BONDABLE TOP METAL CONTACTS FOR GALLIUM NITRIDE POWER DEVICES
An embodiment of a semiconductor device includes a gallium nitride (GaN) substrate having a first surface and a second surface. The second surface is substantially opposite the first surface, at least one device layer is coupled to the first surface, and a backside metal is coupled to the second surface. A top metal stack is coupled to the at least one device layer. The top metal stack includes a contact metal coupled to a surface of the at least one device layer, a protection layer coupled to the contact metal, a diffusion barrier coupled to the protection layer, and a pad metal coupled to the diffusion barrier. The semiconductor device is configured to conduct electricity between the top metal stack and the backside metal.
This application is a continuation of U.S. patent application Ser. No. 13/611,467, filed on Sep. 12, 2012, the disclosure of which is incorporated by reference herein in its entirety for all purposes.
BACKGROUND OF THE INVENTIONPower electronics are widely used in a variety of applications. Power electronic devices are commonly used in circuits to modify the form of electrical energy, for example, from AC to DC, from one voltage level to another, or in some other way. Such devices can operate over a wide range of power levels, from milliwatts in mobile devices to hundreds of megawatts in a high voltage power transmission system. Despite the progress made in power electronics, there is a need in the art for improved electronics systems and methods of operating the same.
SUMMARY OF THE INVENTIONThe present invention relates generally to electronic devices. More specifically, the present invention relates to providing a bondable top metal contact for vertical semiconductor devices. Merely by way of example, the invention has been applied to methods and systems for manufacturing vertical GaN power devices. The methods and techniques can be applied to a variety of vertical semiconductor devices, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar transistors (BJTs, HBTs), diodes, and the like.
An embodiment of a semiconductor device, according to the disclosure, can include a gallium nitride (GaN) substrate having a first surface and a second surface. The second surface is substantially opposite the first surface, at least one device layer can be coupled to the first surface, and a backside metal can be coupled to the second surface. A top metal stack can be coupled to the at least one device layer. The top metal stack can include a contact metal coupled to a surface of the at least one device layer, a protection layer coupled to the contact metal, a diffusion barrier coupled to the protection layer, and a pad metal coupled to the diffusion barrier. The semiconductor device can be configured to conduct electricity between the top metal stack and the backside metal.
An embodiment of a method for fabricating a vertical GaN power device, according to the disclosure, can include providing a GaN substrate having a first surface and a second surface, where the second surface is substantially opposite the first surface. The method also can include forming at least one device layer coupled to the first surface, forming a backside metal coupled to the second surface, and forming a top metal stack. The top metal stack can a contact metal coupled to a surface of the at least one device layer, a protection layer coupled to the contact metal, a diffusion barrier coupled to the protection layer, and a pad metal coupled to the diffusion barrier. The semiconductor device can be configured to conduct electricity between the top metal stack and the backside metal.
A second embodiment of a semiconductor device, according to the disclosure, can include a GaN substrate having a first surface and a second surface, where the second surface is substantially opposite the first surface. At least one device layer can be coupled to the first surface. A backside metal can be coupled to the second surface. The backside metal can comprise an adhesion layer coupled to the second surface of the GaN substrate, a first diffusion barrier coupled to the adhesion layer, and a first protection layer coupled to the first diffusion barrier. A top metal stack can be coupled to the at least one device layer. The top metal stack can comprise a contact metal coupled to a surface of the at least one device layer, a second protection layer coupled to the contact metal, a second diffusion barrier coupled to the second protection layer, and a pad metal coupled to the diffusion barrier. The semiconductor device can be configured to conduct electricity between the top metal stack and the backside metal.
Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention enable a top metal contact (e.g., bond pad) of a semiconductor device to be bonded with high current, low resistivity aluminum bond wires. Techniques provided herein further allow for passivation before the top metal contact's metal stack is complete, enabling the semiconductor device to be exposed to other processing steps before the complete metal stack is formed. These and other embodiments of the invention, along with many of its advantages and features, are described in more detail in conjunction with the text below and attached figures.
In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTSThe present invention relates generally to electronic devices. More specifically, the present invention relates to providing a bondable contact metal for vertical semiconductor devices. Merely by way of example, the invention has been applied to methods and systems for manufacturing vertical GaN power devices. The methods and techniques can be applied to a variety of vertical semiconductor devices, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar transistors (BJTs, HBTs), diodes, and the like.
Furthermore, techniques for providing a bondable contact metal may be used in conjunction with techniques for providing a solderable back metal, also disclosed herein. These techniques for providing a solderable back metal are also provided in U.S. patent application Ser. No. 13/285,271, filed Jul. 19, 2012, entitled “GAN POWER DEVICE WITH SOLDERABLE BACK METAL” (Attorney Docket No. 93444-840540(003500US), which is incorporated by reference into this application for all purposes.
GaN-based electronic and optoelectronic devices are undergoing rapid development, and generally are expected to outperform competitors in silicon (Si) and silicon carbide (SiC). Desirable properties associated with GaN and related alloys and heterostructures include high bandgap energy for visible and ultraviolet light emission, favorable transport properties (e.g., high electron mobility and saturation velocity), a high breakdown field, and high thermal conductivity. In particular, electron mobility, μ, is higher than competing materials for a given background doping level, N. This provides low resistivity, ρ, because resistivity is inversely proportional to electron mobility, as provided by equation (1):
where q is the elementary charge.
Another superior property provided by GaN materials, including homoepitaxial GaN layers on bulk GaN substrates, is high critical electric field for avalanche breakdown. A high critical electric field allows a larger voltage to be supported over smaller length, L, than a material with a lower critical electric field. A smaller length for current to flow together with low resistivity give rise to a lower resistance, R, than other materials, since resistance can be determined by equation (2):
where A is the cross-sectional area of the channel or current path.
These superior properties can give rise to improved semiconductor devices, such as vertical semiconductor devices. Traditional semiconductor devices are typically lateral devices that utilize only the top side of a semiconductor wafer, locating electrical contacts such that electricity travels laterally along the semiconductor surface. This tends to consume a large footprint on the semiconductor. Vertical semiconductor devices, on the other hand, utilize a smaller footprint to achieve the same performance as lateral devices. Vertical semiconductor devices have electrical contacts on both the top surface of the semiconductor and on the bottom surface, or backside, such that electricity flows vertically between the electrical contacts. Vertical power devices are vertical semiconductor devices that can be utilized in high power and/or high voltage applications.
The form and function of the device layer(s) 110 can vary significantly, depending on desired functionality. Device layer(s) 110 can include, for example, one or more active regions, drift regions, PN junctions, P-I-N junctions, doped regions, intrinsic regions, insulating regions, and/or the like, depending on the functionality of the vertical semiconductor device. Examples of vertical semiconductor devices can include JFETs, MOSFETs, MESFETS, BJTs, HBTs, diodes, and the like.
Vertical semiconductor devices can have one or more metal contacts 130, 140, 150 on a top surface, depending on the type of device. The embodiment in
The vertical semiconductor devices of
Often, backside metals are not solderable. Traditional, non-vertical devices, for example, typically include all electrical contacts on a top surface of the semiconductor. Therefore, there is not a need to electrically connect a backside metal to a lead frame. Accordingly, in many cases, devices are attached to a package with electrically insulating epoxy or electrically conductive (e.g. silver filled) epoxy, which is much less thermally conductive and has a much higher electrical resistivity than solder. Solder, on the other hand, has very good electrical and thermal conductivity. It is also known for good reliability under temperature cycling and environmental testing using high humidity levels at elevated temperatures. Therefore, for semiconductors requiring electrical and mechanical backside connections, such as vertical power devices, the backside metal of the semiconductor device can be soldered to the metal lead frame of an electronic package. Because vertical power devices using GaN and/or other III-nitride materials are only now in development, little has been done to form solderable backside metals to these devices.
The manufacture of vertical semiconductor devices in GaN and/or other III-nitride materials can be carried out in a variety of ways.
Referring to
The thickness 215 of the adhesion layer 210 can vary. For example, the thickness 215 of the adhesion layer 210 could be in the range of 5-300 nanometers thick. Furthermore, the respective thicknesses 105, 115 of the GaN substrate 100 and the device layer(s) 110, as well as other properties (e.g., dopant concentration, shape, composition, etc.), also can vary depending on the type of semiconductor devices being fabricated, desired functionality, manufacturing concerns, and/or other factors. Although the GaN substrate 100 is illustrated as including a single material composition, multiple layers can be provided as part of the substrate. Moreover, adhesion, buffer, and other layers (not illustrated) can be utilized during an epitaxial growth process. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.
Some embodiments may include processes that involve polishing and/or other processing the bottom surface 114 of the GaN substrate 100 before the adhesion layer 210 is formed. For example, a GaN substrate 100 can be thinned, after the majority of device processing is completed, to reduce the on-state resistance of the device and lessen the thermal resistance of the device to lower operating temperature and improve its efficiency. The GaN substrate 100 can be thinned using lapping, mechanical polishing, and/or chemo-mechanical polishing (CMP), in which a slurry such as silica is used because it has a chemical etch component. Furthermore, semiconductor device wafers or diced pieces can undergo surface preparation prior to depositing the adhesion layer 210. Surface preparation treatment may include solvent baths, dry etching, and/or sulfuric acid baths to remove organic contaminants. A wet GaN etch, using for example tetramethyl ammonium hydroxide (TMAH), may also be used. Other treatments, such as hydrofluoric and/orhydrochloric acids may also be used to remove oxides from the bottom surface of the substrate. One purpose of the surface treatment can be to provide a clean surface that adheres well and has low electrical contact resistance to adhesion layer 210. In some embodiments, the roughness of the back surface of GaN substrate 100 is optimized to promote acceptable adhesion of the adhesion layer and acceptable specific contact resistance between GaN substrate 100 and adhesion layer 210. In one embodiment, the root-mean-square (RMS) back surface roughness is between 1 and 200 nanometers.
Referring to
Referring to
The form of the solder 510 can vary, depending on desired functionality. For example, the solder 510 can be in the form of a solder paste and/or solder preform. The composition of the solder 510 can also vary. In one embodiment, solder 510 comprises at least 80% lead (Pb). Solders with high lead content are relatively pliable, which can help prevent failure due to the different coefficient of thermal expansion (CTE) of the lead frame and the semiconductor material. In some embodiments the solder may comprise 85-98 wt % Pb, 0-10 wt % tin (Sn), and 0-5 wt % Ag. Two specific embodiments of solders are 2 wt % Sn, 95.5 wt % Pb, and 2.5 wt % Ag; and 1 wt % Sn, 97.5 wt % Pb, and 1.5 wt % Ag. In other embodiments, the solder may contain a combination of gold, tin, indium, or other suitable materials. The thickness 615 of the solder 510 can vary, depending on the desired solder joint thickness, composition, manufacturing concerns, and/or other factors. In one embodiment, the thickness 615 may be in the range of 10-100 microns.
Once the package, solder 510, and vertical semiconductor device are in place, they can be heated until the solder 510 melts. Soldering temperatures are typically in the range of 200-350° Celsius, and the heating process can take from about 1 to 30 minutes. The solder 510 solidifies as the package, solder, and chip are then cooled. As discussed above, at least a portion of protection layer 410 of the backside metal can dissolve in the solder 510 during this heating and cooling process.
The processes described in relation to
A metal contact on a top surface of the GaN power device is formed (715). As discussed previously, the metal contact can be one of a plurality of metal contacts on a top surface of the GaN power device to provide an electrical contact to the GaN power device. Furthermore, the metal contact can comprise one or more metals and/or layers that may be formed before, after, and/or during the formation of a backside metal. Moreover, subsequent layers may be formed on the top surface of the GaN power device, depending on desired functionality.
A backside metal is formed (720). As discussed above, the backside metal can comprise three layers of metal formed on the GaN substrate. These metal layers can be formed, for example, by evaporation, sputtering, and/or electroplating. Although embodiments provided herein describe three metal layers, other embodiments may include a larger or smaller number of metal and/or other layers, depending on desired functionality, manufacturing concerns, and/or other factors.
According the method 700 of
A diffusion barrier can be coupled to the Ohmic adhesion layer, as part of the backside metal. The diffusion barrier can comprise one or more materials to help protect the Ohmic adhesion layer from melting, thereby helping prevent diffusion between the Ohmic adhesion layer and subsequently-formed layers. As indicated elsewhere herein, in some embodiments, the diffusion layer substantially comprises Ni, which does not dissolve as readily in molten solder as other elements.
A protection layer can be coupled to the diffusion barrier, as part of the backside metal. The protection layer can comprise one or more materials to help protect the diffusion barrier from oxidation, contamination, and/or other processes that could deteriorate the solderability of the diffusion layer or the electrical or thermal performance of the vertical GaN power device. Some embodiments contemplate the use of a diffusion barrier that substantially comprises Ag because Ag can dissolve in solder without adversely affecting the solder's mechanical performance over time.
It should be appreciated that the specific steps illustrated in
In a manner similar to the techniques discussed in relation to
Similar to the embodiments shown in
The top metal contact 820 of the vertical semiconductor device of
Referring to
The contact metal 910 can provide a Schottky and/or Ohmic contact with the device layer(s) 110, depending on desired functionality of the vertical semiconductor device. In one embodiment, for example, a contact metal 910 comprising Pt, Pd, and/or Ni can be used to form an Ohmic contact to highly doped p-type GaN device layer(s) 110. Such an Ohmic contact can be formed at room temperature without requiring a high temperature anneal. In some embodiments, the Ohmic contact can be further improved by a rapid-thermal anneal (RTA) of, for example, between about 200° C. and 800° C. for approximately one minute.
In addition, or as an alternative, to an Ohmic contact to highly doped p-type GaN, contact metal 910 can provide a Schottky contact to lightly doped n-type GaN. Again Pt, Pd, and/or Ni can be used to provide the Schottky contact, although embodiments may use other materials additionally or alternatively. The contact metal 910, as deposited, can provide a Schottky contact without subsequent anneals. In some embodiments, a top surface of GaN device layer(s) 110 comprise some areas of highly doped p-type GaN and some areas of lightly doped n-type GaN. The same contact metal 910, with or without annealing, can simultaneously form an Ohmic contact to the p-type GaN and a Schottky contact to the n-type GaN. In one embodiment, this construction may be used to fabricate a vertical merged PN Schottky (MPS) device.
In other embodiments, device layer(s) 110 may comprise highly doped n-type GaN and an Ohmic contact may be formed by contact metal 910 comprising Ti and/or Al., Such an Ohmic contact can be formed at room temperature without requiring a high temperature anneal. In some embodiments, the Ohmic contact may be further improved by an RTA of between about 200° C. and 800° C. for approximately one minute. Additionally or alternatively the contact metal 910 and/or other layers may be annealed during subsequent high-temperature steps (e.g., passivation). Embodiments not utilizing an anneal, however, can be beneficial due to simplified processing of the devices and removal of the possibility of an anneal negatively affecting other aspects of the device.
The thickness 915 of the contact metal 910 can vary. In some embodiments, for example, thickness 915 of the contact metal 910 can be between approximately 20 and 100 nanometers thick. Furthermore, as with other embodiments provided herein, respective thicknesses of the GaN substrate 100 and the device layer(s) 110, as well as other properties (e.g., dopant concentration, shape, composition, etc.), also can vary depending on the type of semiconductor devices being fabricated, desired functionality, manufacturing concerns, and/or other factors. Although the GaN substrate 100 is illustrated as including a single material composition, multiple layers can be provided as part of the substrate. Moreover, adhesion, buffer, and other layers (not illustrated) can be utilized during an epitaxial growth process. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.
The vertical semiconductor device can also include a protection layer 920 coupled to the contact metal 910. In some embodiments, the protection layer 920 can protect the contact metal 910 from oxidation. For example, the protection layer 920 may be a layer substantially comprising Au, which can be highly resistant to many wet and dry etching, cleaning, and deposition processes. As with the contact metal 910, the thickness 925 of the protection layer 920 can vary. The thickness 925 can be approximately 80 nanometers in some embodiments. In some embodiments, the thickness 925 of the protection layer 920 can be between 25 and 250 nanometers.
Because the protection layer 920 can shield the contact metal 910 from processing steps, it can provide flexibility in processing by allowing for a pause in the formation of the top metal contact's metal stack. For example, referring to
Referring to
The pad metal 1120 provides a bondable surface to which wire (and/or other types) of bonds may be formed. Thick aluminum (Al) wires bonds are commonly used, for example, to form contacts in power electronics. Larger diameter (e.g. 50-500 micron) Al wires provide a high current and low resistance path to the semiconductor device. In some embodiments, the pad metal can comprise Al, which is easily deposited, inexpensive, and readily bondable to Al bond wires. Additionally or alternatively, other materials, such as Cu, can be used. Furthermore, physical features and/or patterns of the pad metal 1120 and/or the diffusion barrier 1110 can be defined by material removal processes, such as a lithographical wet etch.
The thickness 1125 of the pad metal 1120 can vary, depending on composition, desired functionality, and/or other factors. The pad metal 1120 can be relatively thick to help ensure the structural integrity of the pad metal 1120 can withstand a subsequent wire bonding process. In particular, the Al wire bonding process exerts large forces on the pad metal. A thick pad metal can absorb these forces to prevent damage to the underlying GaN device layers. In some embodiments, for example, the thickness 1125 of the pad metal 1120 can be between 2 and 6 microns. In one embodiment, thickness 1125 is in the range of 3.5 to 4.5 microns.
In some embodiments, pad metal 1120 and diffusion barrier 1110 are patterned into one or more isolated regions that function as the top electrodes of the vertical GaN power device (e.g. a source electrode and a gate electrode). This patterning may be accomplished using lift-off processes and/or masking and etching processes. In one embodiment, pad metal 1120 comprises Al with a thickness of approximately 4 microns, diffusion barrier 1110 comprises Ni with a thickness or approximately 200 nanometers, and one or more wet etching steps are used to pattern both of these layers using a single photomask. In another embodiment, diffusion barrier 1110 further comprises a layer of Ti with a thickness of about 20 nanometers underlying the Ni, and the Ti is patterned using a lift-off process. In another embodiment, the pad metal has a thickness of at least 2 microns and comprises Al and/or Cu.
The processes described in relation to
A backside metal is coupled to the second surface of the GaN substrate (1415). The backside metal can provide an electrical contact to the GaN power device, and can comprise one or more metals and/or layers, such as the solderable metal stack provided herein above. The backside metal, or portions thereof, may be formed before, after, and/or during the formation of a metal stack of a top metal contact. Moreover, subsequent layers may be formed on the second surface of the GaN power device, depending on desired functionality.
A top metal stack of is formed (1420), which can provide a top metal contact to the vertical GaN power device. As discussed above, the top metal can include four layers comprising metal formed on the at least one device layer. These metal layers can be formed, for example, by evaporation, sputtering, and/or electroplating. Although embodiments provided herein describe four metal layers, other embodiments may include a larger or smaller number of metal and/or other layers, depending on desired functionality, manufacturing concerns, and/or other factors.
According the method 1400 of
A protection layer can be coupled to the contact metal, as part of the metal stack. The protection layer can comprise one or more materials to help protect the contact metal from oxidation, contamination, and/or other processes that could deteriorate the performance of the vertical GaN power device. Some embodiments contemplate the use of a diffusion barrier that comprises Au.
A diffusion barrier can be coupled to the protection layer, as part of the metal stack. The diffusion barrier can comprise one or more materials to help protect the protection layer from diffusing with the subsequently-formed pad metal. As indicated elsewhere herein, in some embodiments, the diffusion layer can comprise Ni, Cr, Mo, Ti, W, and/or TiN.
A pad metal is coupled to the diffusion barrier. The pad metal can comprise a material, such as Al and/or Cu, which provides a surface to which a bond wire, ribbon, lead, and/or other electrical conductor can be bonded. The pad metal can be relatively thick to help ensure that the device layers are not damaged during the bonding process.
It should be appreciated that the specific steps illustrated in
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
Claims
1. A semiconductor device, comprising:
- a gallium nitride (GaN) substrate having a first surface and a second surface, the second surface being substantially opposite the first surface;
- at least one device layer coupled to the first surface;
- a backside metal coupled to the second surface; and
- a top metal stack coupled to the at least one device layer and comprising: a contact metal coupled to a surface of the at least one device layer; a protection layer coupled to the contact metal; a dielectric layer comprising a gap; a diffusion barrier over the dielectric layer and contacting the protection layer in the gap of the dielectric layer; and a pad metal coupled to the diffusion barrier;
- wherein the semiconductor device is configured to conduct electricity between the top metal stack and the backside metal.
2. The semiconductor device of claim 1 wherein the contact metal forms an Ohmic contact with at least a portion of the at least one device layer.
3. The semiconductor device of claim 1 wherein the contact metal forms a Schottky contact with at least a portion of the at least one device layer.
4. The semiconductor device of claim 1 wherein the contact metal comprises at least one of:
- platinum,
- palladium,
- titanium,
- titanium nitride,
- aluminum,
- silver, or
- nickel.
5. The semiconductor device of claim 1 wherein the protection layer comprises gold.
6. The semiconductor device of claim 1 wherein the pad metal has a thickness of at least 2 microns and comprises at least one of:
- aluminum, or
- copper.
7. The semiconductor device of claim 1 wherein the diffusion barrier comprises at least one of:
- nickel,
- chromium,
- molybdenum,
- tungsten, or
- titanium.
8. The semiconductor device of claim 1 further comprising an aluminum bond wire configured to electrically connect the pad metal to a package.
9. The semiconductor device of claim 1 wherein the backside metal comprises a solderable material.
10. The semiconductor device of claim 9 further comprising a solder mechanically and electrically connecting the backside metal to a package.
Type: Application
Filed: Dec 19, 2014
Publication Date: Apr 16, 2015
Inventors: Brian Joel Alvarez (San Jose, CA), Donald R. Disney (Cupertino, CA), Hui Nie (Cupertino, CA), Patrick James Lazlo Hyland (San Jose, CA)
Application Number: 14/577,875
International Classification: H01L 29/20 (20060101); H01L 29/417 (20060101); H01L 29/49 (20060101); H01L 29/45 (20060101); H01L 29/47 (20060101);