Wafer holder for electrically contacting brittle semiconductor wafers and use

Abstract

The invention relates to a wafer holder for large-area electrical contact of a semiconductor wafer (40), comprising a metal body (10), the invention relates to a metal body (10) having a metal flat side (12) protruding at a protrusion height L on the upper side, a gas connection (16) arranged on the lower side, and at least one gas channel (14) opening in the metal flat side (12) and leading to the gas connection (16), the metal body (10) being designed to apply an electric current on the order of kA; and a flexible pad (20) which is placed on the metal body (10) and is made of an inert polymer, which pad (20) has a recess for guiding the metal flat side (12) through and is arranged fixedly parallel to the metal flat side (12) around an edge of the metal flat side (12), according to the invention, on the side facing away from the metal body (10), the pad (20) has one of at least three pad thicknesses along a plurality of closed, mathematically similar concentric contours (22, 24, 26), and the first contour (22) is designed to contact an edge region of the wafer (40), the edge of the metal foil (30) placed on the metal flat side (12) is provided on a smaller second contour (24), and the first and second contours (22, 24) each have two contours (26) directly adjacent to each other. In addition, the invention also relates to an application of the wafer holder.

Description

Technical Field

[0001] The invention relates to a wafer holder for large-area electrical and / or thermal contacting of a wafer by means of negative pressure, which wafer holder can be used in particular in electrochemical or wet-chemical processing of semiconductor wafers. Background Art

[0002] In the sense of the invention, contact is understood to mean that a non-permanent force-locking contact is produced between the flat side of the wafer and a metal electrode, which should be understood to be a good electrical and / or thermal conductor. The contact should be large-area in the sense that the electrode contacts a substantial part of the flat side of the wafer, usually even almost the entire flat side of the wafer.

[0003] For various post-processing processes of the wafer, large-area contact is required. Here, deposition or coating methods and etching processes can be mentioned. In particular, large-area hole etching requires very uniform contact on the back side of the wafer, that is, the flat side facing away from the electrolytic cell. Without such uniform contact, the etching result is usually uncontrollable and etching will only destroy the wafer.

[0004] Furthermore, it is important for many chemical baths to produce well-controlled and uniform temperature conditions, especially when the chemical baths have to be operated at temperatures that are significantly increased relative to room temperature.

[0005] A device that is very suitable for contacting, for example, silicon (Si) wafers has been proposed in document DE 10 2013 104 469 B4. This device is based on a frame-type design, in which the frame has a metal block that is arranged inside the frame and is movably supported as an electrode. The frame has a surrounding groove with an O-ring arranged in the groove. The metal block itself is hermetically sealed with the frame in any position of its range of motion. If a wafer is placed on the O-ring in the frame, the space between the wafer and the metal block can be loaded with negative pressure. As a result, the metal block is attracted to the wafer together with the metal foil placed on the metal block when necessary, and large-area contact is achieved when the wafer is only slightly bent. The metal block can be loaded with a high current intensity from several 100 A to kA, which is usually necessary, especially in etching processes.

[0006] The transfer of thin single-crystalline semiconductor layers is of great interest for many applications in microelectronics, photovoltaics, but also in lithium-ion batteries. Here, multiple layers are removed from the original wafer, whereby the wafer always becomes thinner and thus more prone to breakage. In addition, many III-V semiconductors, such as indium phosphide (InP), are very brittle, so that even thick wafers can be prone to breakage. An effective measure for separating such a semiconductor layer from a single-crystalline wafer consists in carrying out a controlled etching to a predetermined depth of the wafer, for example by means of electrochemical channel etching, followed by an electropolishing step for peeling off the perforated layer located thereon. For this purpose, good backside contact of the wafer is necessary and required.

[0007] In the meantime, the inventors' tests have shown that very brittle semiconductor wafers, especially those made of InP, usually break when attempting to come into contact with the device described above. This is especially the case when the initial thickness of the InP wafer is 500 micrometers or less. The still excessive mechanical shock load is considered to be the cause, which is generated by the metal block approaching the wafer when establishing a negative pressure.

[0008] In the search for a remedy, the inventors referred to the 1977 document US 4 043 894 A, in which it is evident that the wafer is held by a negative pressure in an annular region between two grooves with O-rings, and the intermediate electrical contact is activated by a liquid electrolyte. Here, these concepts are initially unsuitable, for example due to the required high current intensity. However, the document incidentally mentions to some extent in column 2, lines 59 to 33: "The O-rings provide a plurality of coplanar ridges protruding from the upper surface of the disk for receiving the wafer. Elastic O-rings are preferred because such O-rings provide a good seal for the wafer and can be replaced regularly to ensure a continuous seal. However, if coplanarity can be maintained, it is also possible to consider forming the ridges as part of the disk as a whole. It should be noted that as long as the ridges are continuous or annularly closed, the ridges do not have to be concentric or circular, and each continuous ridge has an increasing circumference as it extends towards the outer periphery of the disk surface."

[0009] A solution to the problem can be derived therefrom by using today's available technologies for material processing and structuring. Summary of the Invention

[0010] The object of the present invention is to propose a new design of a wafer holder by the present invention, whereby even for very brittle semiconductor wafers, it is possible to achieve large-area electrical and / or thermal contact with the flat side of the wafer by means of negative pressure without the risk of breakage.

[0011] The object is achieved by a wafer holder for large-area electrical contacting of semiconductor wafers, the wafer holder comprising: a metal body having a metal flat side protruding with a protrusion height L on the upper side, a gas connection provided on the lower side, and at least one gas channel opening in the metal flat side and leading to the gas connection, the metal body being configured to be loaded with a current on the order of kA; and a flexible gasket formed of an inert polymer placed on the metal body, the gasket having a notch for guiding the metal flat side therethrough and the gasket being fixedly arranged parallel to the metal flat side around the edge of the metal flat side, the gasket having on the side facing away from the metal body at least one of at least three gasket thicknesses M, R, G along a plurality of closed, mathematically similar concentric contours, and the gasket thicknesses being predetermined such that M > R > L > G, so that a first contour having a gasket thickness M forms for contacting the edge region of the wafer and the edge of the metal foil placed on the metal flat side is arranged on a smaller second contour having a gasket thickness R, and the first and second contours each have two directly adjacent contours having a gasket thickness G.

[0012] The dependent claims relate to advantageous designs of the wafer holder.

[0013] The inert polymer can in particular be a fluoropolymer or silicone.

[0014] In a preferred design, the gasket thickness M can be a few millimeters (where a few millimeters can be about 1 to 4 millimeters), particularly preferably 2 to 3 millimeters.

[0015] The gasket thickness M preferably can be between 200 and 300 microns greater than the protrusion height L of the metal flat side.

[0016] The gasket thickness G can in particular be equal to half of the gasket thickness M.

[0017] The first and second contours can each have a width of at least 1 millimeter.

[0018] The metal foil can be formed of gold or aluminum.

[0019] The thickness of the metal foil can in particular be 30 to 100 microns.

[0020] The gasket thickness R can be between 50 and 150 microns greater than the protrusion height L of the metal flat side.

[0021] Furthermore, according to the invention, the wafer holder is for the application of large-area electrical contacting of brittle semiconductor wafers, in particular indium phosphide wafers.

[0022] The basic concept of the present invention lies in: (i) replacing the O-ring in the frame of the device according to document DE 10 2013 104 469 B4 with a back structure integrated into a flexible gasket made of an inert polymer and extending along a closed contour, and (ii) the metal flat sides for contact being completely immovable. When negative pressure is established through the gas channels, the wafer placed on the back structure approaches the metal flat side, i.e., the metal block, in such a way that the wafer descends onto the metal flat side when the flexible back structure yields. That is to say, only the small mass of the wafer itself and the even smaller mass of one or more back structures in the flexible gasket move. The force load and bending of the wafer are minimized by the elastic deformation of the inert polymer.

[0023] The inert polymer is in contact with a metal that becomes hot due to the application of electricity and is also in contact with an electrolyte at least outside the outermost back structure. Therefore, the inert polymer must be chemically and thermally stable. Fluorine-containing polymers, such as Teflon®, or silicones are preferably considered as the inert polymer. Currently, for example, the gasket made of an inert polymer can be machined very precisely by laser ablation and structured along a pre-determined contour. In the context of this specification, the structuring includes laser ablation of the polymer material so that the gasket thickness is locally reduced from an initial value, such as M, to a smaller value, such as R and G.

[0024] In this specification, a closed contour means a two-dimensional line in the gasket plane that leads back to itself and has a pre-determined line width, i.e., the contour width. A plurality of contours are provided on the upper side of the flexible gasket. These contours should be arranged concentrically and are similar in the mathematical sense, i.e., they can be made congruent by rotation and / or stretching. According to the invention, different gasket thicknesses M, R, G are assigned to the individual contours, which are achieved, for example, by laser ablation along the contour. Therefore, the contours can no longer intersect each other, and in particular, all contours also have different diameters. If two contours extend completely parallel to each other in the gasket plane without a gap, these two contours are said to be directly adjacent. Each contour can have at most two directly adjacent contours, i.e., a smaller inner contour and a larger outer contour, where the inner contour extends closer to the common center as a whole, and the outer contour extends further away from the center as a whole.

[0025] In the simplest case, all closed contours are circular, that is, a layout structure of concentric rings with different gasket thicknesses is implemented on the gasket. However, as can be seen from US4 043 894 A, the structures integrated into the gasket can also be designed differently, for example, designed as 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 are different from the ideal circular shape because these wafers have at least one flat edge for orientation in production equipment, that is, a circular section on the wafer edge is removed. Different from common O-rings, the contours of the present invention can perfectly follow the edge trend of the wafer, and thus ensure the best gas seal even when the wafer is in contact with the maximum ground area at this time.

[0026] One of the main insights obtained by the inventor from experiments using the integrated back structure in a flexible gasket in the form of US 4 043 894 A is that even if the gasket has only a very small lateral distortion, a stable negative pressure cannot be achieved between the wafer and the metal block. However, this kind of distortion may occur when the gasket is fixed on the metal block and when the back structure deforms when sucking the wafer, because in each deformation, there are also small force components acting in the direction parallel to the gasket, and these force components may cause lateral distortion. At this time, the distortion usually has the effect that the integrated back structure in contact with the wafer loses the required flatness, so that an airtight seal can no longer be achieved.

[0027] To avoid unwanted distortion, the present invention here introduces directly adjacent contours (with gasket thicknesses G < R < M) for every two load-bearing and deformable structures (contours with gasket thicknesses M, R). The directly adjacent contours can also be called groove structures. The purpose is to mechanically decouple the load-bearing structure from the lateral neighborhood in the gasket, so that the force components parallel to the gasket cannot act on the neighborhood. With the help of these grooves on both sides of the load-bearing back structure, a significantly improved airtightness between the wafer and the metal block is achieved; the applied negative pressure is stable and continuous. Brief Description of the Drawings

[0028] The present invention will be described in detail below with reference to exemplary embodiments and with reference to the drawings. Among them:

[0029] Figure 1 A sketch showing a cross-sectional view perpendicular to the metal flat side and the placed wafer, for showing the gasket thickness according to the present invention. Detailed Description of the Specific Embodiment

[0030] In Figure 1A disproportionate cross-sectional sketch of an exemplary wafer holder according to the present invention is shown. Here, the cross-section extends perpendicular to the metal flat side 12 of the protrusion of the metal block 10. The metal flat side 12 protrudes from the rest of the upper side of the metal block 10 by the protrusion height L and is provided in the center of the metal block. In addition, the wafer holder further has at least one opening for the gas channel 14, which internally traverses the metal block 10 and introduces a gas connection 16 on the lower side of the metal block 10. The gas connection 16 can be configured as a closable cock. The gas connection is used to connect a device for sucking gas starting from the metal flat side 12, that is, for loading a negative pressure.

[0031] The metal block 10 bears all other components of the wafer holder, especially the flexible gasket 20 made of an inert polymer. It should be noted here that the part of the metal block 10 where the gasket 20 is placed can also be replaced by another material, for example, it can also be replaced by an electrically insulating material, such as plastic. In this case, the protrusion height L of the metal flat side 12 can be understood in the sense as the height difference between the metal flat side 12 and the placement surface of the gasket 20, which can also be Figure 1 easily seen from.

[0032] The gasket 20 has an initial gasket thickness M > L and a notch for guiding the metal flat side 12 through, that is, the area in the center of the gasket 20 is cut away, and the gasket 20 is placed on the upper side of the metal block 10 such that the gasket surrounds the edge of the metal flat side 12. Thereby, the gasket 20 is oriented parallel to the metal flat side 12 and is fixed in its position by suitable fixing elements, such as screws or clips (not shown), on the edge of the gasket 20. The gasket 20 generally does not protrude beyond the edge of the metal block 10.

[0033] On the side of the gasket 20 facing away from the metal block 10, a plurality of concentric contours are provided, which are configured as circular rings in this example. By means of laser ablation (laser ablation), different gasket thicknesses M, R, G are established along these contours. Here, the relationship M > R > L > G is important, where L refers to the protrusion height of the metal flat side 12. In Figure 1 the disproportionate cross-sectional sketch, only the cross-sections of the machined circular ring structures 22, 24, 26 can be seen. The terms "closed concentric contour" and "back structure, groove structure" are generally used as synonyms here and at other positions in the specification, which means that when the gasket 20 is machined accordingly by laser, a three-dimensional structure is formed by assigning the gasket thickness to the contours.

[0034] The initial gasket thickness of the flexible gasket 20 is preferably 2 to 3 millimeters. Thereafter, the gasket thickness can be reduced integrally, i.e., over the entire surface, to a predetermined gasket thickness M, but it is clearly appropriate to make the gasket thickness M equal to the initial gasket thickness. The closed first contour 22 with the gasket thickness M is set for airtight contact with the wafer 40. The second contour 24 with a smaller diameter and thus more inwardly located and having a gasket thickness R is for supporting the edge of the thin metal foil 30, which would otherwise be placed on the metal flat side 12 and extend beyond the edge of the metal flat side 12, but here does not reach the first contour 22. The first contour 22 preferably has a contour width of approximately 1 millimeter, and the second contour 24 can also be designed to be wider than 1 millimeter in order to establish the largest possible contact surface between the metal foil 30 and the edge of the wafer 40. For the sake of simplicity of the sketch, in Figure 1 the "overhang" common within the supporting contour 24 of the metal foil 30, which causes the metal film 30 to adhere to the metal flat side 12, is not shown.

[0035] Furthermore, the gasket thickness M is preferably 200 to 300 micrometers greater than the protrusion height L of the metal flat side 12. In other words, in the unloaded state, the first contour 22 protrudes 200 to 300 micrometers beyond the fixedly positioned metal flat side 12. The metal foil 30 should be slightly higher at its edge on the second contour 24 than on the metal flat side 12; the gasket thickness R of the second contour is preferably set to be 50 to 150 micrometers greater than the protrusion height L of the metal flat side 12.

[0036] The metal foil 30 is preferably made of an electrically good conductor material, preferably one of the metal elements gold or aluminum. The thickness of the metal foil 30 is preferably between 30 and 100 micrometers. The purpose of the metal foil 30 is to energize the edge region of the wafer 40 between the edge of the metal flat side 12 and the first contour 22. This edge region constitutes the main part of the area of the wafer 40; but all the current from this region can be transmitted through the metal foil 30 in a shorter path to the metal body 10, so that the ohmic losses are small and the potential difference is negligible.

[0037] According to the present invention, the first and second profiles 22, 24 each have two directly adjacent profiles 26 with a pad thickness of G, namely the so-called trench structure. Here, a single trench structure 26 provided between the first and second profiles 22, 24 is directly adjacent to these two profiles 22, 24. The pad thickness G is preferably half of the pad thickness M, and in particular G is less than the protrusion height L of the metal flat side 12. As already explained, the profile 26 with the pad thickness G should prevent the propagation and diffusion of the force action parallel to the pad plane, especially when the force action is caused by the deformation of the back structures 22, 24 in the case of force loading caused by establishing a negative pressure. The profile width of the profile 26 can be 1 mm, but can also be pre-determined to be significantly larger. Here, the user has the freedom of choice, and the user can use this freedom of choice to optimize the flexibility characteristics of its pad 20. Depending on the user's choice of the initial pad thickness and material (inert polymer), the user can define the profile and the pad thickness by himself based on this specification, and can find a solution with the best results through a series of simple preliminary tests.

[0038] Using the wafer holder according to the present invention, when establishing a negative pressure, the brittle wafer 40 is gently and minimally loaded to be lowered onto the metal flat side 12 with the metal foil 30. As a result, in the experiments carried out by the inventor using, for example, 300-micron-thick InP wafers for electrochemical etching, no wafers were broken, while before using the present invention, all wafers were broken.

Claims

1. A wafer holder for large-area electrical contacting of a semiconductor wafer (40), the wafer holder include: A metal body (10) having a metal flat side (12) protruding on the upper side with a protrusion height L, a gas connection piece (16) arranged on the lower side, and at least one gas channel (14) opening in the metal flat side (12) and leading to the gas connection piece (16), wherein the metal body (10) is configured to be loaded with a current of the order of kA; and a flexible pad (20) formed of an inert polymer and placed on the metal body (10), the pad (20) having a notch for guiding the metal flat side (12) through and the pad being fixedly arranged around the edge of the metal flat side (12) parallel to the metal flat side (12), the pad (20) having one pad thickness of at least three pad thicknesses M, R, G along a plurality of closed, mathematically similar concentric contours (22, 24, 26) on the side facing away from the metal body (10), and the pad thicknesses being predetermined by the characteristic M>R>L>G so that a first contour (22) having a pad thickness M is configured for contacting an edge region of a wafer (40) and an edge of a metal foil (30) placed on the metal flat side (12) is arranged on a second contour (24) having a smaller pad thickness R, and the first and second contours (22, 24) each have two directly adjacent contours (26) having a pad thickness G.

2. The wafer holder according to claim 1, It is characterized in that The inert polymer is a fluoropolymer or a silicone.

3. The wafer holder according to claim 1 or 2, It is characterized in that The pad thickness M is several millimeters, preferably 2 to 3 millimeters.

4. The wafer holder according to claim 3, It is characterized in that The pad thickness M is between 200 and 300 microns greater than the protrusion height L of the metal flat side (12).

5. Wafer holder according to any one of the preceding claims, It is characterized in that The pad thickness G is equal to half of the pad thickness M.

6. Wafer holder according to any one of the preceding claims, It is characterized in that The first and second contours (22, 24) each have a width of at least 1 millimeter.

7. Wafer holder according to any one of the preceding claims, It is characterized in that The metal foil (30) is formed of gold or aluminum.

8. The wafer holder according to claim 7, It is characterized in that The thickness of the metal foil is 30 to 100 micrometers.

9. The wafer holder according to claim 8, It is characterized in that The pad thickness R is between 50 and 150 microns greater than the protrusion height L of the metal flat side. 10 . Use of the wafer holder as claimed in claim 1 for large-area electrical contacting of fragile semiconductor wafers ( 40 ), in particular indium phosphide wafers.

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