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Definitions

• the present invention relates to semiconductor device fabrication. More particularly, the present invention relates to a method of fabricating vertical devices using a metal support film.
• LEDs Light emitting diodes
• the color of the light (wavelength) that is emitted by an LED depends on the semiconductor material that is used to fabricate the LED. This is because the wavelength of the emitted light depends on the semiconductor material's band-gap energy, which represents the energy difference between valence band and conduction band electrons.
• GaN Gallium-Nitride
• InGaN/GaN semiconductor layers that emit green, blue, and white visible light.
• This wavelength control ability enables an LED semiconductor designer to tailor material characteristics to achieve beneficial device characteristics.
• GaN enables an LED semiconductor designer to produce blue LEDs and blue laser diodes, which are beneficial in full color displays and in optical recordings, and white LEDs, which can replace incandescent lamps.
• GaN-based opto-electronic device technology has rapidly evolved since their commercial introduction in 1994. Because the efficiency of GaN light emitting diodes has surpassed that of incandescent lighting, and is now comparable with that of fluorescent lighting, the market for GaN based LEDs is expected to continue its rapid growth.
• GaN devices are too expensive for many applications.
• One reason for this is the high cost of manufacturing GaN-based devices, which in turn is related to the difficulties of growing GaN epitaxial layers and of subsequently dicing out completed GaN-based devices.
• GaN-based devices are typically fabricated on sapphire substrates. This is because sapphire wafers are commercially available in dimensions that are suitable for mass-producing GaN-based devices, because sapphire supports high- quality GaN epitaxial layer growths, and because of the extensive temperature handling capability of sapphire. Typically, GaN-based devices are fabricated on 2" diameter sapphire wafers that are either 330 or 430 microns thick. Such a diameter enables the fabrication of thousands of individual devices, while the thickness is sufficient to support device fabrication without excessive wafer warping.
• the sapphire crystal is chemically and thermally stable, has a high melting temperature that enables high temperature fabrication processes, has a high bonding energy (122.4 Kcal mole), and a high dielectric constant.
• sapphires are crystalline aluminum oxide, Al 2 O 3 .
• MOCVD metal oxide chemical vapor deposition
• MBE molecular beam epitaxy
• sapphire substrates are difficult to dice. Indeed, dicing typically requires that the sapphire substrate be thinned to about 100 microns by mechanical grinding, lapping, and/or polishing. It should be noted that such mechanical steps are time consuming and expensive, and that such steps reduce device yields. Even after thinning sapphires remain difficult to dice. Thus, after thinning and polishing, the sapphire substrate is usually attached to a supporting tape. Then, a diamond saw or stylus forms scribe lines between the individual devices. Such scribing typically requires at least half an hour to process one substrate, adding even more to the manufacturing costs.
• the scribe lines have to be relatively wide to enable subsequent dicing, the device yields are reduced, adding even more to manufacturing costs.
• the sapphire substrates are rolled using a rubber roller to produce stress cracks that propagate from the scribe lines and that subsequently dice out the individual semiconductor devices. This mechanical handling reduces yields even more.
• sapphire substrates or other insulating substrate have other drawbacks.
• sapphire is an insulator
• the device topologies that are available when using sapphire substrates (or other insulating substrates) are limited.
• lateral and vertical In practice there are only two device topologies: lateral and vertical. In the lateral topology the metallic electrical contacts that are used to inject current are both located on upper surfaces. In the vertical topology the substrate is removed, one metallic contact is on the upper surface and the other contact is on the lower surface.
• Figures 1 A and IB illustrate a typical lateral GaN-based LED 20 that is fabricated on a sapphire substrate 22.
• an n-GaN buffer layer 24 is formed on the substrate 22.
• a relatively thick n-GaN layer 26 is formed on the buffer layer 24.
• An active layer 28 having multiple quantum wells of aluminum-indium-gallium-nitride (AlInGaN) or of InGaN/GaN is then formed on the n-type GaN layer 26.
• a p-GaN layer 30 is then formed on the active layer 26.
• a transparent conductive layer 32 is then formed on the p-GaN layer 30.
• the transparent conductive layer 32 may be made of any suitable material, such as Ru/Au, Ni/Au or indium-tin-oxide (ITO).
• a p-type electrode 34 is then formed on one side of the transparent conductive layer 32. Suitable p-type electrode materials include Ni/Au, Pd/Au, Pd/Ni and Pt.
• Apad 36 is then formed on the p-type electrode 34. Beneficially, the pad 36 is Au.
• the transparent conductive layer 32, the p-GaN layer 30, the active layer 28 and part of the n-GaN layer 26 are etched to form a step. Because of the difficulty of wet etching GaN, a dry etch is usually used to form the step. This etching requires additional lithography and stripping processes. Furthermore, plasma damage to the GaN step surface is often sustained during the dry-etch process.
• the LED 20 is completed by forming an n-electrode pad 38 (usually Au) and pad 40 on the step.
• Figure IB illustrates a top down view of the LED 20.
• lateral GaN-based LEDs have a significant draw back in that having both metal contacts (36 and 40) on the same side of the LED significantly reduces the surface area available for light emission.
• the metal contacts 36 and 40 are physically close together.
• the pads 36 are often Au.
• Au spreading can bring the electrical contacts even closer together.
• Such closely spaced electrodes 34 are highly susceptible to ESD problems.
• FIGs 2A and 2B illustrate a vertical GaN-based LED 50 that was formed on a sapphire substrate that was later removed.
• the LED 50 includes a GaN buffer layer 54 having an n-metal contact 56 on a bottom side and a relatively thick n-GaN layer 58 on the other.
• the n-metal contact 56 is beneficially formed from a high reflectively layer that is overlaid by a high conductivity metal (beneficially Au).
• An active layer 60 having multiple quantum wells is formed on the n-type GaN layer 58, and a p-GaN layer 62 is formed on the active layer 60.
• a transparent conductive layer 64 is then formed on the p-GaN layer 62, and a p-type electrode 66 is formed on the transparent conductive layer 64.
• a pad 68 is formed on the p-type electrode 66.
• the materials for the various layers are similar to those used in the lateral LED 20.
• the vertical GaN-based LED 50 as the advantage that etching a step is not required.
• the sapphire substrate (not shown) has to be removed. Such removal can be difficult, particularly if device yields are of concern.
• vertical GaN-based LEDs have the advantage that only one metal contact (68) blocks light emission.
• lateral GaN-based LEDs must have larger surface areas, which causes lower device yields.
• the reflecting layer of the n-type contact 56 used in vertical GaN-based LEDs reflect light that is otherwise absorbed in lateral GaN-based LEDs.
• a lateral GaN-based LED must have a significantly larger surface area. Because of these issues, a 2" diameter sapphire wafer can produce about 35,000 vertical GaN-based LEDs, but only about 12,000 lateral GaN-based LEDs.
• the lateral topology is more vulnerable to static electricity, primarily because the two electrodes (36 and 40) are so close together. Additionally, as the lateral topology is fabricated on an insulating substrate, and as the vertical topology can be attached to a heat sink, the lateral topology has relatively poor thermal dissipation. Thus, in many respects the vertical topology is operationally superior to the lateral topology.
• GaN-based LEDs fabricated on insulating substrates have a lateral topology. This is primarily because of the difficulties of removing the insulating substrate and of handling the GaN wafer structure without a supporting substrate. Despite these problems, removal of an insulating (growth) substrate and subsequent wafer bonding of the resulting GaN-based wafer on a Si substrate using Pd/In metal layers has been demonstrated for very small area wafers, approx. 1 cm by 1 cm. (reported by the University of California at Berkley and the Xerox Corporation). But, substrate removal and subsequent wafer bonding of large area wafers remains very difficult due to inhomogeneous bonding between the GaN wafer and the 2 nd (substitutional) substrate. This is mainly due to wafer bowing during and after laser lift off.
• a method of removing semiconductor layers from a sapphire substrate, of isolating a wafer having the partially fabricated semiconductor devices such that wafer warping is reduced or prevented, followed by substitution of a metal supporting layer would be useful. More specifically, a method of partially fabricating GaN-based devices on a sapphire (or other insulating) substrate, followed by substitution of a conducting supporting layer, followed by dicing the substituting layer to yield vertical topology GaN-based LEDs would be beneficial.
• the principles of the present invention provide for a method of fabricating semiconductor devices on insulating substrates by first forming semiconductor layers on the insulating substrate, followed by removal of the insulating substrate to isolate a wafer having the formed semiconductor layers, followed by the addition of a metal support substrate (either on top or bottom of semiconductor layers) that will support the wafer, all while supporting the wafer to prevent warping and/or other damage.
• the principles of the present invention further provide for a method of fabricating GaN-based vertical devices on insulating substrates using metal support films.
• semiconductor layers for the GaN-based devices are formed on an insulating (sapphire) substrate using normal semiconductor fabrication techniques.
• trenches are formed through the semiconductor layers and into the insulating substrate.
• the trenches are fabricated using inductive couple (inductively coupled) plasma reactive ion etching (ICPRIE).
• ICPRIE inductive couple plasma reactive ion etching
• a first support structure is attached to the semiconductor layers.
• the first support structure is comprised of silicon, but almost any hard flat surface is acceptable.
• That first support structure is beneficially attached to the semiconductive layers using an epoxy adhesive, possibly with a protective photo-resist layer over the semiconductive layer. Then, the insulating substrate is removed, beneficially using a laser-lift off process. A second supporting structure is then substituted for the insulating substrate. Beneficially, the second supporting structure is comprised of a metal film of Cu, Au or Al, but almost any conductive film is acceptable. If required, a conductive contact can be inserted between the semiconductive layer and the second supporting structure. In the case of LEDs, the conductive contact is beneficially reflective to bounce photons upward to prevent absorption in the bottom lead frame. The first supporting structure is then removed. Individual devices are then diced out, beneficially either by mechanical dicing or wet/dry etching through the second supporting structure.
• the following describes another way of forming metal support films on the semiconductor layers.
• Trench formation through the semiconductor layers and into the insulating substrate is identical to the procedure described above.
• a thick metal support film is deposited on top of the GaN-based devices using chemical and/or physical deposition techniques (such as electroplating or electro- less plating).
• the insulating substrate is removed, beneficially using a laser-lift off process.
• the thick metal support film is comprised of Cu, Au or Al, but almost any conductive film is acceptable. If required, a conductive contact can be inserted between the semiconductive layer and the second supporting structure.
• the conductive contact is beneficially reflective to bounce photons to prevent absorption in the bottom lead frame. Electrical contacts can then be formed on the exposed surface of the semiconductor layers. Individual devices can then diced out, beneficially either by mechanical dicing or wet/dry etching through the thick metal support film.
• the principles of the present invention specifically provide for a method of fabricating vertical topology GaN-based LEDs on sapphire substrates. According to that method, semiconductor layers for the vertical topology GaN-based LEDs are formed on a sapphire substrate using normal semiconductor fabrication techniques. Then, trenches are formed through the semiconductor layers and into the sapphire substrate. Those trenches define the boundaries of the individual vertical topology GaN-based LEDs.
• the trenches are fabricated using ICPRIE. Then, a protective photo-resist layer is located over the semiconductor layers.
• a first support structure is then attached to the semiconductor layers. Beneficially, the first support structure is a silicon plate, but almost any hard flat material is acceptable.
• the first support structure is beneficially attached to the semiconductive layers (or photoresist layer) using an epoxy adhesive.
• the sapphire substrate is removed, beneficially using a laser lift off process.
• a conductive bottom contact is then located on the exposed semiconductor layer. That conductive bottom contact beneficially includes a reflective layer.
• One or more adhesion support layers, such as a Cr and/or and Au layer, is formed over the reflective layer. Then, a second supporting structure is substituted in place of the sapphire substrate.
• the second supporting structure is comprised of a conductive film of Cu, Au or Al, but almost any conductive film is acceptable.
• the first supporting structure is then removed.
• the individual device dies are diced out, beneficially either by mechanical dicing or by wet/dry etching through the second supporting structure. Mechanical rolling or shear cutting can be used to separate the dies.
• the principles of the present invention also provide for another method of fabricating vertical topology GaN-based LEDs on sapphire substrates.
• semiconductor layers for the vertical topology GaN-based LEDs are formed on a sapphire substrate using normal semiconductor fabrication techniques.
• trenches are formed through the semiconductor layers and into the sapphire substrate. Those trenches define the boundaries of the individual vertical topology GaN-based LEDs.
• the trenches are fabricated using ICPRIE.
• a contact layer comprised, for example, of layers of Cr and Au is located over the semiconductor layers.
• a metal support structure is then formed over the contact layer/semiconductor layers.
• the sapphire substrate is removed, beneficially using a laser lift off process.
• Conductive bottom contacts are then located on the recently exposed semiconductor layer.
• the individual device dies are diced out, beneficially either by mechanical dicing or by wet/dry etching through the metal support structure.
• Figure 1 A illustrates a sectional view of a typical lateral topology GaN-based LED
• Figure IB shows a top down view of the GaN-based LED illustrated in Figure 1A;
• Figure 2A illustrates a sectional view of a typical vertical topology GaN-based LED
• Figure 2B shows a top down view of the GaN-based LED illustrated in Figure 2A.
• FIG. 3-25 illustrate steps of forming light emitting diodes that are in accord with the principles of the present invention.
• FIG. 3-25 illustrate methods of manufacturing vertical topology GaN-based light emitting diodes (LEDs) using sapphire substrates.
• LEDs light emitting diodes
• a vertical topology GaN-based LED layer structure 120 that is similar or identical to the semiconductor layers of the vertical GaN-based LED 50 illustrated in Figures 2A and 2B is formed on a 330-430 micron-thick, 2" diameter sapphire substrate 122.
• the vertical topology GaN-based LED layer structure 120 can have an InGaN/GaN active layer (60) having the proper composition to emit blue light.
• the vertical topology GaN-based LED layer structure 120 is beneficially less than 5 microns thick.
• Various standard epitaxial growth techniques, such as vapor phase epitaxy, MOCND, and MBE, together with suitable dopants and other materials, can be used to produce the vertical topology Ga ⁇ -based LED layer structure 120.
• trenches 124 are formed through the vertical topology Ga ⁇ -based LED layer structure 120 and into the sapphire substrate 122.
• the trenches define the individual LED semiconductor structures that will be produced and separated.
• Each individual LED semiconductor structure is beneficially a square about 200 microns wide.
• the trenches are beneficially narrower than about 10 microns wide and extend deeper than about 5 microns into the sapphire substrate 122.
• the trenches 124 are beneficially formed in the structure of Figure 3 using reactive ion etching, preferably inductively coupled plasma reactive ion etching (ICP RIE).
• ICP RIE inductively coupled plasma reactive ion etching
• Forming trenches using ICP RIE has two main steps: forming scribe lines and etching. Scribe lines are formed on the structure of Figure 3 using a photo-resist pattern in which areas of the sapphire substrate 122 where the trenches 124 are to be formed are exposed. The exposed areas are the scribe lines and all other areas are covered by photo-resist.
• the photo-resist pattern is beneficially fabricated from a relatively hard photo-resist material that withstands intense plasma.
• the photo-resist could be AZ 9260, while the developer used to develop the photo-resist to form the scribe lines could be AZ MIF 500.
• the photo-resist is beneficially spin coated to a thickness of about 10 microns.
• the photo-resist thickness should be about the same as the thickness of the vertical topology GaN-based LED layer structure 120 plus the etch depth into the sapphire substrate 122. This helps ensure that the photo-resist mask remains intact during etching. Because it is difficult to form a thick photo-resist coating in one step, the photo-resist is beneficially applied in two coats, each about 5 microns thick. The first photo-resist coat is spin coated on and then soft baked at approximately 90 °F for about 15 minutes.
• the second photo-resist coat is applied in a similar manner, but is soft baked at approximately 110 °F for about 8 minutes.
• the photo-resist coating is then patterned to form the scribe lines. This is beneficially performed using lithographic techniques and development. Development takes a relatively long time because of the thickness of the photo-resist coating.
• the photo-resist pattern is hard baked at about 80 °F for about 30 minutes.
• the hard baked photo-resist is beneficially dipped in a MCB (Metal Chlorobenzene) treatment for about 3.5 minutes. Such dipping further hardens the photo-resist.
• MCB Metal Chlorobenzene
• the structure of Figure 3 is etched.
• the ICP RIE etch process is performed by placing the structure of Figure 3 on a bottom electrode 350 in a RIE chamber 352 having an insulating window 354 (beneficially a 1 cm-thick quartz window).
• the bottom electrode 350 is connected to a bias voltage supply 356 that biases the structure of Figure 3 to enable etching.
• the bias voltage supply 356 beneficially supplies 13.56 MHz RF power and a DC-bias voltage.
• the distance from the insulating window 354 to the bottom electrode 350 is beneficially about 6.5cm.
• a gas mixture of Cl 2 and BC1 3 , and possibly Ar, is injected into the RIE chamber 352 through a reactive gas port 360.
• a 2.5-turn or so spiral Cu coil 364 is located above the insulating window 354.
• Radio frequency (RF) power at 13.56 MHz is applied to the coil 364 from an RF source 366. It should be noted that magnetic fields are produced at right angles to the insulating window 354 by the RF power.
• thin transparent contacts 190 are formed on the individual LED semiconductor structures of the vertical topology GaN-based LED layer structure 120.
• Those transparent contacts 190 are beneficially comprised of Ru/Au, Ni/Au, or of indium tin oxide (ITO)/Au and are less than lOnm.
• metal contact pads 192 are placed on each transparent contact 190.
• the metal contact pads 192 are beneficially comprised of Pd, Pt, Au, or Al.
• Each metal contact pad 192 has a diameter of about 100 microns and a thickness of about 1 micron.
• a thin Cr/Au inter layer can be used to improve adhesion between transparent contacts 190 and the metal contact pad 192.
• a protective photo-resist film 196 is formed over the structure of Figure 7. That photo-resist film is to protect the GaN- based LED layer structure 120 and to assist subsequent bonding.
• An epoxy adhesive 198 is then used to attach a first supporting structure that takes the form of a temporary supporting wafer 200.
• the temporary supporting wafer 200 is beneficially a silicon plate that is larger than the sapphire wafer. However, almost any hard, flat surface with a sufficient thickness to support a wafer containing the individual LED semiconductor devices during substrate swapping (described subsequently) is acceptable.
• the first substrate swapping processes is surface polishing and sand blasting (or surface roughening with a dry etching processes) the backside (the bottom side in Figure 8) of the sapphire substrate 122. This step helps to ensure uniform laser beam heating during a laser lift off step that is subsequently performed.
• FIG. 9 the structure shown in Figure 8 is then attached to two vacuum chucks.
• a first vacuum chuck 210 attaches to the supporting wafer 200 and the second vacuum chuck 212 attaches to the sapphire substrate 122.
• a laser beam 214 is directed through the sapphire substrate 122.
• the laser beam 214 is beneficially from a 248 nm KrF laser having a 3 mm x 50 mm rectangular beam and beam energy between 200 ⁇ 600 mJ/cm .
• the vacuum chucks 210 and 212 which are made of materials transparent to the 248 nm KrF laser beam, beneficially sapphire, bias the sapphire substrate 122 away from the supporting wafer 200.
• the combination of laser irradiation and bias causes the sapphire substrate 122 to separate as shown in Figure 10.
• the bottom of the resulting structure (the side opposite the temporary supporting wafer 200) is first cleaned with HCl to remove Ga droplets (the laser beam 214 causes heating which separates the GaN into Ga+N).
• ICP RIE etching (see above) and polishing are performed. This etching and polishing exposes and produces an atomically flat surface of pure n-GaN. The flat surface is particularly beneficial in producing high reflectivity from a reflective structure that is deposited subsequently.
• the etched n-GaN surface is further cleaned and etched with aqua regia solution (mixture of H 2 SO and HCl) to enhance the adhesion between n-GaN and Ti/Al metal layers.
• a conductive reflective structure comprised of a titanium layer 230 and an aluminum layer 232 is then formed on the bottom of the structure of Figure 11. That reflective structure will reflect light from completed LEDs that is directed toward the bottom of the LEDs back out of the top of the LEDs.
• These bottom metal layers also serve as an n-type contact layer for the LED device.
• a second supporting structure in the form of a Cu, Au or Al thick film support 240 is formed on the Au adhesion layer 238.
• the thick film support 240 can be formed by physical vapor deposition by electroplating, by electro-less plating, or by other suitable means.
• This thick film support 240 is beneficially less than about 100 microns thick. While a Cu, Au or Al thick film support is beneficial, almost any electrically conductive, and beneficially thermally conductive, material is acceptable.
• the epoxy adhesive 198 and the temporary supporting wafer 200 are removed, reference Figure 15. Such removal is beneficially achieved by heating the structure of Figure 14 to weaken the epoxy adhesive such that the temporary supporting wafer 200 can be removed. After the temporary supporting wafer 200 is removed the resulting structure is immersed in acetone to remove any photo-resist and residual epoxy adhesive 198.
• a transparent metal layer 290 is formed on the vertical topology GaN-based LED layer structures 120. Then, an adhesion layer 338 comprised of Cr and Au layers is located on the transparent metal layer 290. Then, the thick metal support film 300, beneficially comprised of Cu, Au or Al, is formed on the adhesion layer 338.
• the thick metal support film 300 can be formed by physical vapor deposition, electro/ electro-less plating, or by other suitable means. This thick metal support film 300 is beneficially less than about 100 microns thick. While a Cu, Au or Al thick metal support film 300 is beneficial, almost any electrically conductive, and beneficially thermally conductive, material is acceptable.
• FIG 17 the structure shown in Figure 16 is then attached to two vacuum chucks.
• a first vacuum chuck 210 attaches to the thick metal support film 300 and the second vacuum chuck 212 attaches to the sapphire substrate 122.
• a laser beam 214 is directed through the sapphire substrate 122.
• the laser beam 214 is beneficially from a 248 nm KrF laser with 3 mm x 50 mm rectangular beam and beam energy in between 200 ⁇ 600 mJ/cm2.
• the vacuum chucks 210 and 212 which are made of materials transparent to the 248 nm KrF laser beam, beneficially sapphire, bias the sapphire substrate 122 away from the GaN-LED devices backed with thick metal support film 300.
• the combination of laser irradiation and bias causes the sapphire substrate 122 to separate as shown in Figure 18.
• the bottom of the resulting structure (the side opposite the thick metal film 240) is first cleaned with HCl to remove Ga droplets (the laser beam 214 causes heating which separates the GaN into Ga+N).
• ICP RIE etching (see above) and polishing are performed. This etching and polishing exposes and produces an atomically flat surface of pure n-GaN.
• the etched n-GaN surface is further cleaned and etched with aqua regia solution (mixture of H 2 SO and HCl) to enhance the adhesion between n-GaN and Ti/Al metal layers.
• electrical contacts are formed on the individual vertical topology GaN-based LED layer structures 120.
• Those electrical contacts beneficially include a Ti/Al interface layer 330 to the vertical topology GaN- based LED layer structures 120, and a Cr/Au contact pad 332 on the Ti/Al interface layer 330.
• dicing is beneficially accomplished by depositing a photo-resist pattern 250 on the thick film support 240. That photo-resist pattern 250 is then developed to expose areas of the thick film support 240 that align with the trenches 124. Openings 254 are then etched through the thick film support 240. The photo-resist pattern 250 is then removed.
• a mounting tape 260 can be placed on top of the structure of Figure 21. Then, a roller can roll over the mounting tape to stress the remaining intact layers such that the individual devices are diced out.
• the mounting tape 260 can be located on the bottom of the stmcture of Figure 21. Then, a diamond-cutting wheel 262 can dice out the individual devices.
• each LED includes a thick film support 240, an adhesion support (Cr adhesion layer 236 and Au adhesion layer 238), a reflective stmcture (titanium layer 230 and aluminum layer 232), semiconductor layers 120 and top contacts (transparent contact 190 and metal contact pad 192).
• Those semiconductor layers include semiconductor layers as shown in Figure 2A.
• the result is the LED 399 shown in Figure 25.
• That LED includes a thick metal support film 300, an adhesion layer 338, a reflective and p-type transparent contact 290, semiconductor layers 120, an n-type top interface layer 330, and a contact pad 332.
• Those semiconductor layers include semiconductor layers as shown in Figure 2A.

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