Method for producing a metal layer on a semiconductor body using a printing process

Abstract

A process that has: Providing (101) a semiconductor wafer (200) with a coating (201); Printing (102) a metal particle paste on the semiconductor wafer (200), thereby forming a metal layer; and Heating (105) the metal layer in a reducing gas to sinter the metal particle paste in a furnace, wherein printing the metal particle paste comprises: Applying a photoresist layer (202') to the semiconductor wafer (200); Structuring the photoresist layer (202') using a photolithographic process; and Applying the metal particle paste to the patterned photoresist layer (202'); the method further comprising: Pre-sintering (104) the metal particle paste before heating; and Removing the photoresist layer (202') after pre-sintering; the method comprising, before heating: Drying (103) the metal particle paste.

Description

Embodiments of the present invention relate to a method for producing a semiconductor wafer or a chip, in particular to a method for producing copper layers on a semiconductor body.The workflow typically involves depositing particle layers on existing layers in the manufacture of semiconductor chips. This may include depositing metallization layers for interconnection (interconnection) or bonding, wherein copper is increasingly used as a substitute for aluminum due to its low electrical resistance and high thermal conductivity. Such metallization layers can be produced with the aid of a particle deposition process (particle deposition process). However, high mechanical stresses may occur as the temperature changes due to significantly different coefficients of thermal expansion (CTE) of copper and silicon. Delamination of the copper layer and cracking may result. The deposition of porous copper layers can alleviate the above-mentioned problem of thermomechanical stresses. However, the plasma or electrochemical processes currently used for the deposition of porous copper layers are comparatively complex and expensive. The publication U.S. Pat. No. 4,388,346 describes the production of contact electrodes on the surface of photovoltaic cells by means of a screen printing method. The publication JP 2010 053 377 A discloses a sintering method for producing a metal composite body consisting of a plurality of layers. Furthermore, the publication US 2013 / 0 206 225 A1 describes a photovoltaic cell which is coated with a diffusion barrier. An electrical contact layer is applied to this layer, for example, with the aid of a screen printing method. The publication US 2012 / 0 175 147 A1 shows a method for the coating of a surface with copper. In addition, the preparation of the powder used will be described.It is an object of the present invention to provide an improved method for processing semiconductor wafers or chips, in particular for producing porous copper layers which form electrical connections on the semiconductor wafer or chips. This object is achieved by the method according to claim 1 or 15. Exemplary embodiments or further developments of the present invention are covered by the dependent claims.A method of processing a wafer will be described. According to an example of the invention, the method comprises providing a semiconductor wafer with a coating (coating) and printing a metal particle paste on the semiconductor wafer to form a porous or nonporous metal layer. A heat treatment for sintering the metal particle paste or annealing (annealing) the sintered metal particle paste is performed in an atmosphere including a reducing gas. Printing the metal particle paste includes applying a photoresist layer to the semiconductor wafer, patterning the photoresist layer using a photolithography process, and applying the metal particle paste to the patterned photoresist layer. In addition, the method comprises pre-sintering the metal particle paste before heating and removing the photoresist layer after pre-sintering. The method also includes drying the metal particle paste prior to heating.Furthermore, a method for producing a metal layer on a substrate is described. According to another example of the invention, the method comprises providing a substrate and printing a metal particle paste on the substrate to form a porous or nonporous metal layer. A heat treatment for sintering the metal particle paste or annealing (annealing) the sintered metal particle paste is performed in an atmosphere including a reducing gas. Printing the metal particle paste includes applying a photoresist layer to the substrate, patterning the photoresist layer using a photolithography process, and applying the metal particle paste to the patterned photoresist layer. In addition, the method comprises pre-sintering the metal particle paste before heating and removing the photoresist layer after pre-sintering. The method also includes drying the metal particle paste prior to heating.Examples are explained below with reference to the drawings. The drawings are provided to illustrate certain principles, and so only aspects are shown that are necessary to understand these principles. The drawings are not to scale. In the drawings, like reference numerals designate like features. FIG. 1 is a flow chart showing an example process for processing a wafer to form a thick copper layer; FIGS. 2A-2E show a cross section through the semiconductor body according to an example not covered by the claims; and FIGS. 3A-3D show a cross-section through the semiconductor body according to an example of the invention.In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the specification and show specific embodiments in accordance with which the invention may be practiced.Copper (Cu) has been introduced several years ago to replace aluminum (Al) interconnects in ultra-large scale integration (ULSI) logic devices to produce small-sized, high-speed devices. The electrical resistance of copper is 1.7·10 -6 Ω·cm compared to 3·10 -6 Ω·cm for aluminum. The high electrical and thermal conductivity of copper is very attractive for applications with respect to ULSI logic devices as well as smart power devices. The high electrical conductivity of copper allows a large current flow per unit area with less heat generation. High thermal conductivity also helps efficiently dissipate the heat generated by the current flow during operation of the device.Due to significantly different thermal expansion coefficients (CTE) of copper and silicon (Si), thermally induced mechanical stresses (thermal stresses) may occur within the wafer or the silicon chip, and delamination or cracking may result. This problem can be alleviated by limiting the thickness of the copper layer to about 10 μm or by forming porous copper layers which, due to their porosity, result in significantly lower thermal stresses. Such porous copper coatings can be applied to silicon wafers for the purpose of electrical contacting and for transporting heat away from semiconductor elements. In order to be able to reproducibly deposit copper layers with a specified layer thickness, selectable porosity and specific resistance values, deposition processes are used to deposit particles from plasma to form a copper layer, e.g. the so-called plasma dust ® processes. However, such deposition processes are comparatively complex and expensive.According to the embodiment described herein, a printing process is used to form a comparatively thick (e.g. up to about 100 μm) metal layer on a silicon wafer or a chip, in particular a copper layer or a nickel layer. In the past, a significant amount of research has been conducted to develop methods for printing electrically conductive structures. This includes not only printing technologies but also the development of materials and post processing methods (post processing methods). For example, inkjet printing processes have been used to print electrically conductive ink to form electrically conductive structures on low cost substrates such as paper, plastic films (e.g., polyimides), textiles, or the like. A common conductive ink includes metal nanoparticles (e.g., silver, gold, nickel, copper, or alloys of these components) surrounded by a thin dispersive material (e.g., some polymers) and diluted in one or more solvents. During the post-treatment (i.e., after printing), solvents and dispersants are removed and a conductive path is formed along the metal particles by applying a heat treatment to the printed ink structure. This post-treatment is usually referred to as sintering, wherein furnace sintering is usually used. Silver has been used very frequently because of its low resistance and ease of handling in post-treatment.As mentioned above, copper is of particular interest as a material in electronics for conductive layers. However, the silver inks usually used cannot be easily replaced with inks containing copper nanoparticles without changing the post-processing of the printed ink structure, since non-conductive copper oxides may be generated during sintering, thereby degrading the desired electrical properties (low electrical resistance) of the copper layer. Silver ink can be sintered in a furnace. A copper ink may additionally require an inert or reductive atmosphere, or an alternative approach to sintering. The same applies when nickel is used to form a thick nickel layer.In a non-inert and non-reducing atmosphere, copper ink must be sintered in a very short time. For this reason, the following two techniques have been used for sintering copper ink: pulsed light (e.g., xenon high energy flash light) and laser sintering. In laser sintering, a laser beam scans the ink structure, moving either the laser beam or the support supporting the substrate. Continuous wave lasers, as well as pulsed lasers, have been successfully used to sinter silver or copper nanoparticle ink. The advantages of laser sintering over traditional furnace sintering are the achievable speed of the sintering process and the possibility of local sintering. However, the conductivity of the resulting copper layer is still too low during the processing of silicon wafers (compared to pure copper), which indicates a comparatively high proportion of copper oxide within the layer. The exemplary embodiments described herein may at least alleviate this problem and contribute to reducing the amount of copper oxide in the resulting porous copper layer.An example method of processing a wafer to produce a copper layer is illustrated in the flow diagram of FIG. 1. Thus, a printing process is used to form copper layers having a thickness between 20 μm and 100 μm (or more) instead of using a plasma deposition process (such as the mentioned plasma dust ® process). According to FIG. 1A, a wafer is provided (step 101), which comprises a thin (e.g. a few micrometers, less than 10 μm) metallic coating (coating, e.g. nickel, tin, aluminum, silver, gold, titanium, tungsten, copper, or alloys thereof) on which the porous copper layer can adhere. The coating may be patterned to produce the desired electrical connections. Subsequently, a copper particle ink or a copper particle paste is printed on the wafer, in particular on the adhesive coating that has been previously deposited on the wafer (step 102). In general, any printing process can be used. For example, screen printing can be used for the production of copper layers with a thickness up to 20 μm, whereas stencil printing can be used for the production of even thicker copper layers with a thickness up to 100 μm or more.The copper printed pattern is then dried at elevated temperature (step 103). For example, the drying process includes exposing the wafer and the printed copper structure to a temperature of 60 degrees Celsius (° C.) for about 30 minutes in a nitrogen atmosphere. After this first drying step, the wafers are ready for (intermediate) storage before being further processed. Optionally, a second drying step may follow. This second drying step may comprise exposing the printed copper structure to a temperature of about 100°C for about 4 hours in a nitrogen atmosphere or a temperature of 60°C to 100°C in a vacuum chamber for about 15 to 30 minutes.The wafer with the dried copper structure is then subjected to a heat treatment. The heat treatment comprises, as a first part, a sintering process (step 104) which can be carried out at elevated temperatures in a furnace (furnace sintering). The sintering process takes about 5 to 60 minutes at temperatures between 300° C. and 450° C. During this sintering process (already during heating), the wafer is bathed in a carrier gas (e.g. nitrogen, helium, argon, xenon, or forming gas) which is saturated (or at least almost saturated) with gaseous formic acid. In the present example, nitrogen is used as the carrier gas for formic acid. As an alternative to formic acid, another substance having a similar reducing property may be used, such alternatives being, for example, carbon monoxide (CO) and atomic hydrogen (obtained by using a catalytic process or as a plasma). To saturate the nitrogen with formic acid, the nitrogen is passed at room temperature through a stirring system (bubbler system) containing 98% formic acid. The saturated gas stream is then fed to the sintering furnace at flow rates between one and five liters per minute. A flow rate of 2.6 liters per minute results in a feed of about 200 mg formic acid per minute into the sintering furnace. As a result, a porous copper layer having a very low electric resistance of about 8-14 μΩ·cm or even less is obtained. The pressure in the furnace may be lower than atmospheric pressure. A reduced pressure can improve the evaporation of volatile components of the particle paste.During the sintering and annealing process (annealing process), a solid and contiguous copper layer is formed from the copper particles in the printed copper paste. The sintering process is usually completed within a few minutes (e.g., 10 to 60 minutes). However, the heat treatment may be continued, i.e., an annealing process (step 105) may follow the sintering process. During annealing at temperatures between 300°C and 450°C (or even 500°C) for about 20 to 180 minutes, the smaller copper grains recrystalize to form larger grains with a preferred grain orientation, resulting in a stable sponge structure with a defined porosity (e.g., 50% porosity). The lower limit (300° C.) of the temperature interval mentioned may be selected to be at least as high as the maximum temperature during the operation of the resulting electronic device. In particular in power devices, copper compounds may be exposed to transient temperature peaks of around 300° C. Generally, the annealing process continues as the resistance falls and until the desired low electrical resistance is achieved. Furthermore, the annealing process may continue until the copper layer has a stable microstructure. An unstable microstructure could be the cause of further changes in the microstructure of the copper layer during operation and thus lead to rapid degradation of the electronic device. By continuing the annealing until a stable microstructure is achieved in the copper layer, mechanical stresses and strains are reduced.By the comparatively long heat treatment (furnace sintering and annealing in a reducing atmosphere) of the copper layer printed on a semiconductor wafer (e.g. silicon, silicon carbide, gallium nitride), an improvement of the physical properties of the copper layer is achieved (such as a reduced specific electrical resistance and reduced mechanical stress) compared to significantly shorter sintering processes such as laser and flash light treatment. During the heat treatment, a low electrical resistance can be achieved in comparatively short times, e.g. 10 to 20 minutes or even less. However, the heat treatment is maintained for a longer period of time, for example, up to 180 minutes or more, to obtain a stable microstructure of the copper layer.The abovementioned sintering step (comprising the feeding of formic acid) can be preceded by pre-sintering (pre-sintering). The pre-sintering can be carried out in a sintering furnace at temperatures of 100° C. to 300° C. in an atmosphere which is reducing, inert, or oxidizing. Additionally or alternatively, laser sintering with suitable laser light wavelengths (in the red or infrared spectrum) and exposure to flash light (e.g. xenon) can also be used.FIGS. 2A to 2E include cross-sectional views of a semiconductor body (e.g., a semiconductor wafer or a semiconductor chip) in various successive steps during the exemplary process described above with reference to FIG. 1. The example of Figures 2A to 2E is not covered by the claims. FIG. 2A illustrates a semiconductor (e.g., silicon) wafer or semiconductor chip commonly referred to as a semiconductor body 200. The semiconductor body 200 has a thin (thickness of less than e.g. 10 μm) metal coating 201 (e.g. nickel, tin, aluminum, silver, gold, titanium, tungsten, copper or alloys thereof) suitable for forming an intermetallic compound with copper such that the porous copper layer may adhere thereto. Before applying the copper particle ink (copper paste), a stencil 202 is pressed onto the metal coating 201 of the semiconductor body 200 with a specific force F in order to ensure a defined contact pressure between the stencil 202 and the semiconductor body 200. The template 200 may be patterned according to the desired copper pattern to be formed on the semiconductor body 200. This situation is illustrated in FIG. 2B. A screen may be used instead of the stencil if a screen printing process is used instead of stencil printing. As an alternative to the metal coating 201, a non-metallic (e.g. polyimide) coating may also be used if electrical insulation between the semiconductor body and the copper layer is desired.In the subsequent printing step (see FIG. 1, step 102), the copper particle paste 302 is applied to the semiconductor body 200, thereby producing a structured (or unstructured) copper layer structured according to the structure of the stencil 202 (or the screen). The situation is shown in FIG. 2C. The stencil 202 may be made of stainless steel or nickel and may be patterned using known laser cutting or electro forming techniques. The thickness of the template may be between 30 μm and 300 μm, in particular between 50 μm and 100 μm. Usually, the copper paste is not applied to the edge of the wafer, a stripe of 1 mm to 3 mm at the edge of the wafer may be left empty. The printing parameters, such as the squeegee used, the contact force between stencil 202 and semiconductor body 200, the speed of lifting the stencil after applying the copper paste, etc., may depend on the actual printing process, as well as on the properties of the copper particle paste (e.g. its viscosity).FIG. 2D shows the printed copper layer 203 on the semiconductor body 200 during drying of the copper paste after the template 202 has been removed (see also FIG. 1, step 103). In the drying step, the temperature is raised (e.g., to about 60° C.) so that volatile components of the copper particle paste may evaporate and the copper paste solidifies. In order to reduce or avoid oxidation of the copper paste, the drying is carried out in a nitrogen atmosphere (i.e. nonoxidizing). Alternatively, the drying can also take place in a vacuum chamber. Furthermore, the drying may comprise phases of different temperature, e.g. 30 minutes to 60°C and subsequently 240 minutes at 100°C in nitrogen atmosphere or alternatively 30 minutes at 60°C in nitrogen atmosphere and subsequently 20 minutes at 80°C in vacuum. However, the exact values may depend on the copper paste used and other parameters of the particular application.FIG. 2E shows the subsequent heat treatment with the sintering process and the subsequent annealing (see also FIG. 1, step 104), which can be carried out at elevated temperatures in a furnace (furnace sintering). The heat treatment (sintering and annealing process) may take about 5 to 60 minutes at temperatures between 300° C. and 450° C., 400° C. being used in the present example. During the sintering process (already during heating), the wafer is bathed in nitrogen or any other carrier gas saturated (at least approximately) with gaseous formic acid. In order to saturate the carrier gas with formic acid, it may be passed through a stirring system (bubbler system) containing formic acid of a specific concentration (e.g., 98%). The saturated carrier gas is then introduced into the sintering furnace at a certain flow rate. The flow rate may be between one and five liters per minute. A flow rate of 2.6 liters per minute results in a feed rate of about 200 mg formic acid per minute into the sintering furnace.As already mentioned, pre-sintering can optionally be carried out, for example in an inert or reducing gas atmosphere, wherein various sintering methods can be used in such a pre-sintering step. During pre-sintering, formic acid may be employed, but need not necessarily be employed. However, the following (main) sintering process is performed as described above including feeding formic acid into the sintering furnace.FIG. 3 shows an alternative approach in which the stencil 202 is replaced by a photoresist layer (photoresist layer) 202'. FIG. 3A shows the semiconductor body 200 with a thin metal coating 201 (coating) as in the preceding example. In addition, the semiconductor body 200 includes a photoresist layer 202' suitable for being patterned using any known photolithographic process. That is, the photoresist layer 202' is exposed (patterned) and then exposed to a developer that removes the exposed portions of the photoresist layer. The remaining patterned photoresist layer 202' may then be cured by exposure to elevated temperatures. The resulting patterned photoresist layer 202' (see FIG. 3B) functions as a kind of stencil in the subsequent printing process.The copper particle paste is applied to the semiconductor chip 200 and the excess paste is removed (e.g., using a type of squeegee). As a result, the copper particle ink 203 remains only in those parts of the surface of the semiconductor body that are defined by the patterned photoresist layer 202'. In the subsequent drying step, the copper particle paste is dried as in the preceding example. After drying, a pre-sintering step may follow before the photoresist layer 202' is removed. The pre-sintering may be a furnace sintering process, or a photonic process such as laser or flash light treatment. Further, the photoresist layer 202' is removed. The sintering and annealing is performed in the same manner as described above with reference to the preceding example.The embodiments described above make it possible to produce metal layers on a semiconductor material with a thickness (e.g. a thickness of more than 10 μm, in particular more than 20 μm) of a metal particle paste. In contrast to known methods which use short laser or flash pulses for sintering the metal particle paste, a comparatively long heat treatment is carried out and maintained until the printed metal layer assumes a stable (non-varying) and low electrical resistance, as well as a stable microstructure. Good results were obtained with a heat treatment between 300°C and 450°C for about 20-180 minutes. Pre-sintering using any conventional sintering process may be performed (e.g., laser treatment). However, on a silicon wafer, a short laser treatment without subsequent heat treatment results in insufficient electrical and mechanical properties. A reducing atmosphere is provided in the furnace during the heat treatment. Good results were obtained when using, for example, nitrogen as carrier gas with saturated formic acid.

Claims

A method comprising: providing (101) a semiconductor wafer (200) with a coating (201); printing (102) a metal particle paste on the semiconductor wafer (200), thereby forming a metal layer; and heating (105) the metal layer in a reducing gas to sinter the metal particle paste in a furnace, wherein printing the metal particle paste comprises: applying a photoresist layer (202') to the semiconductor wafer (200); patterning the photoresist layer (202') using a photolithographic process; and applying the metal particle paste to the patterned photoresist layer (202'); wherein the method further comprises: pre-sintering (104) the metal particle paste prior to the heating; and removing the photoresist layer (202') after the pre-sintering; wherein the method comprises, prior to the heating: drying (103) the metal particle paste.The process of claim 1, wherein the reducing gas comprises gaseous formic acid, atomic hydrogen, or carbon monoxide.The method according to claim 1 or 2, wherein the drying of the metal particle paste is performed in an inert or reducing atmosphere or in a vacuum chamber.The method of claim 3, wherein the drying comprises: applying a temperature between 60°C and 200°C for about 5 to 60 minutes.The method according to any one of claims 1 to 4, wherein the sintering is performed in a sintering furnace and comprises feeding gaseous formic acid or carbon monoxide as a reducing gas into the furnace at a certain flow rate.The method of claim 5, wherein the supplying the gaseous formic acid comprises: passing a carrier gas through liquid formic acid such that the carrier gas is mixed with gaseous formic acid.The method of claim 6, wherein the supplying comprises: supplying nitrogen to the furnace from one to five liters per minute.The method of any one of claims 1 to 7, wherein the heating is performed under a pressure that is lower than ambient pressure.The method of any one of claims 1 to 8, wherein the heating comprises: applying a temperature between 300°C and 450°C until an electrical resistance of the metal layer assumes a substantially constant value.The method of claim 9, wherein the heating comprises: heating the metal layer (201) at a temperature between 300°C and 450°C until a stable microstructure of the metal layer is achieved.The method of any one of claims 1 to 10, wherein the heating comprises: applying a temperature between 300°C and 450°C for about 20 to 180 minutes.The method according to any one of claims 1 to 11, wherein the pre-sintering of the metal particle paste is performed with or without the presence of a reducing gas.The method according to any one of claims 1 to 12, wherein the metal particle paste is a copper or nickel particle paste.The method of any one of claims 1 to 13, wherein the metal layer has a thickness between about 10 μm and 150 μm or between about 20 μm and 150 μm.A method comprising: providing (101) a substrate (200); printing (102) a metal particle paste on the substrate (200) to form a metal layer; and heating (105) in a reducing gas atmosphere to sinter the metal particle paste in a furnace, and wherein printing the metal particle paste comprises: applying a photoresist layer (202') to the substrate (200); patterning the photoresist layer (202') using a photolithographic process; and applying the metal particle paste to the patterned photoresist layer (202'); wherein the method further comprises: pre-sintering (104) the metal particle paste prior to the heating; and removing the photoresist layer (202') after the pre-sintering; wherein the method comprises, prior to the heating, drying (103) the metal particle paste.The method of claim 15, wherein the substrate (200) comprises at least one of: a semiconductor material, a glass, a ceramic, or a metal.

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