Chapter 2 Literature Review
4.2 Device Fabrication
The epitaxial layers of the InGaP/In0.22Ga0.78As PHEMTs were grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrate along the (100) axis. The schematic cross-sectional view of our InGaP/In0.22Ga0.78As HEMT structure with various doping profiles are shown in Fig. 4-1~4-3.
Fig. 4-1 shows the illustration of δ-doped InGaP/In0.2Ga0.78 As HEMT which consists of , from bottom to top, GaAs buffer layer, AlGaAs barrier layer, δ-doped carrier supply layer with Si doping concentration of 4.0×1012/cm2 which provides carriers, AlGaAs spacer layer, δ-doped carrier supply layer, InGaP Schottky layer and n-GaAs cap layer with Si doping concentration of 3.0×1012/cm3. It is worth noticing that the upper barrier material is InGaP which is different from the lower AlGaAs. The former has wider band-gap leading to higher breakdown voltage, and the later AlGaAs/InGaAs hetero-junction causes higher gm
value than that with InGaP spacer.
Fig. 4-2 exhibits the structure of InGaP/In0.22Ga0.78As HEMT with uniformly-doping profile which indicates the carriers are doped in the Schottky layer. Other layers are as the same as the δ-doped for comparison. Fig. 4-3 shows the diagram of channel-doped InGaP/In0.22Ga0.78As HEMT. The carriers are supplied by Si δ-doped layer and lightly doped channel with concentration of 4.0×1012/cm3.
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The detailed manufacturing process on the InGaP/In0.22Ga0.78As HEMT devices are described at the following sections. Besides, the three kinds of devices HEMTs were fabricated by the same process flow for comparison. The flow chart of the process for device fabrication is illustrated in Fig. 4-4.
4.2.1 Wafer cleaning
The purpose of wafer cleaning is to remove undesirable impurities and particles on the surface. The wafers were immersed in Acetone (ACE) and isopropyl alcohol (IPA) each for five minutes, and blown dry by nitrogen gas.
4.2.2 Mesa isolation
The active region of devices is defined by S1818 photoresist, and other potions were wet etched to the buffer layer. The mesa isolation was carried out by HF:H2O2:H2O (5:1:40) solution to etch the GaAs cap layer, and HCl:H2O (1:1) solution to etch the InGaP Schottky layer. Then, the etching depth will reach about 4000Å by utilizing HF:H2O2:H2O solution again. The etching depth is measured by α-step measurement.
4.2.3 Ohmic formation
A low contact resistance junction is formed between the ohmic metal (Au/Ge/Ni/Au) and the cap layer, where germanium is used for doping GaAs during alloy, and nickel acts as a wetting agent in order to prevent the AuGe metal from “balling up”.
The ohmic contact region is defined by AZ5214E photoresist with undercut profile. The wafers are dipped in 20% HCl solution for 15 seconds to remove the native oxide. Ohmic metal was then deposited on the substrates by using an electron-beam evaporator at a pressure of
~1x10-6 Torr. After ACE lift-off procedure, the wafer was thermally alloyed at 340℃ for 30 seconds by using rapid thermal anneal (RTA) system. After all, the contact resistance is obtained via measuring the transmission line method (TLM), and the specific contact resistivity is 1.9176×10-6 Ω cm2.
4.2.4 Recess and gate formation
The double gate recess process is used here.
The 1st recess slot was defined by e-beam photolithography to form the pattern defined by copolymer photoresist. Citric acid (C.A.) based solution (CA:H2O2:H2O) was used to etch the cap layer, and HCl based solution (HCl:H2O) was used to etch the InGaP shottky layer until it reaches the target current which was confirmed by measuring the drain-to-source current.
For high speed application, short gate length with low gate resistance
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obtaining low gate resistance. In the T-gate structure design, the gate length is defined by the small footprint and the wide top offers low gate resistance.
After removing the 1st recess photo-resist, the gate openings are defined by e-beam lithography to form the T-shaped profile consisting of Copolymer/PMMA/Copolymer to obtain a T-gate. Then, the HCl based solution was used to execute the 2nd recess, which can further increase the breakdown voltage. After that, the wafers are dipped in the 20% HCl solution for 15 seconds to remove the native oxide fallowed by depositing Ti/Pt/Au by e-gun evaporation system, where Titanium provides good adhesion to substrate, platinum acts as a barrier to prevent gold diffusing into GaAs, and gold provides high electrical conductivity. Finally, the wafer was immersed into ACE to lift-off the undesired metal. As the result, the gate length of the InGaP HEMT in this chapter is 0.3μm.
4.2.5 Device passivation
In order to protect the devices from environmental contamination and mechanical damages, the silicon nitride film (SiNX) was formed by PECVD. The wafer was first dipped in the solution of NH4OH:H2O=1:50 for 10 seconds to clean the surface and decrease the surface dangling bonds. The silicon nitride film was grown at 250℃. RF power was 35W, and the precursors were SiH4/Ar, NH3 and N2 . The film thickness was about 1000Å and its refractive index was about 2.0, which were measured by ellipsometer.
After the passivation process, the contact via was defined for interconnections. Then the silicon nitride film was etched by reactive ion etching (RIE) system. The reactive plasmas are CF4 and O2, the RF power is 80W, and the pressure is 60 mtorr.
4.2.6 Air-bridge plating
Electrical plating is usually the last major step of the front-side process, and plated air-bridges are commonly used in GaAs devices and MMICs to interconnect source pads of the HEMTs.
First, a layer of photo-resist was spun and patterned to open areas over metal pads. Then, a thin coating of Ti/Au/Ti was applied to the entire wafer, where Titanium is deposited to improve the adhesion. The thin metal layer can conduct the plating current to the whole wafer. Next, a second coating of photo-resist was applied and patterned. Then the wafer was electroplated with gold for 2μm thickness. After plating, the top resist layer, thin Ti/Au/Ti metal, and lower resist layer were removed individually, leaving only the plated air-bridge.