SILICON NITRIDE PASSIVATION WITH AMMONIA PLASMA PRETREAMENT FOR IMPROVING RELIABILITY OF AlGaN/GaN HEMTs
This invention pertains to an electronic device and to a method for making it. The device is a heterojunction transistor, particularly a high electron mobility transistor, characterized by presence of a 2 DEG channel. Transistors of this invention contain an AlGaN barrier and a GaN buffer, with the channel disposed, when present, at the interface of the barrier and the buffer. Surface treated with ammonia plasma resembles untreated surface. The method pertains to treatment of the device with ammonia plasma prior to passivation to extend reliability of the device beyond a period of time on the order of 300 hours of operation, the device typically being a 2 DEG AlGaN/GaN high electron mobility transistor with essentially no gate lag and with essentially no rf power output degradation.
This application is a divisional application of U.S. application Ser. No. 11/311,592, filed Dec. 9, 2005.
BACKGROUND OF THE INVENTION1. Field of the Invention
This invention pertains to improving reliability of heterojunction transistors, particularly high electron mobility transistors, with an ammonia plasma pretreatment prior to passivation.
2. Description of Related Art
Prior methods which have been used to prepare processed AlGaN/GaN high electron mobility transistors (HEMTs) for reliable operation in the past have included using no passivation at all, or direct deposition of a variety of electrically insulating materials intended to passivate surface states. These material films can be deposited by such processes as plasma enhanced chemical vapor deposition. Silicon nitride (SiN) is one of the most commonly used surface passivating films for AlGaN/GaN HEMTs and can be deposited onto the device by the aforementioned method. However, silicon nitride passivation, while an improvement over no passivation at all, still results in a decrease in performance of the device after dc and rf bias stress. This is a significant limitation and disadvantage in the reliability of AlGaN/GaN HEMT electronic devices.
AlGaN/GaN high electron mobility transistors have shown exceptional microwave power output densities, with a recently reported continuous wave power density of 30 W/mm and 50% power added efficiency at a frequency of 8 GHz, In addition, a 36-mm gate-width GaN HEMT has been demonstrated with a total power output of 150 W and a power added efficiency of 54%. However, device reliability remains a major concern for III-N HEMTs. In AlGaN/GaN HEMTs, degradation of the dc, transient, and microwave characteristics are often seen after relatively short periods of normal device operation. Although reliability is improving, microwave power output typically degrades by more than 1 dB in less than 1000 hours of operation.
OBJECTS AND BRIEF SUMMARY OF THE INVENTIONIt is an object of this invention to pretreat an AlGaN/GaN heterojunction field effect transistor with ammonia plasma prior to passivation in order to improve reliability thereof.
Another object of this invention is to pretreat an AlGaN/GaN transistor with a low-power ammonia plasma prior to silicon nitride deposition for high power applications.
Another object of this invention is to improve reliability of an AlGaN/Gan transistor adapted for use in radars and communication equipment.
Another object of this invention is to improve reliability of AlGaN/GaN HEMTS characterized by the presence of 2 DEG channel.
Another object of this invention is prevention of drain current collapse caused by electron traps.
Another object of this invention relates to reduction of surface and bulk traps in high power and high electron mobility AlGaN/GaN heterojunction transistors that can potentially operate at voltages exceeding 100 volts and with electron mobility in excess of 1000 cm2/v sec.
These and other objects of this invention can be attained by pretreatment of a 2 DEG AlGaN/GaN heterojunction transistor with a low-power ammonia plasma prior to passivation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 2 (a) and (b) show induced current collapse of unpassivated HEMT in (a) before stress and after stress and in (b) of passivated HEMT devices with silicon nitride only and with silicon nitride passivation, stressed for 60 hours and 176 hours, respectively.
The purpose of this invention, in a preferred embodiment, is to improve the reliability of aluminum gallium nitride/gallium nitride (AlGaN/GaN) high electron mobility transistors by incorporating an ammonia (NH3) plasma pre-treatment prior to silicon nitride (SiN) passivation of the heterojunction transistors after all other processing has been completed.
This invention pertains to an electronic device and to a method for making it. The device is a heterojunction transistor, particularly a high electron mobility transistor characterized by the presence of a 2 DEG channel. Transistors of this invention contain an AlGaN barrier and a GaN buffer, with the channel disposed, when present, at the interface of the barrier and the buffer. The method pertains to treatment of the device with ammonia plasma prior to passivation to extend reliability of the device beyond a period of time on the order of 300 hours of operation, the device typically being a 2 DEG AlGaN/GaN high electron mobility transistor with essentially no gate lag and with essentially no rf power output degradation.
Pursuant to one embodiment of the method, an in-situ ammonia plasma treatment is used before a silicon nitride deposition on an Alx Ga1-xN/GaN (where x is 0.20 to 0.30), after all other processing has been completed. The ammonia plasma pretreatment and the silicon nitride deposition can both be performed in a plasma enhanced chemical vapor deposition system. The substrate temperature is maintained at 250° C. for both. For the ammonia plasma pretreatment, a relatively low 35 W power level is typically used. An example of resulting process parameters are 200 mT chamber pressure, 400 sccm N2+SiH4 (95:5) gas flow, 9 sccm ammonia gas flow, 35 W ICP power, and OW RIE power. A silicon nitride film thickness of 750 A is adequate, with an optical index (n) of approximately 2.0, which indicates the approximate composition of the Si3N4 passivation layer.
The nitride deposition process was performed after all of the processing steps and was followed by etching openings to the metal contacts and deposition and patterning of a Ti—Au overlay metal. The ammonia plasma pretreatment and the silicon nitride deposition were performed in an inductively coupled plasma (ICP) configured plasma-enhanced chemical vapor deposition system with a bottom electrode diameter of 8 inches. The silicon nitride film, using about 5% by volume SiH4 in a balance of N2, was formed using a SiH4:NH3:N2 plasma recipe (N2+SiH4 of 300 to 400 sccm; NH3 of 9 to 16 sccm). The substrate temperature was maintained at 250° C. for both the ammonia plasma pretreatment and the silicon nitride deposition processes. The silicon nitride film thickness and the refractive index were measured by ellipsometry, and were 740-800 A and 2.03-2.09, respectively, for different device runs. For the pretreatment, the chamber pressure was 50 mT and the duration was 180 seconds. The ICP power was set to 35 W at 13.56 MHz, while the bottom electrode was set to 0 W.
The invention can be described in connection with
Completing the schematic transistor of
Shown in FIGS. 2 (a) and (b) are the measured drain current for three consecutive sweeps of VDS from 0 to 30V with VGS held at 0 V. The characteristics in
Corresponding results are seen with gate lag measurements as shown in FIGS. 3 (a-d). The gate lag measurements are for VDS=1 V and VGS=VTH−2 V, pulsed to VGS=0 V. The drain current pulse shown in each figure is normalized to the steady state value of drain current. The ideal response characteristic is for the pulsed current to rise to the steady state value, as shown in
The radio frequency (rf) degradation rate, in dB/hour, is also improved by more than 100 times for the ammonia pretreated sample as compared to devices on the same split wafer with silicon nitride only.
An added benefit of the invention is believed to be that the ammonia plasma provides hydrogen (H) atoms at the AlGaN surface. Passivation of surface defects by hydrogen has been used extensively in the past for silicon and gallium arsenide technologies. Hydrogen has been found to penetrate well into the AlGaN and reduce the density of bulk n-AlGaN deep level traps. These traps are thought to play a leading role in the current collapse phenomena.
Ammonia plasma ionizes gas into charged particles that cause the surface to be cleaned and/or charged, as is well known. The chamber used herein for ammonia plasma treatment was a typical for semiconductor equipment manufacturers. Treatment duration was 3 minutes, although it can be higher or lower, but is typically in the range of 3-5 minutes. Other parameters that may be used to produce the desired ammonia plasma include power of 10-35 watts, ammonia flow rate of 30 to 70 sccm. Plasma frequency is 13.56 MHz.
Rf stress conditions included conditions for dc bias and a microwave signal that goes into the device. The input power of this signal is typically 10-15 dBM at a frequency of 2-12 GHz. The dc stress conditions include drain voltage typically of 20-30 volts and a drain current typically of 100-300 mA/mm.
In conclusion, reliability of AlGaN—GaN high eletron mobility transistors that exhibit induced trapping effects due to extended dc bias or microwave operation has been improved by the incorporation of an ammonia plasma treatment prior to passivation. This processing step suppresses increases in current collapse and eliminates gate lag reductions after extended dc bias and significantly lessens degradation under microwave operation. It is believed that the interaction of the plasma with the exposed surface and H ions and/or atoms diffusion into the epitaxial layers are responsible for the improved device characteristics after extended dc bias and microwave operation.
While presently embodiments of the invention have been shown of the novel transistors and treatment with ammonia plasma, and of the several modifications discussed, persons skilled in this art will readily appreciate that various additional changes and modifications can be made without departing from the spirit of the invention as defined and differentiated by the following claims.
Claims
1. A high electron mobility transistor, comprising:
- a substrate;
- a GaN buffer layer disposed on the substrate;
- an AlGaN barrier layer disposed on the GaN buffer layer such that a 2 DEG channel is disposed at the interface of the AlGaN barrier layer and GaN buffer layer; and
- a passivation layer disposed on the AlGaN barrier layer, wherein an upper surface of the AlGaN barrier layer has been pre-treated with an ammonia plasma prior to a deposition of the passivation layer.
2. The transistor of claim 1, wherein the transistor has essentially no measurable drain current collapse for up to about 176 hours of dc bias stress.
3. The transistor of claim 1, wherein the transistor has essentially no measurable gate lag for up to about 176 hours of dc bias stress.
4. The transistor of claim 2, wherein the dc bias stress is characterized by a drain current of 100-300 mA/mm and a drain voltage of about 23-30 volts.
5. The transistor of claim 2, wherein the dc bias stress is provided by a microwave signal having an input power Pin of 5-20 dBm at a frequency of 2-12 GHz.
6. The transistor of claim 1, wherein the AlGaN barrier layer is about 150 A to 300 A thick.
7. The transistor of claim 1, wherein the GaN buffer layer is about 5000 μm to 15,000 μm thick.
8. The transistor of claim 1, wherein the transistor has experienced essentially no decrease in rf power output (Pout) up to about 70 hours.
9. The transistor of claim 1, wherein the AlGaN barrier layer is made from AlxGa1-xN, wherein 0.15≦x≦0.40.
10. The transistor of claim 1, wherein the rate of degradation of microwave power output, while under continuous microwave operation, is at least 100 times smaller than a conventional high electron mobility transistor that is not pre-treated with ammonia plasma.
11. The transistor of claim 1, wherein the transistor has electron mobility in excess of 1000 cm2/V sec.
12. The transistor of claim 1, further comprising a AlN buffer layer disposed between the GaN buffer layer and the substrate.
Type: Application
Filed: Dec 21, 2007
Publication Date: May 8, 2008
Inventors: Andrew Edwards (Cary, NC), Steven Binari (Annandale, VA), Jeffrey Mittereder (Annandale, VA)
Application Number: 11/962,259
International Classification: H01L 29/778 (20060101);