Method for treating surface of substrate and method for bonding such substrate to further substrate, and device for carrying out such method

Abstract

The invention relates to a method for the surface treatment of a substrate (B), in particular as a preparation step for a method for connecting the substrate to a further substrate, comprising:-providing a substrate (B) having a bonding surface provided for connection,-treating (100) the substrate (B) at least in partial regions of the bonding surface in order to form an amorphous layer (C) having a first thickness (D1), wherein primary particle radiation is preferably 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

Technical Field

[0001] The present invention relates to a method for surface treatment of a substrate and a method for bonding the substrate to another substrate, as well as a device for carrying out such a method. Background Art

[0002] In the semiconductor industry, various bonding techniques have been used for many years to connect substrates to one another. This connection process is called bonding. A distinction is made between temporary and permanent bonding methods. In fusion bonding, two flat, pure substrate surfaces are bonded, for example, by contact. In a first step, the two substrates are brought into contact, with the two substrates being pre-fixed by van der Waals forces. To create a permanent fusion bond, the substrate stack is subjected to a heat treatment.

[0003] In order to enable bonding at low temperatures, a plasma treatment can be performed prior to fusion bonding to clean and activate at least one of the substrate surfaces.

[0004] US Pat. No. 10,312,217 B2 pre-treats the substrate surface at room temperature before bonding to achieve better bond strength. To this end, a bonding layer is deposited onto the substrate surface, flattened by chemical-mechanical polishing, and modified by plasma etching. The surface is activated by an RIE plasma process or by reaction with a reactive gas or liquid, resulting in an activated surface composed of activated oxygen or nitrogen compounds.

[0005] US 2021 / 0 225 803 A1 discloses a system that combines wet chemical and vacuum-based processes for pre-treating substrates at the lowest possible temperature before bonding. In US 2021 / 0 225 803 A1, the passivating oxide layer is largely removed without forming an amorphous layer or implanting particles on the substrate surface. To this end, a low-energy plasma is used in a plasma chamber in combination with a series of previously performed wet chemical processes for treating the substrate surface.

[0006] US Pat. No. 10,985,204 B2 uses a low-energy plasma to remove surface oxides. The formation of an amorphous layer is undesirable. In addition, low-energy hydrogen is implanted into the surfaces to be bonded. Possible local defects are passivated by the implanted hydrogen during a subsequent heat treatment. These local defects, which can lead to higher resistance at the bond boundary at temperatures below 300°C after substrate bonding, are removed.

[0007] For example, in US Pat. No. 10,312,217 B2, an additional bonding layer is applied, which is additionally activated using physical and / or (wet) chemical methods. US Pat. No. 202 1 / 0 225 803 A1 also discloses a system that allows for the combination of wet chemical and vacuum-based processes for pre-treating substrates at the lowest possible temperature before bonding. In US Pat. No. 10,985,204 B2, low-energy hydrogen is additionally implanted into the surfaces to be bonded.

[0008] Another method for modifying the substrate surface before bonding is the ion beam method, in which accelerated ions impact the substrate material to be modified at the end of their flight trajectory.

[0009] Conventional ion guns include a source of charged particles that are accelerated by an externally applied electric field generated between a pair of grids. Typically, three grids are used to produce a low-energy ion beam. Different grid arrangements can be used, with their potentials adjusted individually. In a multi-grid system, the first grid, onto which the ions impact, is typically positively biased and the second grid, onto which the ions impact. Another grid can be used to slow down the ions leaving the ion source, producing a collimated beam with more or less uniform energy.

[0010] Preferably, before bonding the substrate surfaces, oxide removal is performed on materials that form native oxides in an oxygen-containing atmosphere. However, this does not apply to substrate surfaces that are naturally oxygenated, such as silicon oxide. In particular, it is preferred to at least primarily, and more preferably, only remove harmful, unnecessary, and / or native oxides, especially metal oxides. Preferably, such oxides are removed as far as possible, especially completely, before the bonding process so that they do not become embedded in the bonding interface (the contact surface between the two substrates). Such embedded oxides can lead to mechanical instability and very low bond strength. Oxide removal is performed, in particular, by physical and / or chemical methods.

[0011] The pretreatment also modifies the microscopic roughness of the substrate surface. Maximum bonding energy is achieved when the surface roughness is optimal. This allows for spontaneous covalent bonding at low temperatures, particularly at room temperature. Contacting is performed under high vacuum.

[0012] EP 3 161 855 B1 describes a method and apparatus for surface treatment of substrates. The basic concept of EP 3 161 855 B1 is to create a predominantly amorphous layer on the surface of the substrate to be bonded. In EP 3 161 855 B1, amorphization of the substrate surface leads to better bonding results, especially at relatively low temperatures. Surface cleaning to remove oxides and amorphization are preferably performed simultaneously.

[0013] In the prior art, the formation of an amorphous surface layer due to a phase change of the substrate material during the pretreatment of the substrate surface prior to bonding is undesirable. If an amorphous layer is formed in the prior art, it is not applied by a transformation of the substrate material itself, but rather by depositing an additional amorphous layer on the substrate surface by a chemical and / or physical deposition process.

[0014] In order to prevent possible damage or to achieve better bonding strength at room temperature, additional process steps are required in the prior art.

[0015] EP 3 161 855 B1 relates in particular to a method for permanently bonding two substrates, wherein at least one, preferably both, substrates are treated prior to bonding. Surface regions, in particular the contact side, of the two substrates or of at least one of the two substrates, preferably over the entire surface, are amorphized prior to the bonding process. The amorphization produces a nanometer-thick layer in which the atoms of at least one of the surfaces to be bonded are arranged randomly. In order to produce the bond according to the invention, the surface is cleaned, in particular to remove oxides. An important aspect of EP 3 161 855 B1 is the use of energetic particles, in particular ions, to induce the amorphization.

[0016] In EP 3 161 855 B1, an amorphous layer is also produced while oxides are removed using an ion beam. The amorphous layer on the surface of a substrate pretreated and activated according to EP 3 161 855 B1 has a positive effect on bond strength during room temperature bonding. However, residual amorphous layers in the bonded substrate stack can interfere with the bond due to increased electrical resistance at the bonding interface. Furthermore, after bonding, the residual amorphous layer can negatively impact the interaction of the (multi-layered) substrate stack with electromagnetic radiation. Relevant parameters include, for example, the thickness of the amorphous layer and various optical properties.

[0017] While amorphous layers have a positive impact on the bonding process, they can also negatively or disruptively affect other process steps in the processing of the (multilayer) substrate stack after the bonding process. As mentioned above, residual amorphous layers at the bonding boundary layer can cause undesirable electrical resistance in the bonded substrate stack. As an additional layer in a bonded multilayer system, an amorphous layer can also interfere with further process steps. For example, the wavelength dependence of the refractive index of crystalline silicon (c-Si) and amorphous silicon (a-Si) differs. Therefore, during laser debonding, for example, a difference in refractive index between crystalline and amorphous silicon occurs under laser irradiation of the bonded multilayer system. Parameters of the (bonded) substrate stack that are relevant for the interaction with electromagnetic radiation, particularly laser radiation, include the number of layers in the multilayer system, the thickness of all layers, and the optical constants of all media. Therefore, amorphous layers cannot be ignored.

[0018] Furthermore, sputtering oxide removal requires very high ion energies to remove material from the substrate surface. This causes the ions to partially implant into the substrate, damaging layers near the surface. The damaged layers can typically be several nanometers thick, typically even 5 to 10 nm or more. This damage negatively impacts the electrical and optical properties of the bonded connection, making it undesirable and problematic in practice. Summary of the Invention

[0019] The object of the present invention is to provide a method and a device for pre-treating substrate surfaces, by means of which the substrates can be bonded with as little disruptive influence as possible from amorphous residual layers.

[0020] This object is achieved by means of a method according to claim 1 or claim 11 and a device according to claim 15. Preferred exemplary embodiments can be gathered from the description, the dependent claims and the drawings.

[0021] According to a first aspect of the present invention, a method for surface treatment of a substrate is provided, in particular as a preparatory step for a method for connecting the substrate to another substrate, the method comprising:

[0022] - providing a substrate having a bonding surface provided for connection,

[0023] - processing the substrate at least in a partial region of the bonding surface to form an amorphous layer having a first thickness, wherein primary particle irradiation is preferably used to form the amorphous layer, and

[0024] - reducing the thickness of the amorphous layer, preferably to a second thickness.

[0025] Compared to methods known from the prior art, the present invention additionally proposes reducing the thickness of the amorphous layer. Reducing the thickness of the amorphous layer makes it possible to significantly reduce the extent of the adverse effects of the amorphous layer. This allows for advantageous use of primary particle radiation, which has proven particularly advantageous for removing oxides and thus cleaning the substrate surface, particularly in the area of ​​the intended bonding plane. The positive effects of the amorphous layer can also be simultaneously utilized in the bonding method. Therefore, the combination of processing the substrate to form the amorphous layer on the one hand and reducing the layer thickness on the other hand has proven particularly advantageous.

[0026] In particular, it is proposed that the substrate be treated to clean the bonding surface before the thickness of the amorphous layer is reduced. In this case, the bonding surface is at least partially free of native and / or unnecessary oxides formed at the substrate surface. In particular, it is proposed that the amorphous layer be formed by a phase transition, in which the composition of the crystalline substrate surface of the provided substrate changes in the treated area of ​​the bonding surface. Therefore, the method is particularly different from those solutions in which the amorphous layer is applied to the substrate surface in a targeted manner. Instead, the crystalline phase is transformed into the amorphous phase by means of primary particle radiation. Accordingly, the first kinetic energy of the primary particle radiation is set so as to ensure the transformation into the amorphous phase. In this regard, reference is explicitly made to the disclosure of EP 3 161 855 B1. In this case, the substrate and the further substrate are preferably bonded to each other via corresponding bonding surfaces.

[0027] Although the formation of an amorphous layer is necessary for better bonding results when bonding substrates at low temperatures, particularly at room temperature, this amorphous layer can interfere with other process steps in the process of bonding multilayer systems. In particular, the amorphous residual layer negatively affects the electrical properties of the bonding boundary layer. Therefore, it has proven advantageous to further process at least the bonding surfaces of the substrates, preferably both substrates, particularly by dynamic sputtering or layer thickness reduction using particle radiation, so that the amorphous layer initially produced by the pretreatment is removed as gently as possible, particularly reduced to the necessary minimum.

[0028] The substrate comprises at least one of the following (crystalline) materials:

[0029] - Semiconductor materials, such as Si, Ge,

[0030] - III-V compound semiconductors, such as GaAs and InP

[0031] - II-VI compound semiconductors,

[0032] - semiconductor alloys,

[0033] - oxides, especially crystalline oxides of SiO2 and titanium,

[0034] - Nitrides, especially SiN x ,

[0035] - Metals such as Al, Cu, Au, Ti,

[0036] - alloys,

[0037] - ceramics,

[0038] - Carbides, especially SiC.

[0039] The substrate can have any shape, but is preferably round. Common diameters in industry for substrates, especially so-called wafers, are 1 inch, 2 inches, 3 inches, 4 inches, 5 inches, 6 inches, 8 inches, 12 inches, and 18 inches. However, the preferred embodiment can, in principle, process any substrate, regardless of its diameter.

[0040] Preferably, the particle radiation is ion radiation provided by an ion source or ion gun. In this case, preferably a gas or a gas mixture is ionized. It is particularly preferred that the particle radiation is used to achieve layer thickness reduction, in particular layer thickness reduction in the form of sputtering.

[0041] In a first process step within the meaning of the method according to EP 3 161 855 B1, the first kinetic energy of the ions is set 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 particularly restricted to the region near the surface of the substrates to be bonded together, preferably by selecting the process parameters during the amorphization, i.e. temperature, pressure, ion energy and / or ion current density. In this case, the material of the substrate remains at least predominantly, preferably completely, crystalline, with the exception of the amorphized layer. In a second process step, i.e. during the layer thickness reduction, the parameters important for the ion beam, in particular the ion energy and / or the ion current intensity, are continuously or stepwise reduced or varied, so that the amorphous layer produced in the first process step is gently further reduced without additionally and incidentally changing the region near the surface of the substrate (e.g. implantation, further amorphization or sputtering of the underlying substrate material itself).

[0042] Amorphization is preferably performed by particle collision with the substrate surface by means of primary particle radiation. The particles are charged particles or uncharged particles. Acceleration is preferably performed by means of charged particles, especially ions, because charged particles can be accelerated more easily technically.

[0043] When ions penetrate the area near the substrate surface, various interactions occur between the ions and the target atoms. The particle bombardment first causes desorption of weakly bonded adsorption layers, then sputtering of impurity and reaction layers (e.g., oxides with a thickness of 1 to 10 nm), and finally sputtering of the substrate material itself.

[0044] Surface cleaning with the aid of primary particle radiation (sputtering) is the ion-induced emission of atoms, atomic clusters, or molecules from a substrate surface and is used to remove oxides. The sputtering yield depends, among other things, on the ion energy, ion type, ion incidence angle, and substrate material.

[0045] Radiation damage and dislocation of substrate atoms are mainly caused in the substrate material by the collision process between the invading ions and the target atoms. The impact cascade triggered in the substrate and the layer produces a high defect concentration, which can cause amorphization. If ions with sufficiently high energy, especially energy greater than 100 eV, are shot at the solid, the ions penetrate the surface of the solid and interact with the target material. Therefore, for example, depending on the material and the ion beam, point defects can appear when more than 25 eV is transferred to the lattice atoms. As the implantation dose increases, the individual defect areas begin to overlap and eventually cause amorphization of the substrate material. If less than 25 eV is transferred to the lattice atoms, the energy is converted into phonon energy and causes a temperature increase.

[0046] In regions where the substrate material has been amorphized, the long-range order of the atoms can no longer be determined. The following parameters influence the transition of a material, such as silicon, from the crystalline phase to the amorphous phase after bombardment with heavy argon ions:

[0047] - ion mass,

[0048] - ion energy,

[0049] - ion current intensity or ion current density,

[0050] - injection dose,

[0051] - ion incident angle, and

[0052] - Substrate temperature.

[0053] When sputtering with an ion beam, the energy and current density of the bombarding ions can be varied independently of each other.

[0054] During surface cleaning, after removing the surface oxide, an amorphous surface layer is formed by radiation damage, dislocation and ion implantation. Ion implantation is mainly carried out when using high-energy ions. Implantation depends on the penetration depth of the injected ions into the substrate material, which depends on the ion energy and also on the ion incident angle. The substrate is at a spacing of 10cm to 50cm from the ion exit grid of the ion source.

[0055] By bombarding the substrate with particles using primary particle radiation, an amorphous layer is created within the existing crystalline base material of the substrate. Amorphization also leads, in particular, to flattening of the substrate surface. Therefore, in particular during amorphization, in addition to the flattening provided by the force during the bonding process, a surface flattening also occurs, which has a positive impact on bonding. Setting the ion incidence angle of the primary particle radiation allows for control of the ablation rate and, therefore, the surface roughness. The incidence angle is therefore particularly selected to maximize amorphization, the removal of impurities, in particular oxides, and surface smoothing for the desired result. Furthermore, amorphization ensures greater material mobility at the interface. This allows for better compensation of any residual roughness. In particular, gaps remaining between the substrate surfaces can be closed. Amorphization creates thermodynamically metastable states, particularly at the substrate surfaces (bonding interfaces).

[0056] According to one embodiment of the present invention, the angle of incidence between the substrate surface and the ion beam can be freely selected and set. The angle of incidence is defined as the angle between the substrate surface and the ion beam. The angle of incidence is preferably between 0° and 90°, preferably between 20° and 70°, and more preferably between 30° and 60°. The impact energy of the ions on the substrate surface can be controlled by the angle of incidence of the ion beam.

[0057] Preferably, in a surface cleaning and activation process, a surface region or bonding surface of the two substrates or at least one of the two substrates, in particular the contact side, is amorphized, preferably over the entire surface, before the bonding process. The surface cleaning and activation process can also be performed on surface regions that are smaller than the substrate surface, in particular also on surface regions that are separated from one another.

[0058] Pretreatment is typically performed in one of the pretreatment modules, which can be separated from the pre-fixing module and / or the bonding module, particularly by an airlock. In this pretreatment module, the surface is first cleaned by particle bombardment, and an amorphized substrate surface is produced. In particular, oxide removal, surface smoothing, and amorphization are performed simultaneously.

[0059] The amorphized layer is preferably produced not by a material applied by means of a physical and / or chemical process, but rather by a phase change of the substrate material. This makes it possible to completely avoid the deposition of particularly undesirable or harmful materials.

[0060] According to a preferred embodiment of the present invention, amorphization is controlled by setting a first kinetic energy of the accelerated particles of the primary particle radiation, in particular the ions of the primary ion radiation. The first kinetic energy of the particles is particularly set between 1 eV and 100 keV, preferably between 25 eV and 50 keV, more preferably between 25 eV and 10 keV, and most preferably between 25 eV and 1 keV. In the first embodiment, the low-energy ion beam, such as an argon ion beam, has an energy between 50 eV and 1 keV.

[0061] The current density (number of particles, especially ions, per unit time and unit area) is typically 0.1 mA / cm 2 and 10mA / cm 2 between, preferably between 0.1 mA / cm 2 and 5mA / cm 2 More preferably, it is between 0.1 mA / cm 2 and 2mA / cm 2 Choose between.

[0062] The processing time for processing the substrate to form the amorphous layer is preferably selected between 1 s and 200 s, preferably between 10 s and 200 s, more preferably between 50 s and 200 s, most preferably between 100 s and 200 s. The processing duration is, among other things, process- and material-dependent.

[0063] The first thickness of the amorphous layer after it is produced directly in the substrate surface is in particular less than 100 nm, preferably less than 50 nm, more preferably less than 10 nm, and most preferably less than 5 nm. The first thickness of the amorphous layer is particularly process- and material-dependent. Amorphization produces an amorphous layer a few nanometers thick, in which the atoms are arranged in a disordered manner. This disordered arrangement leads to better bonding results, especially at relatively low temperatures.

[0064] It is preferably provided that, to form the amorphous layer, primary particle irradiation is carried out using particles having a first kinetic energy, and, to reduce the layer thickness of the amorphous layer, secondary particle irradiation is carried out using particles having a second kinetic energy, wherein the second kinetic energy is less than the first kinetic energy. It has been shown that particle irradiation can not only induce the transformation of the substrate surface into an amorphous phase to form the amorphous layer, but it is also possible to achieve layer thickness reduction with the particle irradiation in a substrate-protected manner. It is also conceivable to carry out layer thickness reduction mechanically and / or chemically and / or with the aid of a laser, or to support layer thickness reduction by mechanical and / or chemical ablation and / or ablation induced by the action of a laser. It is particularly preferably provided that the particles having the first kinetic energy and the particles having the second kinetic energy are provided by the same particle source. Advantageously, this allows for a transition from a process for forming the amorphous phase to layer thickness reduction without significant effort.

[0065] In particular, the first kinetic energy and / or the second kinetic energy are to be understood as the energy imparted to the particles when they impact the substrate surface. If the particles are ions, the kinetic energy can be set particularly easily via the acceleration voltage in the ion source. However, it is also conceivable to set the kinetic energy via appropriate braking measures, for example downstream of the ion source exit, or by extending the flight path.

[0066] Typical gases and / or gas mixtures for surface cleaning, amorphization and subsequent dynamic sputtering or layer thickness reduction by means of particle radiation are, in particular:

[0067] - Atomic gases, especially Ar, He, Kr, Ne, Xe,

[0068] - molecular gases, especially H2, N2, CO, CO2,

[0069] - Gas mixtures, in particular gas mixtures FG (argon + hydrogen between 100% and 50%) and / or gas mixtures RFG (hydrogen + argon between 100% and 50%) and / or gas mixtures NFG (argon + nitrogen between 100% and 50%)

[0070] Preferably, an atomic gas is used, preferably an inert gas, in particular Ar, He, Kr, Ne, Xe.

[0071] The treatment, in particular cleaning, removal of undesired oxides and / or amorphization, is preferably carried out in a vacuum chamber as a process chamber. The vacuum chamber can be evacuated to less than 1 bar, preferably less than 1 mbar, more preferably less than 10 mbar by means of a suitable vacuum pump system. -3 mbar pressure. In particular, before using ions for amorphization, the vacuum chamber is evacuated to preferably the above pressure, more preferably completely evacuated. The oxygen content in the process chamber is greatly reduced, making reoxidation of the substrate surface impossible.

[0072] In order to reduce the layer thickness, the parameters important for the ion beam, in particular the ion energy and / or the ion current intensity, are gradually reduced or changed so that the amorphous layer produced in the first process step is gently reduced without additionally modifying the area near the surface of the substrate, such as implantation, further amorphization or sputtering of the substrate material itself.

[0073] The sputtering rate is reduced by reducing the ion energy of the ion beam. Preferably, the ion energy is reduced to allow gentle ablation of the amorphous layer within a suitable time window. The ablation still occurs via the physical sputtering action of the ions. The substrate remains in the same pretreatment module and no additional (wet) chemical and / or physical pretreatment is required to activate the surface.

[0074] In an embodiment in which the sputtering of the amorphous layer is performed without active ion acceleration, the process duration for reducing the layer thickness 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.

[0075] In an embodiment in which the sputtering of the amorphous layer is performed with low ion acceleration, the treatment duration is in particular selected between 1 s and 1200 s, preferably between 20 s and 900 s, more preferably between 30 s and 600 s.

[0076] The proposed use of an ion gun or ion beam without ion acceleration allows for very low ion energies and the lowest possible current density. In addition to low ion energies, short process times are also sought. With optimal selection of the key parameters, the surface roughness or surface smoothness in the second process step remains largely unchanged.

[0077] The thickness and purity of the resulting amorphous layer influence the bond strength during the subsequent bonding process. According to EP 3 161 855 B1, the bond strength increases with increasing amorphous layer thickness. However, in the case of substrates with a very low average surface roughness, the thickness of the amorphous layer can be reduced. The average surface roughness of the substrate is particularly less than 10 nm, preferably less than 8 nm, more preferably less than 6 nm, most preferably less than 4 nm, and most preferably less than 1 nm.

[0078] In particular, it is provided that the second thickness of the amorphous layer of the substrates to be bonded is reduced to a necessary minimum.

[0079] Thus, the second thickness of the amorphous layer at the substrate after further sputtering on the substrate surface is less than 15 nm, preferably less than 5 nm, more preferably less than 2 nm, and most preferably less than 0.5 nm. In a preferred embodiment of the present invention, the second thickness of the amorphous layer at the substrate after sputtering on the substrate surface is between 0.1 nm and 2 nm. Thus, the significantly reduced amorphous layer can still positively influence the bond strength during pre-processed substrate bonding, wherein the second thickness of the amorphous residual layer has been reduced to such an extent that the bonding boundary layer exhibits a significantly lower electrical resistance in the bonded substrate stack.

[0080] 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 for the second kinetic energy to be as low as possible, since this allows for gentle ablation of the amorphous material, so that no further damage occurs at the bonding surface as the thickness of the amorphous layer decreases. In particular, the probability of a subsequent phase transition to the amorphous phase or of ion penetration into the substrate is reduced. It is also recognized that the diffusion velocity of particles impinging on the amorphous layer is sufficient to cause the desired reduction in layer thickness.

[0081] In particular, it is proposed to change the second kinetic energy of the secondary particle radiation during the layer thickness reduction. In contrast, the first kinetic energy of the primary particle radiation remains essentially constant. This reduction in the second kinetic energy has proven to be advantageous because the probability of interaction with the substrate surface decreases with decreasing layer thickness. At the same time, relatively large erosion occurs at the beginning of the layer thickness reduction, which has proven to be advantageous with regard to the duration of the subsequent processing step that results in the layer thickness reduction.

[0082] In a first embodiment, the layer thickness reduction of the amorphous layer is carried out in a plurality of steps, wherein the parameters of the ion beam are varied, in particular reduced, several times so that the ablation of the amorphous layer becomes smaller and smaller until the desired minimum layer thickness of the amorphous layer is reached. In this first embodiment, the dynamic sputtering of the amorphous layer is preferably carried out in two to five steps, wherein in each step the relevant ion beam parameters are varied such that a smaller number of particles with a lower energy impinge on the substrate surface.

[0083] Furthermore, it is preferably provided that the second kinetic energy of the particles of the second particle beam is continuously varied. In one exemplary embodiment, the parameters of the ion beam are continuously varied, in particular reduced. Preferably, after the first process step, the ion beam voltage is continuously reduced for gentle and controlled sputtering of the amorphous layer until the desired residual layer thickness is reached.

[0084] In particular, it is proposed to gradually change the second kinetic energy of the particles of the second particle radiation. In the embodiment, in the second process step, a layer thickness reduction, in particular sputtering of an amorphous layer, is performed by means of an ion beam without ion acceleration (English: idle activation). Here, the ion gun is operated in idle mode or in idle mode control. The ions are not accelerated, and the ion beam voltage is gradually reduced in a first embodiment. In particular, the ion beam voltage is reduced in steps from 200 V to 50 V. For example, the ion beam voltage is reduced in steps of 50 V, more preferably in steps of 20 V. In a second embodiment, the ion beam voltage is reduced from 200 V to 50 V not gradually but continuously. During idling, the plasma source or ion source is maintained, and the potential of the grid of the ion gun is in the order of magnitude of the idle voltage after the ion beam voltage is reduced.

[0085] By reducing the ion energy and the number of ions leaving the ion source during idle operation, the sputtering rate is significantly reduced, so that a gentle reduction in the layer thickness of the amorphous layer is achieved.

[0086] In another embodiment of the second process step, the ions are slightly accelerated and the ion beam voltage is reduced stepwise from 200 V to 50 V in the first embodiment or continuously from 200 V to 50 V in a supplementary embodiment. Compared to an embodiment without active acceleration of the ions, a higher sputtering rate can be achieved by means of the slight acceleration of the ions, so that the processing duration can be shortened. This makes it possible to process thicker amorphous layers from the first process step, thereby shortening the first process step. In particular, the reduction in the thickness of the amorphous layer in the first process step, which is carried out with a much higher ion energy, can be shortened, since the amorphous layer can be etched away more gently and more efficiently in the second process step. The choice of embodiment depends on the substrate material and the ions used, since the layer thickness and the sputtering rate of the amorphous layer can vary greatly.

[0087] In an embodiment with a step-down, the ion beam voltage is reduced in multiple steps, particularly in at least one step, more preferably in at least two steps, more preferably in at least three steps, and most preferably in four steps. Depending on the substrate material and the ions used, the threshold voltage for the onset of sputtering is between 20 V and 50 V. The ion beam voltage is particularly reduced stepwise from 200 V to 50 V. The kinetic energy of the particles is low, particularly between 200 eV and 50 eV. In contrast, in the first process step, the surface cleaning and activation process is performed using accelerated particles with relatively high kinetic energy, particularly up to 1000 eV.

[0088] By individually setting the potentials of the ion gun's grids, in particular the suction grid and the acceleration grid, the acceleration of the ions can be controlled. The object of the present invention in the second process step is to provide or control the ion gun in such a way that very little sputtering of the amorphous layer can be achieved with very low ion beam voltage and by utilizing the diffusion of the generated ions.

[0089] The second kinetic energy of the particles of the secondary particle irradiation is particularly set between 1eV and 1keV, preferably between 1eV and 800eV, more preferably between 20eV and 500eV, most preferably between 20eV and 300eV, and most preferably between 50eV and 200eV. An energy between 50eV and 200eV has proven to be particularly advantageous. Preferably, the second kinetic energy varies within at least two-thirds of the above-mentioned interval. This therefore means a relatively large span of different second kinetic energies for stripping the amorphous phase. It has proven advantageous to find the best possible setting for reducing the layer thickness, which can be achieved quickly without causing damage to the substrate surface again.

[0090] It is particularly preferred that the second thickness has a value of less than 15 nm, preferably less than 5 nm, and most preferably less than 2 nm or even less than 0.5 nm. This small second thickness, in particular less than 2 nm, allows advantageous use of the positive properties of the amorphous layer during joining, i.e., bonding, while the amorphous layer is thin enough to return to the crystalline phase with a sufficiently high probability during bonding. Advantageously, the resulting overall substrate resulting from joining this substrate with the further substrate then has only a small amorphous fraction, or even no amorphous fraction, which is disadvantageous with respect to the physical properties of the overall substrate.

[0091] In particular, it is proposed that the treatment for forming the amorphous layer and the layer thickness reduction be performed in a common module. This advantageously allows the layer thickness reduction to be carried out immediately after the formation of the amorphous phase, in particular without requiring complex transport of the substrate. Furthermore, it is advantageously possible to utilize the same properties of the substrate's environment, such as the temperature and / or vacuum pressure in the process chamber of the apparatus used to carry out the method.

[0092] It is preferably provided that the layer thickness reduction lasts from 1 s to 1500 s, preferably from 20 s to 1200 s, and more preferably from 30 s to 1200 s. It has been shown that a corresponding layer thickness reduction can be achieved relatively quickly. The required time interval depends essentially on the second kinetic energy and the second thickness to be set.

[0093] Another subject matter of the present invention is a method for connecting a substrate pretreated by the method according to the present invention to another substrate, particularly one pretreated by the method according to the present invention, wherein the substrate and the further substrate are connected in a bonding process. All advantages and properties of the above-described method can be similarly transferred to this method, and vice versa. In this method, the substrate and the further substrate are connected or bonded to each other via corresponding bonding surfaces. The contact of the two substrate surfaces with very thin amorphous layers in the low nm or subnanometer range results in improved bond strength, which is advantageous due to the extremely small amorphous boundary layer at the substrate stack, which, due to the greatly reduced layer thickness, does not provide significant resistance at the bonding interface.

[0094] In particular, it is proposed that during the bonding process, the second thickness is reduced to a third thickness. This advantageously avoids further amorphous components in the entire substrate. In particular, it is proposed that the bonding parameters are set so as to promote the transition from the amorphous phase to the crystalline phase.

[0095] Thus, the total thickness of the amorphous layers in the substrate stack formed by the substrate and the further substrate after bonding is less than 30 nm, preferably less than 15 nm, more preferably less than 5 nm, most preferably less than 2 nm, most preferably still less than 1 nm.

[0096] In particular, during bonding and / or after bonding and / or during bonding and / or during a heat treatment after bonding, a phase transition from the amorphous state to the crystalline state can occur, so that the second thickness, i.e., the residual layer thickness of the amorphous layer, is further reduced to a third thickness. In a preferred embodiment, the process parameters are selected such that minimizing the layer thickness of the amorphous layer of the substrates to be bonded causes the amorphous residual layer of the bonded substrate stack to be completely transformed into the crystalline phase during bonding and / or after bonding and / or during a heat treatment.

[0097] After pre-treating at least one of the two substrate surfaces, preferably both substrate surfaces, the two substrates are oriented relative to one another, in particular in an orientation module, preferably directly in a bonding chamber. The pre-treated and oriented substrates are bonded in a bonding chamber, wherein the bonding chamber is preferably integrally connected to a process chamber in a cluster facility for amorphization and for dynamic sputtering, and more preferably can be transferred from the process chamber to the bonding chamber while maintaining a continuous evacuation.

[0098] In a preferred high vacuum environment, especially <10 -7 mbar, preferably <10 -8 mbar, especially at room temperature, by pure contact without additional pressure or at least only very low pressure, the polished and pretreated substrate surfaces spontaneously and covalently bond. If necessary, the bonding process can be enhanced by further force and / or temperature influences.

[0099] In particular, it is proposed that the second thickness be set so that the amorphous layer present during bonding is converted into a crystalline phase. In particular, more than 30%, preferably more than 50%, and in particular more than 70% of the amorphous component of the amorphous layer is recycled into the crystalline phase. It is also conceivable to set the second thickness so that the amorphous component is completely recycled into the crystalline phase.

[0100] It is particularly preferred that the 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, more preferably less than 5 nm, and particularly preferably less than 2 nm. It has been advantageously demonstrated that when the corresponding second thickness is set, no technically disruptive amorphous layer forms across the entire substrate.

[0101] Another subject matter of the present invention is a device for carrying out the method according to the invention, comprising:

[0102] - a particle source, by means of which particles can be accelerated, and

[0103] - A substrate carrier for holding a substrate. All properties and advantages described for the method apply analogously to the device, and vice versa.

[0104] This device is described in EP 3 161 855 B1.

[0105] In particular, the present invention relates to a method and an apparatus for performing a dynamic sputtering process on substrate surfaces to be bonded.

[0106] Here, the ion gun is preferably provided and / or controlled so that low sputtering of the amorphous layer can be achieved by means of a very small and reduced ion beam voltage and by fully utilizing the diffusion of the generated ions, so that the amorphous layer is reduced to a minimum layer thickness without causing further damage to the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0107] Further advantages, features and details of the present invention are apparent from the following description of preferred exemplary embodiments and from the accompanying drawings. The drawings show:

[0108] Figure 1 : A flow chart illustrating an exemplary method for producing a sub-nanometer thin amorphous layer on a substrate surface to be bonded;

[0109] Figure 2 : shows a time diagram with a thickness profile (layer thickness D) for illustrating the removal of an oxide layer (A) and the formation of an amorphous layer (C) on a substrate surface (B) in a first process step (I) of the method according to the invention, and the reduction of the formed amorphous layer (C) in a multi-stage second process step (II);

[0110] Figure 3a : shows a schematic ion beam voltage-time diagram with a step-by-step reduction of the ion beam voltage in the second process step;

[0111] Figure 3b : shows a schematic ion beam voltage-time diagram with continuously decreasing ion beam voltage in the second process step.

[0112] In the drawings, the same components or components having the same functions are denoted by the same reference numerals. DETAILED DESCRIPTION

[0113] Figure 1 A flow chart shows an exemplary method for surface treatment in a plurality of steps according to a preferred embodiment of the present invention. In a first process step I 100, the energy of the low-energy ions is set such that upon impact with the substrate surface, they cause surface cleaning, in particular removal of native oxide layers and amorphization, in particular in the case of phase changes.

[0114] In the second process step II 200, the amorphous layer C produced in the first process step I 100 is gradually reduced by dynamically sputtering the substrate surface with very low ion acceleration or without active ion acceleration and with a reduced ion beam voltage, without substantially damaging the underlying substrate surface. Process steps I 100 and II 200 are performed in the same module. In the second process step II 200, the ion beam voltage is reduced from the first voltage U B1 (First method step 210) Stepwise reduction to fourth voltage U B4 (Fourth method step 240 ), resulting in a gentle reduction in the layer thickness D of the amorphous layer C. Method steps 210 to 240 are preferably part of second process step II 200 . By switching off ion acceleration (idle activation), the diffusion of ions generated in the plasma source is utilized, without any accompanying changes in the region near the surface of substrate B, such as implantation, further amorphization, or sputtering of the underlying crystalline substrate material itself. As the ion beam voltage U decreases, ion sputtering decreases, resulting in less and less material being removed.

[0115] If necessary, for example in the second and third steps ( 220 , 230 ), parameters such as the setting of the ion beam voltage U or the duration t of the treatment at a defined ion beam voltage U can be optimized by means of a control loop.

[0116] The second thickness D2 of the amorphous layer C at the substrate B after sputtering on 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 still less than 0.5 nm.

[0117] In a preferred embodiment of the present invention, the second thickness D2 of the amorphous layer C at the substrate B after dynamic sputtering of the substrate surface is between 0.1 nm and 2 nm.

[0118] according to Figure 2 In a first process step I100, native oxide A on the substrate surface of substrate B is removed by ion irradiation. In the first process step I100, the thickness of oxide layer A decreases until no oxide layer A is present. As sputtering increases and damage accumulation increases, an amorphous layer C is formed.

[0119] The ion source is, for example, a low-energy ion source. Ions generated in an ionization chamber are extracted from the ion generation space by means of an electric field. The ion beam is then focused to a desired diameter, with a uniform current density established within the beam. The ions can be decelerated to the desired energy and refocused. The ion energy ranges, for example, from a few eV to 1 keV.

[0120] In a first embodiment, in a second process step II 200, sputtering of an amorphous layer C of the pretreated substrate surface is performed in the same module using an ion beam without ion acceleration. The ion gun is idle or operated in idle mode and utilizes ion diffusion from the ion source.

[0121] In a second embodiment of the second process step II 200 , the ions are slightly accelerated.

[0122] By individually setting the potentials of the ion gun grids, in particular the potentials of the pumping grid and the acceleration grid, the acceleration of the ions is controlled. The object of the present invention in the second process step II 200 is to provide or control the ion gun in such a way that very little sputtering of the amorphous layer can be achieved with very low ion beam voltage and by utilizing the diffusion of the generated ions.

[0123] according to Figure 2 , the ion energy or the ion beam voltage U is reduced in multiple steps. Figure 2 The reduction of the ion beam voltage is shown 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 high as possible and very gently. The layer thickness D of the amorphous layer C is reduced according to Figure 2 In the second process step II 200, the ion beam voltage is reduced from the first thickness D1 to a second thickness D2, the so-called residual layer thickness D2. B1 ) gradually decreases to the fourth voltage (U B4 ). U B1 >U B2 >U B3 >UB4 In particular, the ion beam voltage U is reduced stepwise from 200V to 50V.

[0124] In an embodiment of sputtering of the amorphous layer C without active ion acceleration, the process duration 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.

[0125] In an embodiment of sputtering of the amorphous layer C with low ion acceleration, the process duration t is preferably selected between 1 s and 1200 s, preferably between 20 s and 900 s, more preferably between 30 s and 600 s.

[0126] Figure 3a and Figure 3b The variation of the ion beam voltage during the second process step II is shown in different embodiments, one being a stepwise variation (see Figure 3a ) and one is continuous (cf. Figure 3b When the ions hit the substrate surface of substrate B, the corresponding second kinetic energy of the ions can be adjusted by the ion beam voltage U, U B1-B4 to set.

[0127] Reference numerals:

[0128] A oxide layer

[0129] B substrate

[0130] C amorphous layer

[0131] D layer thickness

[0132] D1 first thickness

[0133] D2 second thickness, residual layer thickness

[0134] U, U B1-B4 Ion beam voltage

[0135] T time, processing duration

[0136] 100 First process step I, treatment

[0137] 200 Second process step II, layer thickness reduction

[0138] 210 First Method Step

[0139] 220 Second method step

[0140] 230 Third Method Step

[0141] 240 Fourth Method Step

[0142] V Volts

[0143] s seconds

Claims

1. A method for surface treatment of a substrate (B), in particular as a preparatory step for a method for connecting the substrate (B) to another substrate, comprising: - providing the substrate (B), which has a bonding surface provided for performing the connection, - processing (100) 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 - reducing (200) the layer thickness of the amorphous layer (C), preferably to a second thickness (D2).

2. The method according to claim 1 , wherein, in order to form the amorphous layer (C), the primary particle irradiation is carried out by means of particles having a first kinetic energy, and in order to reduce (200) the layer thickness of the amorphous layer (C), the secondary particle irradiation is carried out by means of particles having a second kinetic energy, wherein the second kinetic energy is less than the first kinetic energy. 3 . The method according to claim 2 , wherein 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.

4. Method according to claim 2 or 3, wherein the second kinetic energy is varied during the layer thickness reduction (200). The method according to claim 4 , wherein the second kinetic energy of the particles of the secondary particle radiation is continuously varied. 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 any of the preceding claims, wherein the second thickness (D2) has a value less than 15 nm, preferably less than 5 nm, and most preferably less than 2 nm or even less than 0.5 nm.

8. The method according to any of the preceding claims, wherein the treatment for forming the amorphous layer (C) and the layer thickness reduction (200) are performed in a common module.

9. The method according to any of the preceding claims, wherein the value of the second kinetic energy of the secondary particle radiation is between 1 eV and 1 keV, preferably between 1 eV and 800 eV, preferably between 20 eV and 500 eV, more preferably between 20 eV and 300 eV, most preferably between 50 eV and 200 eV.

10. The method according to any of the preceding claims, wherein the layer thickness reduction (200) lasts from 1 s to 1500 s, preferably from 20 s to 1200 s, more preferably from 30 s to 1200 s.

11. A method for connecting a substrate (B) pretreated by means of a method according to any of the preceding claims to a further substrate, in particular pretreated by means of a method according to any of the preceding claims, wherein the substrate (B) and the further substrate are connected to one another in a bonding process.

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

13. The method according to claim 11 or 12, wherein the second thickness (D2) is set so that the amorphous layer (C) present during bonding is transformed into a crystalline phase throughout the entire substrate in which the substrate (B) and the further substrate are connected to each other.

14. The method according to claim 11 , wherein the 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. A device for performing the method according to any one of the preceding claims, the device comprising: - a particle source, by means of which particles can be accelerated, and - a substrate carrier for holding the substrate (B).

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