary bonding processes and permanent bonding processes. In fusion bonding, for example, two flat, clean substrate surfaces are bonded together by contacting. In a first step, the two substrates are contacted, with the two substrates being pre-fixed by van der Waals forces. To create a permanent fusion bond, the substrate stacks are subjected to heat treatment.

[0004] In order to enable bonding at low temperatures, a plasma treatment can be carried out before fusion bonding to clean and activate at least one of the substrate surfaces.

[0005] In US 10,312,217 B2, the substrate surfaces are pretreated at room temperature prior to bonding to improve bond strength. For this purpose, a bonding layer is deposited on the substrate surface, planarized by chemical-mechanical polishing, and modified by plasma etching. The surface is activated by RIE plasma processes or by reactions with reactive gases or liquids, so that the activated surface consists of an activated oxygen or activated nitrogen compound. US 2021 / 0225 803 A1 discloses systems that enable a combination of wet-chemical processes and vacuum-based processes for the pretreatment of substrates prior to bonding at the lowest possible temperatures. In US 2021 / 0225 803A1, passivating oxide layers are removed largely without the formation of amorphous layers or the implantation of particles on the substrate surface.For this purpose, a low-energy plasma is used in a plasma chamber, in combination with a series of previously performed wet chemical processes to treat the substrate surfaces.

[0006] In US 10,985,204 B2, a low-energy plasma is used to remove surface oxides. The formation of an amorphous layer is undesirable. In US 10,985,204 B2, an additional implantation of low-energy hydrogen is performed in the surfaces to be bonded. Any local defects that could lead to higher electrical resistance at the bond interface after bonding the substrates at temperatures below 300 °C are passivated with the implanted hydrogen during the subsequent heat treatment.

[0007] For example, in US 10,312,217 B2, an additional bonding layer is applied, which is further activated using physical and / or (wet) chemical processes. US 202 1 / 0225 803 A1 also discloses systems that enable a combination of wet chemical and vacuum-based processes for the pretreatment of substrates prior to bonding at the lowest possible temperatures. US 10,985,204 B2 additionally involves the implantation of low-energy hydrogen into the surfaces to be bonded.

[0008] Another method for modifying substrate surfaces prior to bonding is ion beam technology. In this process, the accelerated ions impact the substrate material to be modified at the end of their trajectory.

[0009] A conventional ion gun comprises a source of charged particles accelerated by an externally applied electric field created between a pair of grids. Traditionally, three grids are used to produce low-energy ion beams. Various grid arrangements can be used, with their potentials individually controlled. In a multi-grid system, the first grid encountered by the ions is usually positively biased, and the second grid is negatively biased. Another grid can be used to slow down the ions emerging from the ion source, creating a collimated beam of ions with more or less uniform energy.

[0010] Preferably, oxide removal is performed prior to bonding substrate surfaces for materials in which an oxygen-containing atmosphere forms a native oxide. However, this does not apply to generated oxygen substrate surfaces, such as silicon oxide. In particular, preferably at least predominantly, more preferably exclusively, harmful, unnecessary and / or native, in particular metallic, oxides are removed. Preferably, the aforementioned oxides are removed as largely, in particular completely, before a bonding process to prevent them from being incorporated into the bonding interface (contact surface between two substrates). Incorporating such oxides would lead to mechanical destabilization and very low bond strength. The oxide is removed in particular by physical and / or chemical processes.

[0011] Pretreatment also changes the microroughness of the substrate surfaces, and when optimal surface roughness is achieved, maximum bonding energies can be achieved. This allows spontaneous, covalent bonds to form at low temperatures, especially at room temperature. Contacting takes place in a high vacuum.

[0012] EP 3 161 855 B1 describes a method and a device for the surface treatment of substrates. The basic idea of ​​EP 3 161 855 B1 is to create a predominantly amorphized layer on a substrate surface to be bonded. The amorphization of the substrate surface in EP 3 161 855 B1 leads to a better bonding result, especially at comparatively low temperatures. Preferably, cleaning of the surface to remove oxide and amorphization are carried out simultaneously.

[0013] In the prior art, the formation of an amorphous surface layer due to phase transformation of the substrate material during pretreatment of substrate surfaces prior to bonding is undesirable. If an amorphous layer is formed in the prior art, it is not applied by transformation of the substrate material itself, but by deposition of an additional amorphous layer on the substrate surface using chemical and / or physical deposition processes. To prevent potential damage or to achieve better bond strength at room temperature, additional process steps are necessary in the prior art.

[0014] EP 3 161 855 B1 relates in particular to a method for permanently bonding two substrates, at least one, preferably both, of which are treated prior to bonding. Surface regions, in particular a contact side, preferably over the entire surface, of both or at least one of the two substrates are amorphized prior to the bonding process. Amorphization produces a nanometer-thick layer in which the atoms of at least one of the surfaces to be bonded are randomly arranged. To produce a bond according to the invention, the surfaces are cleaned, in particular to remove oxides. A key aspect of EP 3 161 855 B1 is the use of energetic particles, in particular ions, to effect amorphization.

[0015] In EP 3 161 855 B1, an amorphous layer is created simultaneously with oxide removal using an ion beam. The amorphous layers on the surfaces of the substrates pretreated and activated according to EP 3 161 855 B1 have a positive effect on the bond strength during bonding at room temperature. However, the amorphous residual layer in the bonded substrate stack has a disruptive effect by increasing the electrical resistance at the bonding interface. Furthermore, the amorphous residual layer after bonding can have a negative effect on the interaction of the (multilayer) substrate stack with electromagnetic radiation. Relevant parameters include, for example, the thickness and the different optical properties of the amorphous layer.

[0016] While the amorphous layer has a positive effect on the bonding process, it can have a negative or disruptive effect on further process steps in the processing of the (multilayer) substrate stack after the bonding process. As already mentioned, the residual amorphous layer at the bonding interface can cause undesirable electrical resistance in the bonded substrate stack. Even as an additional layer in a bonded multilayer system, the amorphous layer can interfere with further process steps. For example, the wavelength dependence of the refractive index of crystalline silicon (c-Si) and amorphous silicon (a-Si) is different. Thus, when the bonded multilayer system is laser irradiated, for example during laser debonding, there can be a difference in the refractive index between crystalline and amorphous silicon.The parameters of a (bonded) substrate stack that are relevant for the interaction with electromagnetic radiation, especially laser radiation, include the number of layers in the multilayer system, the thickness of all layers, and the optical constants of all media. Therefore, the amorphous layer cannot be neglected.

[0017] Furthermore, oxide removal by sputtering requires very high ion energies to remove material from the substrate surface. This leads to ions being partially implanted into the substrate, thereby damaging the layer near the surface. This damaged layer can typically be several nm thick, typically even 5 to 10 nm or more. This damage can negatively impact the electrical and optical characteristics of the bond, making it undesirable and problematic in practice.

[0018] The object of the present invention is to provide a method and a device for pretreating a substrate surface, with which substrates can be bonded as far as possible without disturbing influences of the amorphous residual layer.

[0019] The object is achieved by a method according to claim 1 or claim 11 and a device according to claim 15. Preferred embodiments can be taken from the description, the dependent claims and the figures.

[0020] According to a first aspect of the present invention, a method for surface treatment of a substrate, in particular as a preparatory step for a method for bonding the substrate to another substrate, is provided, comprising:

[0021] Providing the substrate with a bonding surface intended for bonding,

[0022] Treating the substrate at least in a partial area of ​​the bonding surface to form an amorphous layer having a first thickness, wherein primary particle radiation is preferably used to form the amorphous layer, and reducing the layer thickness of the amorphous layer, preferably to a second thickness.

[0023] Compared to the methods known from the prior art, the invention additionally provides for reducing the layer thickness of the amorphous layer. Reducing the layer thickness of the amorphous layer makes it possible to significantly reduce the extent of the adverse effects of the amorphous layer. This advantageously allows for the use of primary particle radiation, which has proven particularly advantageous for removing oxides and thus for cleaning the substrate surface, particularly in the region of the intended bonding surface. At the same time, the positive effect of the amorphous layer on the bonding process can also be utilized. Therefore, the combination of treating the substrate to form an amorphous layer on the one hand and reducing the layer thickness on the other hand proves to be particularly advantageous.

[0024] In particular, it is envisaged that the treatment of the substrate prior to the reduction in the thickness of the amorphous layer serves to clean the bonding surface. In this process, the bonding surface is freed, at least in some regions, of a native and / or unnecessary oxide that is formed on a substrate surface. In particular, it is envisaged that the amorphous layer is formed by a phase transformation in which the composition of the crystalline substrate surface of the provided substrate is transformed in the treated region of the bonding surface. This procedure thus differs in particular from approaches in which an amorphous layer is specifically applied to the substrate surface. Instead, the crystalline phase is converted into an amorphous phase by means of the primary particle radiation.Accordingly, a first kinetic energy of the primary particle radiation is adjusted to ensure conversion into the amorphous phase. In this regard, explicit reference is made to the disclosure of EP 3 161 855 B1. The substrate and the further substrate are preferably bonded to each other via corresponding bonding surfaces.

[0025] While the formation of an amorphous layer is necessary for improved bonding results when bonding substrates at low temperatures, particularly at room temperature, this amorphous layer can interfere with subsequent processing steps of the bonded multilayer system. In particular, the residual amorphous layer has a negative impact on the electrical properties of the bonding interface. It is therefore advantageous to further treat at least the bonding surface of the substrates, preferably both substrates, in such a way, particularly by dynamic sputtering or layer thickness reduction using particle radiation, that the amorphous layer initially formed during pretreatment is largely gently removed, particularly reduced to a necessary minimum.

[0026] The substrates comprise at least one of the following (crystalline) materials: semiconductor materials such as Si, Ge, III-V compound semiconductors such as GaAs, InP II-VI compound semiconductors, semiconductor alloys,

[0027] Oxides, especially SiO2 and crystalline oxides of titanium, nitrides, especially SiN x , metals such as Al, Cu, Au, Ti, alloys, ceramics, carbides, especially SiC

[0028] The substrates can be of any shape, but are preferably circular. For substrates, especially so-called 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. However, a preferred embodiment can handle any substrate, regardless of its diameter.

[0029] The particle radiation is preferably ion radiation provided by an ion source or an ion gun. For this purpose, a gas or a gas mixture is preferably ionized. It is particularly preferred that the layer thickness reduction, particularly in the form of sputtering, be achieved by particle radiation.

[0030] In a first process step in the sense of the method from EP 3 161 855 B1, the first kinetic energy of the ions is adjusted such that, upon impact with the substrate surface, they cause surface cleaning, in particular removal of the native oxide layer and amorphization. The amorphization is limited in particular to the near-surface region of the substrates to be bonded together, preferably by selecting the process parameters temperature, pressure, ion energy and / or ion current density during the amorphization. The material of the substrate remains, apart from the amorphized layer, at least predominantly, preferably completely, crystalline. In a second process step, ieDuring layer thickness reduction, the parameters relevant for the ion beam, in particular ion energy and / or ion current, are continuously or stepwise reduced or changed in such a way that the amorphous layer produced in the first process step is gently further reduced without other collateral changes to the near-surface areas of the substrate (ie implantation, further amorphization or sputtering of the underlying substrate material itself).

[0031] Amorphization preferably occurs through particle collision with the substrate surface using primary particle radiation. The particles are either charged or uncharged. Acceleration is preferably carried out with charged particles, especially ions, since charged particles are technically easier to accelerate.

[0032] When the ions penetrate the near-surface region of the substrate, different interactions are triggered between the ion and target atoms. The particle bombardment first causes the desorption of weakly bound adsorption layers, then the sputtering of foreign and reaction layers (e.g., oxides with thicknesses of 1 to 10 nm), and finally the sputtering of the substrate material itself.

[0033] Surface cleaning using primary particle radiation (sputtering) is an ion-induced emission of atoms, atom clusters, or molecules from the substrate surface and is used for oxide removal. The sputtering yield depends, among other things, on the ion energy, the ion type, the ion incidence angle, and the substrate material.

[0034] Beam damage and dislocations of substrate atoms occur in the substrate material predominantly as a result of collision processes between the penetrating ions and the target atoms. The collision cascades triggered in the substrate and the layer generate a high defect concentration, which can lead to amorphization. If an ion is fired at a solid with sufficiently high energy, especially greater than 100 eV, it penetrates its surface and is subjected to interaction processes with the target material. For example, a point defect can arise if more than 25 eV is transferred to a lattice atom, depending on the material and the ion beam. With increasing implantation dose, the individual defect regions begin to overlap and ultimately lead to amorphization of the substrate material. If less than 25 eV is transferred to a lattice atom, this energy is converted into phonon energy and causes a temperature increase.In areas where the substrate material has been amorphized, long-range atomic order is no longer detectable. The following parameters influence the transformation of the crystalline phase into an amorphous phase of the material, for example, silicon, after bombardment with heavy argon ions: ion mass, ion energy, ion current strength, or ion current density.

[0035] Implantation dose, ion incidence angle and substrate temperature.

[0036] When atomizing with ion beams, the energy and current density of the bombarding ions can be varied independently of each other.

[0037] During surface cleaning, an amorphous surface layer forms after the removal of surface oxides due to beam damage, dislocations, and ion implantation. Ion implantation occurs predominantly using high-energy ions. The implantation depends on the penetration depth of the injected ions into the substrate material, which is dependent on the ion energy and the ion incidence angle. The substrate is located at a distance of 10 cm to 50 cm from the ion source's ion exit grid.

[0038] By bombarding the substrate with primary particle radiation, an amorphous layer is created in the existing crystalline base material of the substrate. Amorphization also causes a planarization of the substrate surface. Surface planarization, which has a positive effect on bonding, thus occurs particularly during amorphization, particularly in addition to the planarization by force applied during the bonding process. Adjusting the angle of incidence of the primary particle radiation enables control of the removal rate and thus the surface roughness. The angle of incidence is therefore selected to maximize amorphization, removal of impurities, especially oxides, and surface smoothing for the desired result. Amorphization also ensures greater mobility of the material at the interfaces.This allows for any residual roughness to be better compensated. In particular, remaining gaps between the substrate surfaces can be closed. Amorphization, in particular, creates a thermodynamically metastable state at the substrate surface (bonding interface).

[0039] According to one embodiment of the invention, the angle of incidence between the substrate surface and the ion beam is freely selectable and adjustable. The angle of incidence is defined as the angle between the substrate surface and the ion beam. The angle of incidence is in particular between 0° and 90°, preferably between 20° and 70°, and more preferably between 30° and 60°. The angle of incidence of the ion beam can be used to control the impact energy of the ions on the substrate surface.

[0040] Preferably, surface areas or bonding surfaces, in particular a contact side, preferably over the entire surface, of both or at least one of the two substrates are amorphized in the surface cleaning and activation process prior to the bonding process. The surface cleaning and activation process can also be performed on surface areas that are smaller than the substrate surface, in particular on surface areas separated from one another.

[0041] The pretreatment takes place in a pretreatment module that can be separated from the pre-fixing module and / or the bonding module, in particular by locks. In this module, surface cleaning is first performed by particle bombardment, creating an amorphized substrate surface. In particular, oxide removal, surface smoothing, and amorphization are performed simultaneously.

[0042] Preferably, the amorphized layer is not created by a material applied by physical and / or chemical processes, but rather by a phase transformation of the substrate material. This completely eliminates the need for the deposition of a material, especially an unwanted or harmful one.

[0043] According to a preferred embodiment of the invention, the amorphization is controlled by adjusting the first kinetic energy of the accelerated particles of the primary particle radiation, in particular the ions of a primary ion radiation. The first kinetic energy of the particles is in particular set between 1 eV and 100 keV, preferably between 25 eV and 50 keV, more preferably between 25 eV and 10 keV, most preferably between 25 eV and 1 keV. A low-energy ion beam, such as an argon ion beam, has an energy between 50 eV and 1 keV in a first embodiment. The current density (number of particles, in particular ions, per unit time and area) is in particular set between 0.1 mA / cm 2 and 10 mA / cm 2 , preferably between 0.1 mA / cm 2 and 5 mA / cm 2 , more preferably between 0.1 mA / cm 2 and 2 mA / cm 2 , chosen.

[0044] The treatment time for the substrate to form an amorphous layer is preferably between 1 s and 200 s, preferably between 10 s and 200 s, more preferably between 50 s and 200 s, and most preferably between 100 s and 200 s. The treatment time depends in particular on the process and material.

[0045] A first thickness of the amorphous layer immediately after formation in a substrate surface is in particular less than 100 nm, preferably less than 50 nm, even more preferably less than 10 nm, and most preferably less than 5 nm. The first thickness of the amorphous layer depends in particular on the process and material. Amorphization creates an amorphous layer several nanometers thick in which the atoms are randomly arranged. This random arrangement leads to a better bonding result, especially at comparatively low temperatures.

[0046] Preferably, primary particle radiation is used to form the amorphous layer, using particles with a first kinetic energy, and secondary particle radiation is used to reduce the thickness of the amorphous layer, using particles with a second kinetic energy, wherein the second kinetic energy is less than the first kinetic energy. It has been found that particle radiation can not only cause the substrate surface to convert into the amorphous phase to form the amorphous layer, but it is also possible to achieve the layer thickness reduction with particle radiation in a way that protects the substrate.It is also conceivable for the layer thickness reduction to be carried out mechanically and / or chemically and / or by means of laser light, or for the layer thickness reduction to be assisted by mechanical and / or chemical removal and / or removal induced by the action of laser light. In particular, it is preferably provided that the particles with the first kinetic energy and the particles with the second kinetic energy are provided by the same particle source. This advantageously makes it possible to switch from a treatment to form the amorphous phase to a layer thickness reduction without great effort. In particular, a first and / or second kinetic energy is understood to mean the energy of the particles that can be assigned to the particles when they impact the substrate surface.If the particles are ions, the kinetic energy can be adjusted particularly easily via the accelerating voltage in the ion source. However, it is also conceivable that the kinetic energy can be adjusted by appropriate braking, for example, downstream of an exit from the ion source, or by extending the flight path.

[0047] Typical gases and / or gas mixtures used for surface cleaning, amorphization and subsequent dynamic sputtering or layer thickness reduction using particle radiation include:

[0048] - Atomic gases, in particular o Ar, He, Kr, Ne, Xe, Molecular gases, in particular o H2, N2, CO, CO2, Gas mixtures, in particular o Forming gas FG (Argon between 100% and 50% + Hydrogen) and / or o Forming gas RFG (Hydrogen between 100% and 50% + Argon) and / or o Forming gas NFG (Argon between 100% and 50% + Nitrogen)

[0049] Atomic gases, preferably noble gases, in particular Ar, He, Kr, Ne, Xe, are preferably used.

[0050] The treatment, in particular the cleaning, removal of unwanted oxides, and / or amorphization, preferably takes place in a vacuum chamber as the process chamber. The vacuum chamber is pressurized with a suitable vacuum pump system to a pressure of less than 1 bar, preferably less than 1 mbar, even more preferably less than 10' 3 mbar, can be evacuated. The vacuum chamber is preferably evacuated to a predetermined pressure, more preferably completely, especially before the ions are used for amorphization. The oxygen content in the process chamber is reduced to such an extent that reoxidation of the substrate surfaces is impossible.

[0051] To reduce the layer thickness, the parameters relevant for the ion beam, in particular ion energy and / or ion current, are gradually reduced or changed in such a way that the amorphous layer produced in the first process step is gently reduced without other collateral changes to the near-surface areas of the substrate, such as implantation, further amorphization or sputtering of the substrate material itself.

[0052] By reducing the ion energy of the ion beam, the sputtering rate is reduced. Preferably, the ion energy is lowered so that the amorphous layer can be gently removed within a suitable time frame. Removal still occurs via the physical sputtering effect of the ions. The substrate remains in the same pretreatment module, and no further (wet) chemical and / or physical pretreatment is necessary to activate the surface.

[0053] In the embodiment of sputtering the amorphous layer without active ion acceleration, the treatment time for layer thickness reduction is selected in particular between 1 s and 1500 s, preferably between 20 s and 1200 s, more preferably between 30 s and 1200 s.

[0054] In the embodiment of sputtering the amorphous layer with a low ion acceleration, the treatment duration is selected in particular between 1 s and 1200 s, preferably between 20 s and 900 s, more preferably between 30 s and 600 s.

[0055] The intended use of the ion gun or ion beam without ion acceleration allows for a correspondingly low ion energy and the lowest possible current density. In addition to the low ion energy, a short treatment time is also desired. With the optimal selection of the relevant parameters, the surface roughness or surface smoothness in the second process step does not change significantly.

[0056] The thickness of the produced amorphous layer, as well as the purity of the amorphous layer, influences the bond strength in the subsequent bonding process. According to EP 3 161 855 B1, the bond strength increases with increasing thickness of the amorphous layer. However, the thickness of the amorphous layer can be reduced for substrates with a substrate surface with a very low average roughness. The average roughness of the substrate surface is in particular less than 10 nm, preferably less than 8 nm, even more preferably less than 6 nm, most preferably less than 4 nm, and most preferably less than 1 nm. In particular, it is envisaged that the second thickness of the amorphous layer of a substrate to be bonded is reduced to a necessary minimum.

[0057] The second thickness of the amorphous layer on the substrate is thus less than 15 nm after further sputtering of the substrate surface, preferably less than 5 nm, even more preferably less than 2 nm, most preferably less than 0.5 nm. In a preferred embodiment of the invention, the second thickness of the amorphous layer on the substrate after sputtering of the substrate surface is between 0.1 nm and 2 nm. As a result, the greatly reduced amorphous layer can still positively influence the bond strength when bonding the pretreated substrates, wherein the second thickness of the residual amorphous layer has been reduced to such an extent that the bonding interface shows a much lower electrical resistance in the bonded substrate stack.

[0058] It is preferably provided that the ratio of the second kinetic energy to the first kinetic energy is less than 0.1, preferably less than 0.05, and particularly preferably less than 0.01. It has proven particularly advantageous if the second kinetic energy is as small as possible, since this enables particularly gentle removal of the amorphous material, so that no further damage occurs to the bonding surface during the reduction in the thickness of the amorphous layer. In particular, the probability of a renewed phase transformation into the amorphous phase or the implementation of ions into the substrate is reduced. It has also been found that it is sufficient for the particles to impact the amorphous layer at their diffusion speed in order to bring about the desired reduction in layer thickness.

[0059] In particular, it is intended that the second kinetic energy of the secondary particle radiation is changed during the layer thickness reduction. In contrast, the first kinetic energy of the primary particle radiation is kept essentially constant. The reduction of the second kinetic energy proves advantageous because, as the layer thickness decreases, the probability of interaction with the substrate surface is reduced. At the same time, a comparatively large amount of material is removed at the beginning of the layer thickness reduction, which proves advantageous with regard to the duration of the post-processing step leading to the layer thickness reduction.In a first embodiment, the reduction in the thickness of the amorphous layer occurs in several steps, with the ion beam parameters being changed repeatedly, in particular reduced, so that the removal of the amorphous layer becomes increasingly smaller until the desired minimum thickness of the amorphous layer is reached. In this first embodiment, the dynamic sputtering of the amorphous layer preferably occurs in two to five steps, with the relevant ion beam parameters being changed in each step so that a smaller number of particles impact the substrate surface with less energy.

[0060] Furthermore, it is preferably provided that the second kinetic energy of the particles of the second particle beam is continuously changed. In an exemplary embodiment, the parameters of the ion beam are continuously changed, in particular reduced. Preferably, the ion beam voltage is continuously reduced following the first process step for gentle and controlled sputtering of the amorphous layer until the required residual layer thickness is reached.

[0061] In particular, it is provided that the second kinetic energy of the particles of the second particle beam is changed stepwise. In this embodiment, the second process step involves a layer thickness reduction, in particular a sputtering of the amorphous layer with the ion beam without ion acceleration (idle activation). The ion gun is operated in idle mode or with idle activation. The ions are not accelerated, and in a first embodiment, the ion beam voltage is reduced stepwise. The ion beam voltage is reduced stepwise, in particular, from 200 V to 50 V. For example, the ion beam voltage is reduced in 50 V steps, more preferably in 20 V steps. In a second embodiment, the ion beam voltage is not reduced stepwise, but continuously from 200 V to 50 V.In idle mode, the plasma source or ion source is maintained and the potentials of the grids of the ion gun are in the order of magnitude of the open circuit voltage after reduction of the ion beam voltage.

[0062] By reducing the ion energy and the number of ions leaving the ion source during open-circuit control, the sputtering rate is significantly reduced, resulting in a very gentle reduction in the thickness of the amorphous layer. In a further embodiment of the second process step, the ions are slightly accelerated, and the ion beam voltage is gradually reduced from 200 V to 50 V in a first embodiment, or continuously reduced from 200 V to 50 V in a supplementary embodiment. With a slight acceleration of the ions, higher sputtering rates can be achieved compared to the embodiment without active acceleration of the ions, so that the treatment time can be shortened. This enables the treatment of thicker amorphous layers from the first process step, thereby shortening the first process step.In particular, the reduction of the amorphous layer thickness in the first process step, which is performed with much higher ion energies, can be shortened because the amorphous layer can be removed more gently and effectively in the second process step. The choice of the embodiment depends on the substrate material and the ions used, since the layer thickness of the amorphous layer and the sputtering rate can vary greatly.

[0063] In the embodiment with a stepwise reduction, the ion beam voltage is reduced in several steps, in particular in at least one step, more preferably in at least two steps, even more preferably in at least three steps, and most preferably in four steps. The threshold voltage at which sputtering begins is between 20 V and 50 V, depending on the substrate material and the ions used. The ion beam voltage is reduced step by step, in particular from 200 V to 50 V. The kinetic energy of the particles is low, in particular between 200 eV and 50 eV. In comparison, the surface cleaning and activation process in the first process step is carried out with accelerated particles with a relatively high kinetic energy, in particular up to 1000 eV.

[0064] By individually adjusting the potentials of the ion gun grids, particularly the extraction grid and the acceleration grid, the acceleration of the ions can be controlled. The aim of the present invention in the second process step is to provide an ion gun or to control it in such a way that very low sputtering of the amorphous layer can be achieved with a very low ion beam voltage and by exploiting the diffusion of the generated ions.

[0065] The second kinetic energy of the particles of the secondary particle radiation is set in particular between 1 eV and 1 keV, preferably between 1 eV and 800 eV, even more preferably between 20 eV and 500 eV, most preferably between 20 eV and 300 eV, and most preferably between 50 eV and 200 eV. An energy between 50 eV and 200 eV has proven particularly advantageous. The second kinetic energy preferably changes over at least two-thirds of the above intervals. This thus represents a comparatively large range of different second kinetic energies that are used to ablate the amorphous phase. This has proven advantageous for finding the most optimal setting for the layer thickness reduction that can be implemented quickly without causing further damage to the substrate surface.

[0066] It is particularly preferred if the second thickness has a value which is less than 15 nm, preferably less than 5 nm and most preferably less than 2 nm or even less than 0.5 nm. Such small second thicknesses, in particular less than 2 nm, advantageously make it possible to make use of the positive properties of the amorphous layer during bonding, while the amorphous layer is so thin that there is a sufficiently high probability that the amorphous layer will return to a crystalline phase during bonding. Advantageously, the overall substrate produced from the bonding of the substrate and the further substrate then has only a small amount or even no amorphous components, which are disadvantageous with regard to individual physical properties of the overall substrate.

[0067] In particular, it is envisaged that the treatment for forming the amorphous layer and the layer thickness reduction are carried out in a common module. This advantageously makes it possible to realize the layer thickness reduction immediately after the formation of the amorphous phase, in particular without complex transport of the substrate. Furthermore, the same properties in the environment of the substrate, such as a temperature and / or a vacuum pressure in a treatment chamber of the device for carrying out the process, can be advantageously utilized.

[0068] Preferably, the layer thickness reduction takes between 1 s and 1500 s, preferably between 20 s and 1200 s, and even more preferably between 30 s and 1200 s. It has been found that a corresponding layer thickness reduction can be achieved comparatively quickly. The required time period depends essentially on the second kinetic energy and the second thickness to be set.

[0069] The present invention further provides a method for joining a substrate, which has been pretreated using the method according to the invention, to another substrate, which has in particular been pretreated using the method according to the invention, wherein the substrate and the further substrate are joined in a bonding process. All advantages and properties of the above method can be transferred analogously to this method and vice versa. The substrate and the further substrate are joined or bonded to one another via respective bonding surfaces. Contacting two substrate surfaces with very thin amorphous layers in the lower nm or subnanometer range leads to better bond strength with the advantage of a vanishingly small amorphous boundary layer on the substrate stack, which, due to the greatly reduced layer thickness, does not offer any substantial electrical resistance at the bonding interface.

[0070] In particular, it is intended that the second thickness be reduced to a third thickness during the bonding process. This advantageously avoids further amorphous components in the overall substrate. In particular, it is intended that bonding parameters be adjusted in such a way that the conversion of the amorphous phase into a crystalline phase is favored.

[0071] The total thickness of the amorphous layer in the substrate stack, formed from the substrate and the further substrate, is thus less than 30 nm after bonding, preferably less than 15 nm, more preferably less than 5 nm, most preferably less than 2 nm, most preferably less than 1 nm.

[0072] In particular, a phase transformation from the amorphous state to the crystalline state can occur during and / or after bonding and / or during heat treatment, so that the second thickness, i.e., a residual layer thickness of the amorphous layer, is further reduced to a third thickness. In a preferred embodiment, the process parameters mentioned are selected such that minimizing the layer thickness of the amorphous layer of the substrates to be bonded results in a complete transformation of the amorphous residual layer of the bonded substrate stack into the crystalline phase during and / or after bonding and / or during heat treatment.

[0073] After pretreating at least one of the two substrate surfaces, preferably both substrate surfaces, the two substrates are aligned to each other in an alignment module, preferably directly in the bonding chamber. Bonding of the pretreated and aligned substrates takes place in the bonding chamber, wherein the bonding chamber is preferably integrally connected to the process chamber for amorphization and dynamic sputtering in a cluster system, and more preferably can be transferred from the process chamber to the bonding chamber while constantly maintaining evacuation.

[0074] In a preferred high vacuum environment, especially < 10' 7 mbar, preferably < 10' 8mbar, the polished and pretreated substrate surfaces bond spontaneously and covalently through pure contact without additional pressure, or at least only very low pressure, especially at room temperature. If necessary, the bonding process can be enhanced by additional force and / or temperature.

[0075] In particular, it is provided that the second thickness is set such that the amorphous layer present during bonding is converted into a crystalline phase. In particular, more than 30%, preferably more than 50%, and especially more than 70% of the amorphous components of the amorphous layers are returned to the crystalline phase. It is also conceivable that the second thickness is set such that a complete return of the amorphous components to the crystalline phase occurs.

[0076] It is particularly preferred that a sum of the second thickness of the substrate and the second thickness of the further substrate is less than 30 nm, preferably less than 15 nm, even more preferably less than 5 nm and particularly preferably less than 2 nm. It has been found to be advantageous that when setting the corresponding second thicknesses, no amorphous layer is formed on the entire substrate, which has a technically disruptive effect.

[0077] Another object of the present invention is a device for carrying out a method according to the invention, comprising:

[0078] -particle source with which particles can be accelerated, and

[0079] -Substrate carrier for holding the substrate. All properties and advantages described for the processes apply analogously to the device, and vice versa.

[0080] Reference is made to document EP 3 161 855 B1 with regard to the device.

[0081] The invention particularly relates to a method and apparatus for performing a dynamic sputtering process on a substrate surface to be bonded. An ion gun is preferably provided and / or controlled in such a way that, with a very low and decreasing ion beam voltage and by exploiting the diffusion of the generated ions, a low sputtering of the amorphous layer can be achieved, thus reducing it to a minimal layer thickness without causing further damage to the substrate.

[0082] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. These show:

[0083] Figure 1: Flowchart for an exemplary method for producing a sub-nanometer thin amorphous layer on a substrate surface to be bonded;

[0084] Figure 2: Time diagram with thickness profile (layer thickness D) to illustrate the removal of the oxide layer (A) and formation of the amorphous layer (C) on the substrate surface (B) in the first process step (I) of the method according to the invention as well as reduction of the formed amorphous layer (C) in the second, multi-stage, process step (II);

[0085] Figure 3a: Schematic ion beam voltage-time diagram with a stepwise reduction of the ion beam voltage in the second process step;

[0086] Figure 3b: Schematic ion beam voltage-time diagram with a continuous reduction of the ion beam voltage in the second process step.

[0087] In the figures, identical components or components with the same function are marked with the same reference numerals.

[0088] Figure 1 shows a flow diagram of an exemplary method for surface treatment in several steps according to a preferred embodiment of the present invention. In a first process step I 100, the energy of low-energy ions is adjusted such that, upon impact with a substrate surface, they cause surface cleaning, in particular removal of a native oxide layer and amorphization, in particular with a phase transformation. In a second process step II 200, the amorphous layer C produced in the first process step I 100 is gradually reduced by dynamic sputtering of the substrate surface with very low ion acceleration or without active ion acceleration and with decreasing ion beam voltage, without substantial damage to the underlying substrate surface. The process steps I 100 and II 200 take place in the same module.In the second process step II 200, the ion beam voltage is gradually reduced from a first voltage UBI (first process step 210) to a fourth voltage UB4 (fourth process step 240), so that the layer thickness D of the amorphous layer C is gently reduced. Process steps 210 to 240 are preferably part of the second process step II 200. By switching off the ion acceleration (idle activation), the diffusion of the ions generated in the plasma source is utilized and no collateral changes to the near-surface regions of the substrate B take place, such as implantation, further amorphization, or sputtering of the underlying crystalline substrate material itself. As the ion beam voltage U decreases, ion sputtering decreases, so that less and less material is removed.

[0089] If required, for example in the second and third steps (220, 230), the parameters, such as setting of the ion beam voltage U or duration t of the treatment with a defined ion beam voltage II, can be optimized by a control loop.

[0090] The second thickness D2 of the amorphous layer C on the substrate B after sputtering of the substrate surface is preferably less than 15 nm, preferably less than 5 nm, more preferably less than 2 nm, most preferably less than 1 nm, most preferably less than 0.5 nm.

[0091] In a preferred embodiment of the invention, the second thickness D2 of the amorphous layer C on the substrate B after the dynamic sputtering of the substrate surface is between 0.1 nm and 2 nm.

[0092] According to Figure 2, in a first process step 1100, the native oxide A on the substrate surface of a substrate B is removed by ion irradiation. In the first process step 1100, the thickness of the oxide layer A decreases until finally no oxide layer A remains. With increasing sputtering and thus with increasing damage accumulation, an amorphous layer C forms. The ion source is, for example, a low-energy ion source. The ions generated in the ionization chamber are extracted from the ion formation space using electric fields. The ion beam is then focused to the desired diameter, with a homogeneous current density being set in the beam. The ions can be decelerated and refocused to the desired energy. The ion energies are, for example, between a few eV and 1 keV.

[0093] In a first embodiment, in the second process step II 200, sputtering of the amorphous layer C of the pretreated substrate surfaces is performed with the ion beam without ion acceleration in the same module. The ion gun is operated at idle or with idle control, and the diffusion of the ions from the ion source is utilized.

[0094] In a second embodiment of the second process step II200, the ions are slightly accelerated.

[0095] The acceleration of the ions is controlled by individually adjusting the potentials of the ion gun grids, particularly the extraction grid and the acceleration grid. The aim of the present invention in the second process step II 200 is to provide an ion gun, or rather, to control it in such a way that very low sputtering of the amorphous layer can be achieved with a very low ion beam voltage and by utilizing the diffusion of the generated ions.

[0096] According to Figure 2, the ion energy or the ion beam voltage U is reduced in several steps. Figure 2 shows a reduction of the ion beam voltage in four steps 1 to 4. By gradually reducing the ion energy, the sputtering rate is gradually reduced, so that the layer thickness D of the amorphous layer C is reduced as much as possible and at the same time very gently. The layer thickness D of the amorphous layer C is reduced in the second process step II 200 according to Figure 2 from a first thickness D1 to a second thickness D2, the so-called residual layer thickness D2. The ion beam voltage is gradually reduced from a first voltage (UBI) to a fourth voltage (UB4). UBI > UB2 > UBS > UB4 applies here. The ion beam voltage U is particularly gradually reduced from 200 V to 50 V.In the embodiment of sputtering the amorphous layer C without active ion acceleration, the treatment time t is selected in particular between 1 s and 1500 s, preferably between 20 s and 1200 s, more preferably between 30 s and 1200 s.

[0097] In the embodiment of sputtering the amorphous layer C with a low ion acceleration, the treatment time t is selected in particular between 1 s and 1200 s, preferably between 20 s and 900 s, more preferably between 30 s and 600 s.

[0098] Figures 3a and 3b show the change in the ion beam voltage during the second process step II in different embodiments: one stepwise (see Figure 3a) and one continuously (see Figure 3b). The ion beam voltages II, UBI-B4 can be used to adjust the corresponding second kinetic energies for the ions when they hit the substrate surface of substrate B.

[0099] Reference symbol:

[0100] A oxide layer

[0101] B Substrat

[0102] C Amorphous layer

[0103] D Layer thickness

[0104] D1 first thickness

[0105] D2 second thickness, remaining layer thickness

[0106] U, UBI-B4 ion beam voltages t time, treatment duration

[0107] 100 first process step I, treatment

[0108] 200 second process step II, layer thickness reduction

[0109] 210 first procedural step

[0110] 220 second procedural step

[0111] 230 third procedural step

[0112] 240 fourth process step V Volt s Seconds

Claims

Claims 1 . A method for the surface treatment of a substrate (B), in particular as a preparatory step for a method for bonding the substrate (B) to another substrate, comprising: Providing the substrate (B) with a bonding surface intended for bonding, Treatment (100) of the substrate (B) at least in a partial region of the bonding surface to form an amorphous layer (C) having a first thickness (D1), wherein primary particle radiation is preferably used to form the amorphous layer (C), and layer thickness reduction (200) of the amorphous layer (C), preferably to a second thickness (D2).

2. Method according to claim 1, wherein for the formation of the amorphous layer (C) the primary particle radiation is realized with particles which have a first kinetic energy and for the layer thickness reduction (200) of the amorphous layer (C) a secondary particle radiation is realized with particles which have a second kinetic energy, wherein the second kinetic energy is smaller than the first kinetic energy.

3. The method according to claim 2, wherein a ratio of the second kinetic energy to the first kinetic energy is less than 0.1, preferably less than 0.05 and particularly preferably less than 0.

01.

4. The method according to one of claims 2 or 3, wherein the second kinetic energy is changed during the layer thickness reduction (200).

5. The method according to claim 4, wherein the second kinetic energy of the particles of the secondary particle radiation is continuously changed.

6. The method according to claim 4, wherein the second kinetic energy of the particles of the secondary particle radiation is changed stepwise.

7. Method according to one of the preceding claims, wherein the second thickness (D2) assumes a value which is less than 15 nm, preferably less than 5 nm and is preferably less than 2 nm or even less than 0.5 nm.

8. Method according to one of the preceding claims, wherein the treatment for forming the amorphous layer (C) and the layer thickness reduction (200) are carried out in a common module.

9. Method according to one of the preceding claims, wherein the second kinetic energy of the secondary particle radiation assumes a value between 1 eV and 1 keV, preferably between 1 eV and 800 eV, preferably between 20 eV and 500 eV, even more preferably between 20 eV and 300 eV, most preferably between 50 eV and 200 eV.

10. Method according to one of the preceding claims, wherein the layer thickness reduction (200) lasts between 1 s and 1500 s, preferably between 20 s and 1200 s, more preferably between 30 s and 1200 s.

11. A method for bonding a substrate (B) pretreated by the method according to one of the preceding claims to another substrate, which in particular has been pretreated by the method according to one of the preceding claims, wherein the substrate (B) and the another substrate are bonded together in a bonding process.

12. The method according to claim 11, wherein the second thickness (D2) of the substrate (B) and / or the second thickness (D2) of the further substrate is reduced to a third thickness during the bonding process.

13. The method according to claim 11 or 12, wherein the second thickness (D2) is adjusted such that the amorphous layer (C) present during bonding is converted into a crystalline phase in the overall substrate in which the substrate (B) and the further substrate are bonded to one another.

14. The method according to any one of claims 11 to 13, wherein a sum of the second thickness (D2) of the substrate (B) and the second thickness (D2) of the further substrate before bonding is less than 30 nm, preferably less than 15 nm, more preferably less than 5 nm and particularly preferably less than 2 nm.

15. Apparatus for carrying out a method according to any one of the preceding claims, comprising: Particle source with which particles can be accelerated, and Substrate carrier for holding the substrate (B).

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