Wafer holder for electrically contacting brittle semiconductor wafers and use

Abstract

The invention relates to a wafer holder for extensively electrically contacting a semiconductor wafer (40) comprising a metal body (10) with a metal flat side (12) offset by the offset height L on the upper side and a gas coupling (16) arranged on the underside, as well as at least one gas channel (14) opening in the metal flat side (12) and leading to the gas coupling (16), wherein the metal body (10) is designed for applying with electrical currents of a magnitude kA, and a flexible mat (20) lying on the metal body (10) and formed from an inert polymer, wherein the mat (20) has a recess for leading through the metal flat side (12) and the edge of the metal flat side (12) is arranged such that it is fixed all around in parallel with the metal flat side (12), the mat (20) has a respective one of at least three mat thicknesses on the side facing away from the metal body (10), along a plurality of closed, mathematically similar, concentric contours (22, 24, 26) and a first contour (22) is designed for contacting the edge region of a wafer (40), and the edge of a metal film (30) lying on the metal flat side (12) is arranged on a second smaller contour (24), and the first and second contours (22, 24) have two respective directly adjacent contours (26). The invention also relates to a use of a wafer holder.

Description

[0001] WAFER HOLDER FOR ELECTRICAL CONTACTING OF BRITTLE SEMICONDUCTOR WAFERS AND USE

[0002] The invention relates to a wafer holder for large-area electrical and / or thermal contacting of a wafer by means of negative pressure, particularly applicable in the electrochemical or wet-chemical processing of semiconductor wafers.

[0003] For the purposes of this invention, contacting refers to the creation of a non-persistent, force-locking contact between a flat side of a wafer and a metallic electrode, with the metallic electrode being understood as a good electrical and / or thermal conductor. The contacting should be extensive, meaning that the electrode contacts the majority of the wafer's flat side, typically even almost the entire wafer's flat side.

[0004] Various wafer post-processing processes require large-area contacting. These include, in particular, deposition or coating processes and etching processes. Large-area pore etching, in particular, requires very uniform contacting of the backside of the wafer—that is, the flat side facing away from the electrolyte bath. Without this uniform contacting, the etching results are usually uncontrollable, and the etching simply destroys the wafer.

[0005] Furthermore, the creation of well-controlled and uniform temperature conditions is important for many chemical baths, especially when they have to be operated at a temperature significantly higher than room temperature.

[0006] A device well suited for contacting silicon (Si) wafers, for example, has already been presented in the document DE 102013 104469 B4. It is based on the concept of a frame with a movably mounted metal block arranged inside the frame as an electrode. The frame has a circumferential groove with an O-ring arranged in the groove. The metal block, in turn, forms a gas-tight seal with the frame in every position within its range of movement. If a wafer is placed on the O-ring in the frame, the space between the wafer and the metal block can be subjected to negative pressure. This draws the metal block towards the wafer - possibly with a metal foil resting on the metal block - and brings it into contact over a large area with very little deflection of the wafer. The metal block can be subjected to high currents - several hundred A to kA - which is usually necessary especially in etching processes.

[0007] The transfer of thin monocrystalline semiconductor layers is of great interest for many applications in microelectronics, photovoltaics, and even lithium-ion batteries. This process involves removing multiple layers from the original wafer, making the wafer increasingly thinner and thus more susceptible to breakage. Furthermore, many III-V semiconductors, such as indium phosphide (InP), are very brittle, so that even thick wafers can easily break. One effective method for separating such a semiconductor layer from a monocrystalline wafer is a controlled etching attack at a predetermined depth within the wafer, for example, through electrochemical channel etching followed by an electropolishing step to remove the overlying, drilled layer. For this, good backside contacting of the wafer is necessary and in demand.

[0008] However, experiments conducted by the inventors have revealed that very brittle semiconductor wafers, especially those made of InP, usually break when attempting to contact the device described above. This is especially true when the initial thickness of the InP wafer is 500 micrometers or less. The cause is believed to be the still excessive mechanical impact stress that results from the approach of the metal block to the wafer when the vacuum builds up.

[0009] In their search for a solution, the inventors turned to document US 4 043 894 A from 1977, which ostensibly proposes holding the wafer by means of negative pressure in an annular area between two grooves with O-rings and making electrical contact in the center using a liquid electrolyte. These ideas are not particularly useful here, e.g. because of the high currents required. But the document mentions in passing in column 2, line 59 - column 3, line 3: “O-rings provide a plurality of coplanar ridges projecting from the disk upper surface for receiving the wafer. Resilient O-rings are preferred because they provide a good seal to the wafer and can be periodically replaced to insure a consistent seal. However, it is contemplated that ridges could be formed as an integral part of the disk if coplanarity can be maintained.It should be noted that the ridges need not necessarily be concentric or circular as long as that they are continuous or closed-looped and that each continuous ridge has a progressively larger perimeter as they extend towards the outer periphery of the disk surface.”.

[0010] With the technologies available today for material processing and structuring, it is possible to derive a solution to the problem.

[0011] The invention aims to realize large-area electrical and / or thermal contacting of a flat side of a wafer by means of negative pressure, even for very brittle semiconductor wafers, without risk of breakage, by proposing a new design of a wafer holder.

[0012] The object is achieved by a wafer holder for large-area electrical contacting of a semiconductor wafer, comprising a metal body with a metal flat side offset on the top by the offset height L and a gas coupling arranged on the bottom and at least one gas channel open in the metal flat side and leading to the gas coupling, wherein the metal body is designed to be subjected to electrical currents of the order of kA and a flexible mat lying on the metal body formed from an inert polymer, wherein the mat has a recess for the passage of the metal flat side and is arranged surrounding the edge of the metal flat side and fixed parallel to the metal flat side, wherein the mat on the side facing away from the metal body along a plurality of closed, mathematically similar, concentric contours in each case one of at least three mat thicknesses M, R,G and the mat thicknesses with the property M > R > L > G are predetermined such that a first contour of the mat thickness M is designed to contact the edge region of a wafer and on a second smaller contour of the mat thickness R the edge of a metal foil resting on the metal flat side is arranged and the first and second contours each have two immediately adjacent contours of the mat thickness G.,

[0013] The subclaims are directed to advantageous embodiments of the wafer holder.

[0014] The inert polymer can in particular be a fluoropolymer or a silicone.

[0015] In a preferred embodiment, the mat thickness M can be a few millimeters (a few millimeters can be about 1-4 millimeters), particularly preferably 2-3 millimeters.

[0016] The mat thickness M can preferably be between 200 and 300 micrometers greater than the settling height L of the metal flat side.

[0017] In particular, the mat thickness G can correspond to half the mat thickness M.

[0018] The first and second contours may each have a width of at least 1 millimeter.

[0019] The metal foil can be made of gold or aluminum.

[0020] The thickness of the metal foil can be 30-100 micrometers.

[0021] The mat thickness R can be between 50 and 150 micrometers larger than the settling height L of the metal flat side.

[0022] Further according to the invention is the use of the wafer holder for large-area electrical contacting of brittle semiconductor wafers, in particular indium phosphide wafers.

[0023] The basic idea of ​​the invention is that (i) the O-ring in the device according to document DE 10 2013 104 469 B4 is replaced by a back structure integrated into a flexible mat made of an inert polymer and extending along a closed contour, and (ii) the flat metal side for contacting is completely immobile. The wafer resting on the back structure approaches the flat metal side - the metal block - when the negative pressure is built up through the gas channel, by lowering the wafer onto the flat metal side as the flexible back structure yields. Therefore, only the small mass of the wafer itself and the even smaller mass of one or more back structures in the flexible mat are moved. The force load and deflection of the wafer are minimized by the elastic deformation of the inert polymer.

[0024] The inert polymer comes into contact with hot metal—as a result of current flow—and, at least outside the outermost back structure, also with an electrolyte. It must therefore be chemically and thermally stable. Fluoropolymers such as Teflon® or silicones are preferred inert polymers. A mat made of inert polymer can now be processed very precisely, for example, by laser ablation, and structured along predetermined contours. Structuring in the context of this description involves laser ablation of polymer material to locally reduce the mat thickness from an initial value, e.g., M, to smaller values, e.g., R and G.

[0025] For this description, a closed contour is understood to be a two-dimensional, self-contained line in the plane of the mat with a predetermined line width, the contour width. A plurality of contours is provided on the upper side of the flexible mat. These contours should be arranged concentrically and be mathematically similar, i.e. they should be able to be brought into congruence by rotation and / or stretching. According to the invention, different mat thicknesses M, R, G are assigned to the individual contours, which are set, for example, by laser ablation along the contours. For this reason alone, the contours cannot overlap, and in particular all contours also have different diameters. Two contours are referred to as immediately adjacent if they run completely parallel to one another without any gaps in the plane of the mat.Each contour can have a maximum of two immediately adjacent contours, namely a smaller inner contour that runs closer to the common center everywhere and a larger outer contour that runs further away from the center everywhere.

[0026] In the simplest case, all closed contours are circular, i.e. an arrangement of concentric rings with different mat thicknesses is realized on the mat. However, reference is made to US 4 043 894 A that the structures integrated into the mat can also be designed differently, for example square or even star-shaped. This can be advantageous for processing wafers that are not circular disks. It should be emphasized here that almost all wafers used in industry deviate from the ideal circular shape because they are provided with at least one flat for the purposes of automatic orientation in production systems, i.e. a circular section at the wafer edge has been removed. The contours of the invention can - in contrast to conventional O-rings - follow the edge profile of the wafer perfectly and thus ensure optimal gas sealing even with maximum surface contact with the wafer.

[0027] One of the inventors' key findings from experiments with integrated back structures in a flexible mat as described in US Pat. No. 4,043,894 A is that stable vacuum cannot be achieved between the wafer and the metal block if the mat exhibits even the slightest lateral distortion. Such distortions can occur both when the mat is fixed to the metal block and when the back structures are deformed when the wafer is suctioned in, because with every deformation, small force components act in a direction parallel to the mat, which can result in lateral distortions. These distortions then typically have the effect of eliminating the required planarity of the integrated back structure in contact with the wafer, so that a gas-tight seal is no longer possible.

[0028] To avoid unwanted distortions, the invention introduces two contours of mat thickness G < R < M directly adjacent to the supporting structures to be deformed (contours with mat thicknesses M, R). These contours can also be referred to as trench structures. Their purpose is to mechanically decouple the supporting structures from their lateral neighbors in the mat, so that force components parallel to the mat cannot act on this neighborhood. These trenches on both sides of the supporting back structures result in significantly improved gas tightness between the wafer and the metal block; the applied negative pressure is stable and consistent.

[0029] The invention is explained in more detail below using an exemplary embodiment and a figure. It shows:

[0030] Fig. 1 shows a sketch of a sectional view perpendicular to the metal flat side and the wafer lying on it to illustrate the mat thicknesses according to the invention.

[0031] Fig. 1 shows a sectional sketch, not to scale, through an exemplary wafer holder according to the present invention. The section runs perpendicular to the offset metal flat side 12 of the metal block 10. The metal flat side 12 protrudes by the offset height L from the remaining top side of the metal block 10 and is arranged in its center. It also has at least one opening for a gas channel 14, which traverses the interior of the metal block 10 and leads into a gas coupling 16 on the underside of the metal block 10. The gas coupling 16 can be designed as a closable tap. It is used to connect a device for sucking in gas from the metal flat side 12, i.e., for applying negative pressure. The metal block 10 carries all other components of the wafer holder, in particular the flexible mat 20 made of inert polymer.It should be noted here that the parts of the metal block 10 on which the mat 20 rests could also be replaced with a different material, for example, an electrically insulating material such as a plastic. In this case, the offset height L of the flat metal side 12 would be understood to be the height difference between the flat metal side 12 and the support surface of the mat 20, which is also clearly visible in Fig. 1.

[0032] The mat 20 has an initial mat thickness M > L and a recess for the passage of the flat metal side 12. This means that a central area of ​​the mat 20 is cut out, and the mat 20 lies on top of the metal block 10 such that it surrounds the edge of the flat metal side 12. The mat 20 is thus oriented parallel to the flat metal side 12 and is fixed in position by suitable fixing elements such as screws or clamps at the edge of the mat 20 (not shown). The mat 20 typically does not protrude beyond the edge of the metal block 10.

[0033] On the side of the mat 20 facing away from the metal block 10, several concentric contours are provided, which in this example are designed as circular rings. Various mat thicknesses M, R, G are created along these contours by laser ablation, whereby the relationship M > R > L > G with L as the offset height of the flat metal side 12 is important. In the sectional sketch in Fig. 1, only the cross sections of the machined circular ring structures 22, 24, 26 are visible. The terms "closed concentric contours" and "ridge structures, trench structures" are often used synonymously here and elsewhere in the description, meaning that the three-dimensional structures are created by assigning a mat thickness to the contours when the mat 20 is processed accordingly with the laser.

[0034] The initial mat thickness of the flexible mat 20 can preferably be 2 to 3 millimeters. Although it can then be reduced everywhere - i.e. over the entire surface - to a predetermined mat thickness M, it is clearly expedient to equate the mat thickness M with the initial mat thickness. A first closed contour 22 with mat thickness M is provided for the gas-tight contacting of the wafer 40. A second contour 24, smaller in diameter and thus further inward, with mat thickness R serves to support the edge of a thin metal foil 30, which otherwise rests on the flat metal side 12 and extends beyond the edge of the flat metal side 12, but does not reach as far as the first contour 22.Preferably, the first contour 22 has a contour width of approximately 1 millimeter, while the second contour 24 can also be wider than 1 millimeter in order to establish the largest possible contact area between the metal foil 30 and the edge of the wafer 40. The usual "sagging" of the metal foil 30 within the supporting contour 24, which leads to the metal foil 30 resting on the flat metal side 12, is not shown in Fig. 1 to simplify the sketch.

[0035] Furthermore, the mat thickness M is preferably between 200 and 300 micrometers greater than the settling height L of the flat metal side 12. In other words, the first contour 22 projects beyond the fixedly positioned flat metal side 12 by 200-300 micrometers in the unloaded state. The metal film 30 should be slightly higher at its edge on the second contour 24 than on the flat metal side 12; preferably, the mat thickness R of the second contour is between 50 and 150 micrometers greater than the settling height L of the flat metal side 12.

[0036] The metal foil 30 is preferably made of a good electrical conductor material, preferably one of the element metals gold or aluminum. The metal foil 30 is preferably between 30 and 100 micrometers thick. The purpose of the metal foil 30 is to supply current to the edge region of the wafer 40 between the edge of the flat metal side 12 and the first contour 22. This edge region makes up a significant portion of the surface of the wafer 40; however, the entire current from this region can be transported via short paths through the metal foil 30 into the metal body 10, so that the ohmic losses are small and the potential differences are negligible.

[0037] According to the invention, the first and second contours 22, 24 each have two directly adjacent contours 26 of mat thickness G, so-called trench structures. The individual trench structure 26 arranged between the first and second contours 22, 24 is directly adjacent to both contours 22, 24. Preferably, the mat thickness G is half the mat thickness M, and in particular, G is less than the settling height L of the flat metal side 12. As already explained, the contours 26 with mat thickness G are intended to prevent the propagation of forces parallel to the mat plane, among other things, when these forces arise from the deformation of the supporting back structures 22, 24 under force loading due to the build-up of negative pressure. The contour widths of the contours 26 can be 1 millimeter, but can also be predetermined to be significantly larger. Here, the user has a freedom of choice, which they can use to optimize the flexible behavior of their mat 20.Depending on the user's choice of initial mat thickness and material (inert polymer), the user will be able to easily define contours and mat thicknesses based on the present description and identify the solution with the best results by means of a simple series of preliminary tests.

[0038] The lowering of the brittle wafer 40 onto the metal flat side 12 with metal foil 30 during the build-up of the negative pressure is carried out with the wafer holder according to the invention so gently and with minimal force that in the experiments of the inventors, for example with 300 micrometer thick InP wafers for electrochemical etching, not a single wafer was broken, whereas before the use of the invention all of them broke.

Claims

A N S P R Ü C H E 1. Wafer holder for large-area electrical contacting of a semiconductor wafer (40), comprising a metal body (10) with a metal flat side (12) offset on the top by the offset height L and a gas coupling (16) arranged on the bottom, as well as at least one gas channel (14) open in the metal flat side (12) and leading to the gas coupling (16), wherein the metal body (10) is designed to be subjected to electrical currents of the order of kA, and a flexible mat (20) lying on the metal body (10) formed from an inert polymer, wherein the mat (20) has a recess for the passage of the metal flat side (12) and is arranged fixedly surrounding the edge of the metal flat side (12) parallel to the metal flat side (12), the mat (20) on the side facing away from the metal body (10) along a plurality of closed, mathematically similar, concentric contours (22, 24, 26) each have one of at least three mat thicknesses M, R,G and the mat thicknesses with the property M > R > L > G are predetermined such that a first contour (22) of the mat thickness M is designed to contact the edge region of a wafer (40) and on a second smaller contour (24) of the mat thickness R the edge of a metal foil (30) resting on the metal flat side (12) is arranged and the first and second contours (22, 24) each have two immediately adjacent contours (26) of the mat thickness G., 2. Wafer holder according to claim 1, characterized in that the inert polymer is a fluoropolymer or a silicone.

3. Wafer holder according to claim 1 or 2, characterized in that the mat thickness M is a few millimeters, preferably 2-3 millimeters.

4. Wafer holder according to claim 3, characterized in that the mat thickness M is between 200 and 300 micrometers greater than the settling height L of the metal flat side (12). Wafer holder according to one of the preceding claims, characterized in that the mat thickness G corresponds to half the mat thickness M. Wafer holder according to one of the preceding claims, characterized in that the first and the second contour (22, 24) each have a width of at least 1 millimeter. Wafer holder according to one of the preceding claims, characterized in that the metal foil (30) is formed from gold or aluminum. Wafer holder according to claim 7, characterized in that the thickness of the metal foil is 30-100 micrometers. Wafer holder according to claim 8, characterized in that the mat thickness R is between 50 and 150 micrometers greater than the settling height L of the flat metal side. Use of the wafer holder according to one of the preceding claims for large-area electrical contacting of brittle semiconductor wafers (40), in particular of indium phosphide wafers.

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