B. K. Sehgal

Experimental Study of the Influence of Process Pressure and Gas Composition on GaAs Etching Characteristics in Cl2/BCl3-Based Inductively Coupled Plasma

A study of Cl2/BCl3-based inductively coupled plasma (ICP) was conducted using thick photoresist mask for anisotropic etching of 50 μm diameter holes in a GaAs wafer at a relatively high average etching rate for etching depths of more than 150 μm. Plasma etch characteristics with ICP process pressure and the percentage of BCl3 were studied in greater detail at a constant ICP coil/bias power. The measured peak-to-peak voltage as a function of pressure was used to estimate the minimum energy of the ions bombarding the substrate. The process pressure was found to have a substantial influence on the energy of heavy ions. Various ion species in plasma showed minimum energy variation from 1.85 eV to 7.5 eV in the pressure range of 20 mTorr to 50 mTorr. The effect of pressure and the percentage of BCl3 on the etching rate and surface smoothness of the bottom surface of the etched hole were studied for a fixed total flow rate. The etching rate was found to decrease with the percentage of BCl3, whereas the addition of BCl3 resulted in anisotropic holes with a smooth veil free bottom surface at a pressure of 30 mTorr and 42% BCl3. In addition, variation of the etching yield with pressure and etching depth were also investigated.

Anisotropic Etching of GaAs Using CCl2 F 2 / CCl4 Gases to Fabricate 200 μm Deep Via Holes for Grounding MMICs

In this study we have investigated the reactive ion etching of 60 μm diam, 200 μm deep holes in 3 in. diam semi-insulating GaAs wafer using a combination of CCl 2 F 2 and CCl 4 gases for fabrication of through substrate via holes for grounding in monolithic microwave integrated circuits (MMICs). The effect of process parameters viz. pressure, CCl 4 /CCl 2 F 2 ratio, and power on GaAs etch rate and resultant etch profile was investigated. Two kind of masks, photoresist and Ni, were used to etch GaAs and their performance was compared by investigating effect on etch rate, etch depth, etch profile, and surface morphology. The etch profile, etch depth, and surface morphology of as-etched samples were characterized by scanning electron microscopy. The desired 200 μm deep strawberry profile, with a top diam = 60 ′ 10 μm and bottom diam = 180 ′ 10 μm, was obtained at 40 mTorr process pressure with an average etch rate ∼1.3 μm/min using Ni mask. The vias were then metallized by depositing a thin seed layer of Ti/Au (1000 A) using radio frequency sputtering and Au (5 μm) electroplated to connect the front side pad and back side ground plane. The parasitic inductance offered by these vias was ∼76 pH. The developed process was then integrated into the MMIC process line and a 16-18 GHz amplifier was fabricated using grounding vias with yield >90%.

A Highly Selective Low Pressure Inductively Coupled Plasma EtchingProcess for GaAs Using Photoresist Mask

GaAs etch characteristics like etch rate, etch profile sidewall angle, etch surface morphology and selectivity are studied as a function of Inductively Coupled Plasma (ICP) power and Cl2/BCl3 flow rate ratio in ICP at low pressure (<15mTorr) and low RF bias power (<100W) regime to achieve moderate GaAs etch rate with an-isotropic profiles and smooth surface morphology. The low pressure regime etching at Cl2/BCl3 flow rate ratio of 4:1 has resulted in vertical etch profiles with controlled sidewall angle ~ 84o, smooth surface morphology and good mask selectivity ~15 without significant deposition of CClx polymer on the etched sidewalls but with limited etch depth ~ 100μm using photoresist mask. The mask selectivity is found to be a strong function of RF bias power and ICP power and a weaker function of process pressure. The resultant etch depth increases with an increase in pressure and flow rate ratio at the expense of etch surface morphology, as the desorption of chemical species limits the etching process at higher Cl2 flow rates and leaves some of the residue on the surface.

Study of Cl2/BCl3 inductively coupled plasma for selective etching of GaAs

Inductively Coupled Plasma (ICP) etching has been replacing conventional reactive ion etching (RIE) for GaAs backside via etching to provide low inductance grounding in microwave devices like HEMT and MESFET. ICP tools provide higher throughput with faster etch rates in addition to better dimensional control, repeatability and reproducibility. Generally reported etch depths using ICP for via-hole etching applications in GaAs Monolithic Microwave Integrated Circuits are ≪ 200µm using photoresist mask due to lower etch rate and poor mask selectivity. In this study, we are reporting the ICP etching of 60µm diameter via-holes for etch depth ∼ 200µm, on 3-inch GaAs wafer with photoresist mask using Cl2/BCl3 gases. ICP process parameters like pressure and platen power were varied at a fixed ICP coil power to achieve an etch depth ∼ 200µm at a relatively very high etch rate with good etch surface morphology. The etch rate, etch depth, etch profile and surface morphology of etched holes were determined using scanning electron microscope.

Cl2/Ar based inductively coupled plasma etching of GaN/AlGaN structure

Cl2/Ar based inductively coupled plasma etching of GaN/AlGaN is investigated using photoresist mask in a restricted domain of pressure < 10mTorr and RF power <100W, for selective mesa etching. The etch characteristics and rootmean- square surface roughness are studied as a function of process parameters viz. process pressure, Cl2 percentage in total flow rate ratio, and RF bias at a constant ICP power, to achieve moderately high GaN etch rate with anisotropic profiles and smooth surface morphology. The etch rate and resultant rms roughness of etched surface increased with pressure mainly due to dominant reactant limited etch regime. The etch rate also increased with increasing Cl2 % as a result of increased chlorine radicals that enhances chemical etching. The etch rate and rms roughness showed strong dependence on RF power with former increasing and later decreasing with applied RF power up to 80W. The process etch yield variation with applied RF bias is also reported. Negligible etch selectivity was observed between GaN and AlGaN up to 25% aluminum concentration with etch rate ~120nm/min. The studied etch parameters resulted in highly anisotropic mesa structures with Ga rich etched surface.

High temperature alloyed Ag based ohmic contacts to pseudomorphic high electron mobility transistor (p-HEMT)

The quality of ohmic contacts in pseudomorphic high electron mobility transistors (p-HEMTs) formed by sequential e-beam evaporation of Ni/Ge/Au/Ag/Au was investigated as a function of alloying cycles in the range from 390degC to 460degC for duration of 30 sec. The contact resistance (Rc) and the specific transfer resistance (ohm-mm) values of ohmic contacts so formed were measured by Transmission line method (TLM) and compared with the contacts formed without Ag interlayer. XRD technique revealed the presence of Au-Ga phase at alloying temperatures 420degC and 440degC indicative of the formation of good quality and low Rc ohmic contacts. The Ag based contacts showed improved specific transfer resistance values and smooth surface morphology at wider alloying temperatures compared to the contacts formed without using Ag as a barrier layer. This factor is beneficial in the fabrication of lower resistance ohmic contacts in HEMTs, where a deeper penetration of ohmic metal upto the 2DEG channel is desirable.

A Reproducible High Etch Rate ICP Process for Etching of Via-Hole Grounds in 200μm Thick GaAs MMICs

An inductively coupled plasma etching process to replace an existing slower rate reactive ion etching process for 60 ㎛ diameter via-holes using Cl2/BCl3 gases has been investigated. Process pressure and platen power were varied at a constant ICP coil power to reproduce the RIE etched 200 ㎛ deep via profile, at high etch rate. Desired etch profile was obtained at 40 mTorr pressure, 950 W coil power, 90W platen power with an etch rate ~ 4 ㎛/min and via etch yield >90% over a 3-inch wafer, using 24 ㎛ thick photoresist mask. The etch uniformity and reproducibility obtained for the process were better than 4%. The metallized via-hole dc resistance measured was ~ 0.5 Ω and via inductance value measured was ~ 83 pH.

Parametral dependence of bilevel-interconnect formation in GaAs ICs/MMICs

The formation of multilevel interconnects in GaAs MMIC fabrication depends mainly on the adhesion of the interconnect metals with the intermediate dielectric and with the substrate. The metal lines in our experimental study were formed by depositing multilayer films by rf sputtering technique over spin-coated polyimide and Plasma Enhanced Chemical Vapor Deposited (PECVD) silicon nitride (Si3Nx). It is observed that samples with a poor adhesion between the second level metal and Si3Nx show bubbling of blistering in the metal layer when subjected to subsequent high temperature cycles in the fabrication process. It is also found that the choice of certain metal schemes develops a stress between the first level metal and GaAs at the bond pads which peel off at the time of bonding. The present experimental study was undertaken to avoid the loss of the circuit at such advanced stages of fabrication like bonding or second level interconnect formation. The study brings out the dependence of the bondability of the circuit bond pads and the formation of the interconnect lines on the rf sputtering parameters, the premetallization treatment, the choice of the multilayer metal schemes and the thickness of the metals in the multilayer metallization.