DE102022122634B3 - WAFER HOLDER FOR ELECTRICAL CONTACTING BRITTLE SEMICONDUCTOR WAFER AND USE - Google Patents

Abstract

The invention relates to a 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 upper side by the offset height L and a gas coupling (16) arranged on the underside and at least one in the metal flat side (12) open gas channel (14) leading to the gas coupling (16), the metal body (10) being designed to be subjected to electrical currents of the order of magnitude kA, characterized by a flexible mat (20) resting on the metal body (10). made of an inert polymer, - the mat (20) having a recess for the passage of the metal flat side (12) and surrounding the edge of the metal flat side (12) is arranged fixed parallel to the metal flat side (12), - the mat (20) on the The side facing away from the metal body (10) along a plurality of closed, mathematically similar, concentric contours (22, 24, 26) each has one of at least three mat thicknesses and a first contour (22) is designed for contacting the edge region of a wafer (40) and - The edge of a metal foil (30) resting on the metal flat side (12) is arranged on a second smaller contour (24) and the first and second contours (22, 24) each have two immediately adjacent contours (26). The invention further relates to a use of a wafer holder.

Description

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

For the purposes of this invention, contacting is to be understood as the generation of a non-persistent non-positive contact between a flat side of a wafer and a metallic electrode, whereby the metallic electrode is to be understood as a good electrical conductor and/or as a heat conductor. The contacting should be over a large area in the sense that the electrode contacts the majority of the wafer flat side, typically even almost the entire wafer flat side.

Various post-processing processes for wafers require large-area contacting. The main ones that should be mentioned here are deposition or coating processes and etching processes. Large-area pore etching in particular requires very uniform contacting of the back, i.e. the flat side of the wafer facing away from the electrolyte bath. Without this uniform contacting, the etching results are usually not controllable and the etching merely destroys the wafer.

Furthermore, creating well-controlled and uniform temperature conditions is important for many chemical baths, especially if they have to be operated at a temperature that is significantly higher than room temperature.

A device that is well suited for contacting, for example, silicon (Si) wafers is already described in the publication DE 10 2013 104 469 B4 presented. 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, for its part, 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. As a result, the metal block is pulled towards the wafer - possibly with a metal foil resting on the metal block - and brought into large-area contact with very little deflection of the wafer. The metal block can be subjected to large currents - several 100 A to kA - which is particularly necessary in etching processes.

The transfer of thin monocrystalline semiconductor layers is of great interest for many applications in microelectronics, photovoltaics and also Li-ion batteries. In the process, multiple layers are removed from the original wafer, making the wafer increasingly thinner and therefore more susceptible to breakage. In addition, many III-V semiconductors such as indium phosphide (InP) are very brittle, so even thick wafers can break easily. An effective means of separating such a semiconductor layer from a monocrystalline wafer consists of a controlled etching attack at a predetermined depth of the wafer, for example by electrochemical channel etching followed by an electropolishing step to remove the overlying drilled layer. For this purpose, good contact on the back of the wafer is necessary and in demand.

However, experiments by the inventors have revealed that very brittle semiconductor wafers, especially those made of InP, usually break when attempting to make contact with the device described above. This is especially true if the initial thickness of the InP wafers is 500 micrometers or less. The reason is believed to be the still excessive mechanical impact load that results from the metal block approaching the wafer when the negative pressure is built up.

The inventors of the pamphlet were looking for a solution US 4,043,894 A from 1977, in which primarily a holding of the wafer using negative pressure in an annular area between two grooves with O-rings and a central electrical contact using a liquid electrolyte are encouraged. These ideas are initially not effective here, for example because of the large current levels required. But the publication mentions, so to speak, 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.”

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

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

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 settling height L and a gas coupling arranged on the underside and at least one gas channel open in the metal flat side and leading to the gas coupling, the metal body is designed to apply electrical currents of the order of magnitude kA and a flexible mat resting on the metal body is formed from an inert polymer, the mat having a recess for the passage of the metal flat side and surrounding the edge of the metal flat side is arranged fixedly parallel to the metal flat side, the mat being on the side facing away from the metal body along a plurality of closed, mathematically similar, concentric contours each has one of at least three mat thicknesses M, R, G and the mat thicknesses with the property M > R > L > G are predetermined in such a way that a first contour of the mat thickness M is designed to contact the edge region of a wafer and the edge of a metal foil lying on the flat metal side is arranged on a second smaller contour of the mat thickness R and the first and second contours each have two immediately adjacent contours of the mat thickness G.

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

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

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.

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

The mat thickness G can in particular correspond to half the mat thickness M.

The first and second contours can each have a width of at least 1 millimeter.

The metal foil can be made of gold or aluminum.

The thickness of the metal foil can be in particular 30-100 micrometers.

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

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.

The basic idea of the invention can be seen in the fact that (i) the O-ring in the context of the device according to the publication DE 10 2013 104 469 B4 is replaced by a back structure integrated into a flexible mat made of an inert polymer and running along a closed contour and (ii) the metal flat side is completely immovable for contacting purposes. The wafer resting on the back structure approaches the metal flat side - the metal block - when the negative pressure is built up through the gas channel by lowering the wafer onto the metal flat side while the flexible back structure gives way. So 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 is minimized by the elastic deformation of the inert polymer.

The inert polymer comes into contact with - through energization - hot metal 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 as inert polymers. A mat made of inert polymer can now be processed quite precisely using laser ablation and structured along predetermined contours. Structuring in the context of this description includes the 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.

This description understands a closed contour to be a two-dimensional, self-recurring line in the mat plane with a predetermined line width, the contour width. A plurality of contours are provided on the top of the flexible mat. These contours should be arranged concentrically and be similar in a mathematical sense, that is, they should be able to be brought to congruence by rotation and/or stretching. According to the invention, the individual contours are assigned different mat thicknesses M, R, G, which are set up, for example, by laser ablation along the contours. For this reason, the contours cannot overlap, and in particular all the contours have different diameters. Two contours are said to be immediately adjacent if they have no intermediate space in the mat plane are completely parallel to each other. Each contour can have a maximum of two immediately adjacent contours, namely a smaller inner one that runs closer to the common center everywhere and a larger outer one that runs further away from the center everywhere.

In the simplest case, all closed contours are circular, ie an arrangement of concentric rings with different mat thicknesses is created on the mat. However, reference is made to the US 4,043,894 A pointed out 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 non-circular wafers. It should be emphasized here that almost all industrially used wafers deviate from the ideal circular shape because they are provided with at least one flat for the purposes of automatic orientation in production systems, ie a circular section on the edge of the wafer has been removed.

The contours of the invention can - in contrast to conventional O-rings - perfectly follow the edge of the wafer and thus ensure optimal gas sealing with maximum surface contact of the wafer.

One of the inventors' key findings from experiments with integrated back structures in a flexible mat like the US 4,043,894 A is that no stable negative pressure can be achieved between the wafer and the metal block if the mat has even the slightest lateral distortions. However, such distortions can occur both when the mat is fixed on the metal block and when the back structures are deformed when the wafer is sucked in, because with every deformation small force components also act in the direction parallel to the mat, which can result in lateral distortions. The distortions then usually have the effect that they cancel the required planarity of the integrated back structure in contact with the wafer, so that gas-tight sealing is no longer possible.

In order to avoid undesirable distortions, the invention introduces two contours of the mat thickness G < R < M which are immediately adjacent to the supporting structures - to be deformed (contours with mat thicknesses M, R), which can also be referred to as trench structures. Its purpose is to mechanically decouple the supporting structures from the lateral neighborhood in the mat so that force components parallel to the mat cannot act on this neighborhood. With the help of these trenches on both sides of the supporting back structures, there is a significantly improved gas tightness between the wafer and the metal block; the applied negative pressure is stable and consistent.

• 1 die Skizze eines Schnittbildes senkrecht zu Metallflachseite und aufliegendem Wafer zur Darstellung der erfindungsgemäßen Mattendicken.

The invention is explained in more detail below using an exemplary embodiment and using a figure. This shows:

• 1 the sketch of a sectional view perpendicular to the flat metal side and the wafer lying on it to show the mat thicknesses according to the invention.

In 1 1 is a sectional sketch, not to scale, of an exemplary wafer holder according to the present invention. The cut runs perpendicular to the offset metal flat side 12 of the metal block 10. The metal flat side 12 protrudes from the rest of the top side of the metal block 10 by the offset height L and is arranged in its center. It also has at least one opening for a gas channel 14, which passes through 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 lockable tap. It is used to connect a device for sucking in gas from the metal flat side 12, i.e. to apply 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 by another material, for example by an electrically insulating material, such as a plastic. In this case, the settling height L of the metal flat side 12 would be understood to mean the height difference between the metal flat side 12 and the support surface of the mat 20, which is also evident from the 1 is easy to see.

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

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. Laser ablation is used to create different shapes along these contours The mat thicknesses M, R, G are set up, whereby the relation M > R > L > G with L as the settling height of the metal flat side 12 is essential. In the sectional sketch of the 1 only the cross sections of the worked out circular ring structures 22, 24, 26 can be seen. The terms “closed concentric contours” and “back structures, trench structures” are often used synonymously here as well as 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 connected to the Laser processed accordingly.

The initial mat thickness of the flexible mat 20 can preferably be 2 to 3 millimeters. Although it can then be reduced to a predetermined mat thickness M everywhere - that is, over the entire surface - 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 with a mat thickness R that is smaller in diameter and therefore further inward serves to support the edge of a thin metal foil 30, which otherwise rests on the metal flat side 12 and extends beyond the edge of the metal flat side 12, but not to the first contour 22 is enough. The first contour 22 preferably has a contour width of approximately 1 millimeter, while the second contour 24 can also be designed to 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 metal flat side 12, is in 1 not shown to simplify the sketch.

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

The metal foil 30 preferably consists 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 area of the wafer 40 between the edge of the metal flat side 12 and the first contour 22. This edge area makes up a significant portion of the area of the wafer 40; However, the entire current from this area can be transported over short distances through the metal foil 30 into the metal body 10, so that the ohmic losses are small and the potential differences are negligible.

According to the invention, the first and second contours 22, 24 each have two immediately adjacent contours 26 of the 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. The mat thickness G is preferably half of the mat thickness M, and in particular G is smaller than the settling height L of the metal flat 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 if these forces come from the Deformation of the supporting back structures 22, 24 occurs under force load 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 freedom of choice which he can use to optimize the flexible behavior of his mat 20. Depending on the user's choice of initial mat thickness and material (inert polymer), the user will easily define contours and mat thicknesses based on the present description and will be able to identify the solution with the best results through a simple series of preliminary tests.

The brittle wafer 40 is lowered onto the metal flat side 12 with metal foil 30 when the negative pressure is built up using the wafer holder according to the invention so gently and with minimal force that in the inventors' experiments, for example with 300 micrometer thick InP wafers for electrochemical etching, no only wafer is broken anymore, where they all broke before using the invention.

Claims (10)

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 upper side by the offset height L and a gas coupling (16) arranged on the underside and at least one open in the metal flat side (12). , gas channel (14) leading to the gas coupling (16), the metal body (10) being designed to be subjected to electrical currents of the order of magnitude kA, and a flexible mat (20) resting on the metal body (10) formed from an inert polymer, the Mat (20) has a recess for the metal flat side (12) to pass through and the edge the metal flat side (12) is arranged fixedly 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 one of has 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 formed for contacting 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. wafer holder Claim 1 , characterized in that the inert polymer is a fluoropolymer or a silicone. wafer holder Claim 1 or 2 , characterized in that the mat thickness M is a few millimeters, preferably 2-3 millimeters. wafer holder 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 second contours (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 made of gold or aluminum. wafer holder Claim 7 , characterized in that the thickness of the metal foil is 30-100 micrometers. wafer holder Claim 8 , characterized in that the mat thickness R is between 50 and 150 micrometers greater than the settling height L of the metal flat 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 indium phosphide wafers.

Images