BONDING OF DIAMOND WAFERS TO CARRIER SUBSTRATES

Abstract

A method of bonding a diamond wafer to a carrier substrate. The diamond wafer is placed on the carrier substrate, the diamond wafer having a diameter of at least 50 mm. A voltage is applied to the carrier substrate which induces an electrostatic force which bonds the diamond wafer to the carrier substrate. The voltage applied to the carrier substrate is removed, leaving the diamond wafer bonded to the carrier substrate via residual electrostatic force. A mounted diamond wafer comprises a diamond wafer having a diameter of at least 50 mm and a carrier substrate, wherein the diamond wafer is bonded to the carrier substrate via a residual electrostatic force.

Claims

1. A method of bonding a diamond wafer to a carrier substrate, the method comprising: placing a diamond wafer on a carrier substrate, the diamond wafer having a diameter of at least 50 mm; applying a voltage to the carrier substrate which induces an electrostatic force which bonds the diamond wafer to the carrier substrate; and removing the voltage applied to the carrier substrate leaving the diamond wafer bonded to the carrier substrate via residual electrostatic force.

2. A method according to claim 1, wherein the diamond wafer is selected from the group consisting of: a plain free-standing diamond wafer; a coated diamond wafer; and a semiconductor-on-diamond wafer.

3. A method according to claim 1, wherein the diamond wafer is formed of a diamond material selected from the group consisting of: polycrystalline CVD diamond material; polycrystalline HPHT diamond material; single crystal CVD diamond material; single crystal HPHT diamond material; and natural single crystal diamond material.

4. A method according to claim 1, wherein an electrically conductive layer is provided on a side of the diamond wafer which is bonded to the carrier substrate.

5. A method according to claim 4, wherein the electrically conductive layer is selected from the group consisting of: a metal layer; a graphite layer; or a hydrogen terminated diamond surface.

6. A method according to claim 1, wherein the diamond wafer is polished on a side of the diamond wafer which is bonded to the carrier substrate prior to electrostatic bonding to have a surface roughness of no more than 0.5 m, 0.4 m, 0.3 m, 0.2 m, 0.1 m, or 0.05 m.

7. A method according to claim 1, wherein the diamond wafer has a thickness in a range 50 m to 200 m.

8. A method according to claim 1, wherein the diamond wafer has a diameter of at least 75 mm, 100 mm, or 150 mm.

9. A method according to claim 1, wherein the diamond wafer has a thickness variation of no more than 40 m.

10. A method according to claim 1, wherein the diamond wafer is bowed prior to electrostatic bonding and the electrostatic bonding pulls the diamond wafer flat, the bowing of the diamond wafer prior to electrostatic bonding being in a range 50 m to 300 m.

11. A mounted diamond wafer comprising: a diamond wafer having a diameter of at least 50 mm; a carrier substrate; wherein the diamond wafer is bonded to the carrier substrate via a residual electrostatic force.

12. A mounted diamond wafer according to claim 11, wherein the mounted diamond wafer has the following characteristics: a total thickness variation of no more than 40 m; a wafer bow of no more than 100 m; and a wafer warp of no more than 40 m.

13. A mounted diamond wafer according to claim 12, wherein the mounted diamond wafer meets the requirements for total thickness variation, wafer bow, and wafer warp over a diameter of at least 75 mm, 100 mm, or 150 mm.

14. A mounted diamond wafer according to claim 11, wherein the diamond wafer is selected from the group consisting of: a plain free-standing diamond wafer; a coated diamond wafer; and a semiconductor-on-diamond wafer.

15. A mounted diamond wafer according to claim 11, wherein the diamond wafer is formed of a diamond material selected from the group consisting of: polycrystalline CVD diamond material; polycrystalline HPHT diamond material; single crystal CVD diamond material; single crystal HPHT diamond material; and natural single crystal diamond material.

16. A mounted diamond wafer according to claim 11, wherein an electrically conductive layer is provided on a side of the diamond wafer which is bonded to the carrier substrate.

17. A mounted diamond wafer according to claim 16, wherein the electrically conductive layer is selected from the group consisting of: a metal layer; a graphite layer; or a hydrogen terminated diamond surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0031] FIG. 1 shows a schematic diagram of the steps involved in bonding a plain free-standing diamond wafer to a carrier substrate;

[0032] FIG. 2 shows a schematic diagram of the steps involved in bonding a diamond wafer to a carrier substrate where the diamond wafer comprises an electrically conductive layer provided on a side of the diamond wafer which is bonded to the carrier wafer; and

[0033] FIG. 3 shows a schematic diagram of the basic steps involved in bonding a semiconductor-on-diamond wafer to a carrier substrate.

DETAILED DESCRIPTION

[0034] As described in the summary of invention section, the present invention is based on the surprising finding that it is possible to bond a diamond wafer to a carrier substrate using electrostatic bonding and that the electrostatic bonding is sufficiently strong to allow processing of the diamond wafer after bonding to the carrier substrate.

[0035] The diamond wafer may be a plain free-standing diamond wafer, a coated diamond wafer (e.g. a metal coated diamond wafer or a diamond wafer with an optical coating such as antireflective coating), or a composite wafer such as a semiconductor-on-diamond wafer (e.g. GaN-on-diamond). In certain embodiments, the diamond material is in the form of polycrystalline diamond material deposited via chemical vapour deposition (i.e. polycrystalline CVD diamond wafers). However, the present invention may also be applied to other forms of diamond material including sintered, high pressure, high temperature (HPHT) synthetic polycrystalline diamond material (PCD) or single crystal diamond materials including CVD synthetic, HPHT synthetic and natural single crystal diamond materials.

[0036] The diamond wafer may be bowed prior to electrostatic bonding and the electrostatic bonding pulls the diamond wafer flat.

[0037] The carrier substrate is typically a thin (e.g. 100 m to 2 mm thickness) stand-alone substrate with columbic, Johansen-Rahbek, or any other typical electrostatic bonding design. In one example the bulk of the carrier substrate consisting of a silicon wafer which may be patterned, metalized, and coated with a dielectric according to the specific design of the supplier. Additionally, the stand-alone electrostatic carrier substrate can be designed as a perforated carrier or a different variant to facilitate handling, attachment, mounting, dismounting, etc. Suitable carrier substrates can be obtained from Beam Services, Inc.

[0038] FIG. 1 illustrates the basic method steps. A carrier substrate 2 is first placed on an electrostatic chuck 4. A diamond wafer 6 is then placed on the carrier substrate 2. A voltage is applied to the electrostatic chuck 4 which induces an electrostatic force EF which pulls the diamond wafer 6 flat and bonds the diamond wafer 6 to the carrier substrate 2. This step may be aided by use of a vacuum arrangement to pull the diamond wafer 6 flat prior to, and/or during, the application of the voltage. Finally, the diamond wafer 6 and carrier substrate 2 are removed from the electrostatic chuck 4 with the diamond wafer 6 bonded and held flat to the carrier substrate 2 via residual electrostatic force.

[0039] FIG. 2 shows a similar method to that shown in FIG. 1 but in this case an electrically conductive layer 8 (e.g. a layer of conductive material such as a layer of metal or graphite, or a hydrogen terminated diamond surface) is provided on a side of the diamond wafer 6 which is bonded to the carrier substrate 2. As before, a carrier substrate 2 is first placed on an electrostatic chuck 4. The diamond wafer 6 is then placed on the carrier substrate 2 with the electrically conductive layer 8 proximal to the carrier substrate 2. A voltage is applied to the electrostatic chuck 4 which induces an electrostatic force EF which pulls the diamond wafer 6 flat and bonds the diamond wafer 6 to the carrier substrate 2 via the electrically conductive layer 8. Finally, the diamond wafer 6 and carrier substrate 2 are removed from the electrostatic chuck 4 with the diamond wafer 6 bonded and held flat to the carrier substrate 2 via residual electrostatic force. In this configuration, the electrically conductive layer 8 aids electrostatic bonding of the diamond wafer 6 to the carrier substrate 2.

[0040] FIG. 3 shows a similar method to that shown in FIGS. 1 and 2 but in this case the diamond wafer 6 is a semiconductor-on-diamond wafer comprising a layer of diamond 10 bonded to a layered semiconductor structure 12, e.g. a GaN epilayer structure. As before, a carrier substrate 2 is first placed on an electrostatic chuck 4. The diamond wafer 6 is then placed on the carrier substrate 2 with the diamond layer 10 proximal to the carrier substrate 2 and the semiconductor layer 12 distal to the carrier substrate 2. A voltage is applied to the electrostatic chuck 4 which induces an electrostatic force which pulls the diamond wafer 6 flat and bonds the diamond wafer 6 to the carrier substrate 2 via the diamond layer 10. Finally, the diamond wafer 6 and carrier substrate 2 are removed from the electrostatic chuck 4 with the diamond wafer 6 bonded and held flat to the carrier substrate 2 via residual electrostatic force. In this configuration, the semiconductor layer structure 12 is exposed for device fabrication. Optionally, an electrically conductive layer can also be provided on the diamond prior to electrostatic bonding to aid electrostatic bonding of the semiconductor-on-diamond wafer 6 to the carrier substrate 2 as described previously with reference to FIG. 2.

[0041] While FIGS. 1 to 3 illustrate the application of a voltage to the carrier substrate by placing the carrier substrate on an electrostatic chuck, the voltage can be applied to the carrier substrate via other means such as pins or other electrical connections to the carrier substrate. In this case, the carrier substrate itself can function as a free-standing electrostatic chuck. In all of the aforementioned embodiments, electrostatic bonding is improved by careful preparation of the rear side of the diamond wafer which is to be bonded to the carrier substrate. In this regard, the diamond wafer can be polished on a side of the diamond wafer which is bonded to the carrier substrate prior to electrostatic bonding to have a surface roughness (R.sub.a) of no more than 7 m, 5 m, 3 m, 1 m, 0.5 m, 0.4 m, 0.3 m, 0.2 m, 0.1 m, or 0.05 m. Finer surface finishes can achieve even lower surface roughnesses of no more than 50 nm, 30 nm, 20 nm, 10 nm, or 5 nm. For many applications, it is important that the diamond wafer is processed to a precise thickness with little thickness variation (e.g. less than 25 m variation, but preferably less than 2 m/2 cm linear and radial length of travel across the wafer). This is particularly important when a high degree of flatness is required after electrostatic bonding of the diamond wafer to the carrier substrate. For example, when mounting semiconductor-on-diamond wafers on a carrier substrate for subsequent semiconductor device fabrication the mounted wafer must meet strict flatness requirements. As such, the diamond wafer may have a thickness in a range 50 m to 500 m, preferably 50 m to 200 m. The diamond wafer may also have a thickness variation of no more than 40 m. Since the diamond wafer may have a diameter of at least 50 mm, 75 mm, 100 mm, or 150 mm, then the wafer should be processed to meet such requirements over relatively large areas.

[0042] One complication with the electrostatic bonding process and requirements such as processing of a rear surface of the diamond wafer to meet flatness, roughness, thickness, and thickness variation requirements is that as-grown diamond wafers such as large area polycrystalline CVD diamond wafers, are typically bowed. As such, where the diamond wafer is bowed prior to electrostatic bonding then the electrostatic bonding requires the diamond wafer to be pulled flat to the carrier substrate. If the bow of the initial wafer is too large then this may be difficult to achieve, especially given the rigid nature of the diamond material and especially if the diamond wafer is relatively thick. Accordingly, the state of the initial diamond wafer is important to ensure good electrostatic bonding. For example, the bowing of the diamond wafer prior to electrostatic bonding may in a range 50 m to 300 m. Thin diamond wafers may have a significant bow towards the upper end of this range while thicker diamond wafers may require a lower initial bow towards the lower end of this range to achieve good electrostatic bonding. If the diamond wafer is too thick and bowed then electrostatic bonding may not be possible. Ultimately, the flattenability of the wafer is the determining factor. Flattenability is a function of diamond thickness, free-standing bow/warp and grain size. Accordingly, diamond growth conditions play an important role in generating material that is suitable for mounting on a carrier substrate via electrostatic bonding. According to certain examples, a suitable thickness of diamond material is of the order of 50 m to 150 m, with a free-standing bow/warp of <1 mm.

[0043] In order to dealing with the bowing issue, the electrostatic chuck and/or carrier substrate may also incorporate a vacuum system for pulling the diamond wafer flat. In this regard, one or more holes may be provided in the carrier substrate such that when the diamond wafer is placed on the carrier substrate, a vacuum system can be utilized to pull the diamond wafer flat against the carrier substrate prior to electrostatic bonding.

[0044] In addition to the effect of bowing in relation to the requirement to pull the diamond wafer flat as part of the electrostatic bonding process, the bowing also makes surface processing of the rear side of the diamond wafer prior to electrostatic bonding more problematic. The diamond wafer cannot necessarily be surface processed on a rear surface to have a flat configuration prior to bonding as the bow may be too large to process out and/or the requirement to have a uniform thickness may prevent an approach in which the bowed rear surface is surface processed until it is flat. As such, processing of the rear surface to achieve the desired levels of surface roughness and thickness variation must account for the bowing of the diamond wafer. For example, a bowed polishing wheel which is complimentary to the bowed rear surface of the diamond wafer may be utilized or otherwise the bowed diamond wafer may be pushed into a plat configuration for the surface processing. Ideally, in addition to achieving desired values for surface roughness and thickness uniformity, the prepared surface should have a large fraction of the surface area which is flat once electrostatic bonding is applied. For example, one approach for a GaN-on-diamond wafer is to mount the free-standing GaN-on-diamond wafer onto an optical flat via the GaN side of the wafer and directly polish the rough side of diamond. It is possible to successfully mount such a processed GaN-on-diamond wafer to a carrier substrate via electrostatic bonding with as little as 15% total area of diamond polished in this manner. However, there are two important factors governing the success or failure. One is the total thickness variation of the GaN-on-diamond wafer and the other is the average diamond thickness. The thicker the diamond wafer the harder it is to flatten the wafer and electrostatically bond it.

[0045] A second approach is to perform pre-silicon handle etch polishing of a diamond-on-GaN-on-silicon wafer using a bowed polishing wheel.

[0046] The applied voltage to be applied to achieve electrostatic bonding will depend on a number of factors including the nature of the carrier substrate, the stiffness, the thickness, bow, diameter, and surface finish of the diamond wafer, the strength of the electrostatic bond required for an application, and the requirement to de-bond the diamond wafer from the carrier substrate in certain applications after the desired usage has been completed. Typically, a voltage in a range 500 V to 8000 V may be applied to achieve electrostatic bonding of a diamond wafer to a carrier substrate depending on the aforementioned variables. For certain applications the applied voltage will be at least 1000, 2000, 3000, 4000, 5000, or 6000 V.

[0047] Using the methodology as described herein, it is possible to fabricate a mounted diamond wafer comprising: a diamond wafer; and a carrier substrate, wherein the diamond wafer is bonded to the carrier substrate via a residual electrostatic force. Advantageously, for certain applications, such as semiconductor-on-diamond applications, the mounted diamond wafer has the following characteristics: a total thickness variation of no more than 40 m; a wafer bow of no more than 100 m; and a wafer warp of no more than 40 m. Furthermore, for many applications the mounted diamond wafer meets the requirements for total thickness variation, wafer bow, and wafer warp over a diameter of at least 50 mm, 75 mm, 100 mm, or 150 mm.

[0048] In relation to the above, it may be noted that an XYZ automated optical comparator can be used to establish the Z-direction height of 300-500 points on a given diamond wafer for various X and Y positions. Consequently, it is possible to build a surface contour map of each diamond wafer before and after mounting and for various electrostatic mounting methodologies.

[0049] According to certain examples, the diamond wafer has a thickness of no more than 130 microns and at least 30% of the rear surface of the diamond wafer is polished for electrostatic bonding. A voltage of 6000 V in then applied to electrostatically bond the diamond wafer to a coated silicon carrier substrate and achieve a mounted diamond wafer which is sufficiently flat for lithography applications.

[0050] While this invention has been particularly shown and described with reference to embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appending claims.