GaN Etch Rates Research Articles

Influence of the carrier wafer during GaN etching in Cl2 plasma

In this study, we have performed a thorough characterization of the GaN surface after etching up to 100 nm in Cl2 plasma under various bias voltages and according to the carrier wafer used (Si, SiO2, Si3N4, and photoresist). The objective of this article is to evaluate the etch damage and contamination of the GaN surface when materials with other chemical nature are present during etching. The effects of etching conditions on surface morphology and chemical compositions of etched GaN films are studied in detail using XPS and AFM measurements. To this aim, a universal methodology is proposed to estimate accurately by XPS the stoichiometry of the GaN surface exposed to reactive plasmas when only an Al Kα x-ray source is available. The results indicate that the GaN etching mechanisms are very sensitive to the chlorine radical density present in the plasma, the latter being strongly influenced by the carrier wafer. Substrates that are more chemically reactive with Cl2 plasma such as silicon or photoresist compared to SiO2 or Si3N4 will lead to a greater loading of atomic chlorine, which in turn will lead to lower GaN etch rates. Moreover, the GaN surface contamination will depend on the etch by-products ejected by the carrier wafer. The GaN surface exposed to Cl2 plasma shows a Ga-depleted surface because of the more important reactivity of Cl with Ga rather than N, except in the SiO2 carrier wafer case. In this latter case, the formation of Ga–O bond limits the Ga removal. Regarding the surface roughness, it seems that the contaminants play a little role in the roughness formation except for the oxygen released by the SiO2 carrier wafer. On the other hand, the surface roughness evolution is clearly driven by the chlorine radical flux reaching the GaN surface. At low bias voltage, a preferential crystalline orientation etching driven by the Cl radicals leads to the formation of hexagonal shaped defects that are associated to screw-type threading dislocations already present in the pristine GaN material. At higher bias, the enlargement of the defects is limited, leading to a very low surface roughness value but to amorphized surfaces.

Cascaded Ni hard mask to create chlorine-based ICP dry etched deep mesas for high-power devices

A highly selective cascaded Ni hard mask without stress and film damage issues suitable for the formation of deep vertical mesas for high-power devices is presented. A Cl2/BCl3/Ar inductively coupled plasma dry etch was used to investigate the effect of photoresist, and patterned Ni hard masks applied via four different methods on the etch rate of GaN, and mask selectivity. Strain in the e-beam only deposited Ni films produced low yield due to poor adhesion. Electroplated Ni showed rough mask morphology. Sputtered Ni left damage/chemically reacted layers on the GaN under the mask. A cascaded 50 nm e-beam evaporated Ni followed by 2 µm of sputtered Ni mask method improved the yield to 100%, achieved high selectivity and retained an undamaged GaN surface morphology. The cascaded Ni hard mask successfully achieved GaN mesas up to 13 µm. The dry etching rates of the photoresist, GaN and Ni masks were 250, 275 and 13 nm min−1, respectively giving an etch selectivity of the GaN/Ni hard mask of ∼20, much higher than the etch selectivity of 1.1 for the photoresist. Furthermore, yield of the cascaded Ni hard mask was 100% making it suitable for commercial high-power electronics.

Impact of carrier wafer on etch rate, selectivity, morphology, and passivation during GaN plasma etching

The choice of carrier wafer was found to significantly influence etch rates, selectivity, and morphology in GaN micropillar etching in a Cl2-Ar high-density inductively coupled plasma. 7 × 7 mm2 GaN on sapphire chips with a plasma-enhanced chemical vapor deposition SiO2 hard mask was etched on top of 4-in. fused silica, silicon carbide, silicon, sapphire, aluminum nitride, and high purity aluminum carriers. Silicon and silicon carbide carriers reduced GaN:SiO2 selectivity because incidental SiClx and CClx etch products from the carriers attack the SiO2 mask. Aluminum nitride and high-purity aluminum carriers yielded the highest GaN:SiO2 selectivities due to the deposition of Al-based etched by-products, while the highest GaN etch rate was achieved using the sapphire carrier since it was the most inert carrier and did not sink any Cl2. Results indicate that SiO2 and Al may be used as passivation materials during GaN etching, as vertical profiles were achieved when SiO2 or Al is redeposited from the fused silica and aluminum carriers, respectively. Floor pitting, trenching, sidewall roughness, and faceting were all influenced by carrier wafer type and will be discussed.

Polarity dependence in Cl2-based plasma etching of GaN, AlGaN and AlN

This work compares Cl2-based inductively-coupled plasma (ICP) etching of N-polar and III-polar AlxGa1-xN, including GaN and AlN. For the etch processes of this study, etch rates of AlxGa1-xN decreased with increasing Al:Ga alloy ratio. A 750 W ICP power, 175 W platen power Cl2/Ar ICP process exhibited etch rates in the range 160–450 nm/min for AlxGa1-xN with the precise rate depending on alloy composition and material polarity, and etch rate selectivity between AlN and GaN of 3.0 for III-polar material and 1.6 for N-polar material. Cl2/Ar/O2 processes with 650 W ICP power and 50 W platen power showed an overall reduction in etch rate in the range 2–360 nm/min for AlxGa1-xN with precise rate depending on alloy composition and material polarity. These O2 containing chemistries demonstrated significantly greater etch rate selectivity for N-polar materials (8–80) compared to III-polar materials (1.8–1.9). A correlation between GaN etch rate, surface roughening and selectivity was observed, and the possible reasons behind the polarity dependence of etch rate selectivity are discussed.

Chlorine-based dry etching of β-Ga2O3

Reactive ion etching (RIE) and inductively coupled plasma (ICP) etching techniques were used to determine the optimal dry etch conditions for β-Ga2O3. RF power and chamber pressure were examined to study their effects on etch rate and surface roughness for three crystallographic planes, i.e., (100); (010); and ( by RIE. BCl3 etch rate calibrations were performed on all β-Ga2O3 planes studied, in comparison to Cl2. RIE yielded moderate etch rates (<20 nm min−1), and surface roughness showed no clear trend with RF power. Moreover, the effect of bias power, plasma power, and the choice of etchant were studied using ICP. The etches performed by ICP were shown to be superior to RIE in both etch rate and surface roughness, due to the much higher plasma densities and uniformities possible with plasma powers beyond those realized in RIE. The maximum etch rate of 43.0 nm min−1 was achieved using BCl3 in ICP. SF6/BCl3 mixtures, which yield high GaN etch rates, were also studied. However, in contrast to GaN etching, SF6/BCl3 was found to be far less effective than pure BCl3 in etching β-Ga2O3.

Nanometer scale patterning of GaN using nanoimprint lithography and Inductively Coupled Plasma etching

In this paper results on GaN patterning using nanoimprint technology are presented. Direct method of stamp fabrication based on FIB etching was used. Stamp with critical dimensions of 50nm was achieved. Two kinds of polymer materials were used for master stamp replication. Influence of etching parameters using chlorine based plasma on GaN etch rate and surface roughness was discussed. Triple mask consisting of TU2 resist, Cr and SiO2 used for NIL-generated pattern transfer into GaN allowed successful patterning.

Improvement of GaN Deep Etched Surface State by Fluorination Dedicated to Power Devices

In this paper, the etched surface states of GaN using two different etching techniques are reported. In Cl2/Ar ICP based plasma, two types of defect were identified: cavities and columns. A strong decrease of the cavity diameter and density on etched surface was observed with the addition of CHF3 in the chemistry. SF6 addition instead of CHF3 leads to an etched surface free of defects with low GaN etch rate similar to that obtained with IBE technique. XPS and AFM measurements revealed the importance of fluorine species which leads to the formation of a GaxFy passivation layer on the GaN etched surface.

GaN etch rate and surface roughness evolution in Cl2/Ar based inductively coupled plasma etching

Cl2/Ar based inductively coupled plasma (ICP) etching of GaN is investigated using photoresist mask in a consequential restricted domain of pressure<1.2Pa and radio frequency (RF) sample power<100W, for selective mesa etching. The etch characteristics and root-mean-square (rms) surface roughness are studied as a function of process parameters viz. process pressure, Cl2 percentage in total flow rate ratio, and RF sample power at a constant ICP power, to achieve moderate GaN etch rate with anisotropic profiles and smooth surface morphology. The etch rate and resultant surface roughness of etched surface increased with pressure mainly due to dominant reactant limited etch regime. The etch rate and surface roughness show strong dependence on RF sample power with the former increasing and the later decreasing with the applied RF sample power up to 80W. The process etch yield variation with applied RF sample power is also reported. The studied etch parameters result in highly anisotropic mesa structures with Ga rich etched surface.

Effect of BCl3 concentration and process pressure on the GaN mesa sidewalls in BCl3/Cl2 based inductively coupled plasma etching

GaN mesa etching is investigated using BCl3/Cl2 based inductively coupled plasma at constant ICP/RF powers for HEMT fabrication. The effect of chamber process pressure (5–15 mTorr) and BCl3/Cl2 flow rate ratio >1 on mesa sidewall profile is studied in detail using less complex photoresist mask. Mesa sidewall sharpness varied strongly with chamber pressure and deteriorated at lower pressure ∼5 mTorr. The etched GaN mesas resulted in severely damaged sidewalls with significant sidewall erosion at BCl3/Cl2 ratio of <1, which reduced gradually as BCl3/Cl2 ratio was increased to values >1 mainly due to decreased Cl ion/neutral scattering at the edges. Finally, the smooth and sharp mesa sidewalls with angle of ∼80° and moderate GaN etch rate of ∼1254 Å/min are obtained at BCl3/Cl2 ratio of 2.5:1 and 10 mTorr pressure due to a better balance between physical and chemical components of ICP etching.

Fabrication and characterisation of microscale air bridges in conductive gallium nitride

Fabrication and electrical characterisation of mi- croscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN-AlInN- GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrica- tion sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Process- ing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported be- tween anchor posts at both ends. Deposition of Ti-Au con- tact pads onto the anchor posts allowed study of the elec- trical characteristics. At low applied voltages, vertical con- duction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current- voltage characteristics at low applied voltages, and had re- sistances of 1050 � . The observed resistance values are compared with the predicted value based on materials para- meters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2 × 10 5 Ac m −2 .

Comparison between extended microtunnels along different crystal orientations in GaN

AbstractTriangular prism‐like extended microtunnels (EMTs) can be easily formed in specially designed epitaxial lateral overgrowth GaN thick films by selective wet chemical etching in molten potassium hydroxide (KOH). In this study, extended microtunnels along the 〈1$ \bar 1 $00〉 and the 〈1$ \bar 2 $00〉 directions were fabricated and compared. If tunnels along the 〈1$ \bar 1 $00〉 direction, the {11$ {\bar 2} {\bar 2} $} planes would be the etch stop plane. While the tunnels along 〈1$ \bar 2 $00〉 direction, the etch stop plane would become to {10$ {\bar 1} {\bar 1} $} planes. The activation energies of wet chemical etch for the {11$ {\bar 2} {\bar 2} $} planes and the {10$ {\bar 1} {\bar 1} $} planes were determined to be 23.6 kcal/mol (1.02 eV) and 22.8 kcal/mol (0.99 eV) using Arrhenius plot. On the other hand, the etching depths of the tunnels along the 〈1$ \bar 1 $00〉 direction were more than twice the depths of the tunnels along the 〈1$ \bar 2 $00〉 direction during the same etching time. The highest etch rate of the tunnels along the axial direction can reach 1000 μm/hr in 〈1$ \bar 1 $00〉 direction, which is believed to be the highest etch rate of GaN ever reported. Additional crystal facet of m‐plane in GaN was also observed in the sample with extended microtunnels along the 〈1$ \bar 2 $00〉 direction. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl2/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 μm and 0.7 μm thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl2/Ar and CF4/Ar/O2 chemistries, and a potassium hydroxide (KOH) aqueous wet etch.

Nitride-based Schottky diodes and HFETs fabricated by photo-enhanced chemical wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. It was found that the maximum etch rates were 510, 1960, 300, and 0 nm/mm for GaN, Al 0.175Ga 0.825N, Al 0.23Ga 0.77N, and Al 0.4Ga 0.6N, respectively. It was also found that we could achieve a high Al 0.175Ga 0.825N to GaN etch rate ratio of 12.6. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. It was found that we could achieve a saturated I D larger than 850 mA/mm and a maximum g m about 163 mS/mm from PEC wet etched HFET with a 0.5 μm gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.

GaN etch enhancement in inductively coupled BCl3 plasma with the addition of N2 and SF6 gas

Etching of GaN using BCl3/N2, BCl3/SF6 and BCl3 by inductively coupled plasma (ICP) was performed and the effects of N2 and SF6 were examined using optical emission spectroscopy. With the addition of 40% N2 and 20% SF6 to BCl3, the etch rate of GaN was increased from 600 to 2800 and 2000 Å min−1 at 700 W ICP power, respectively. There was a clear relationship between the increase of GaN etch rate and reactive Cl with the addition of N2 and SF6 gas to BCl3. In addition, B emission intensity was increased also with Cl emission intensity and negligible BN emission was observed. This means that N2 and SF6 reduce the recombination probability of B and reactive Cl and result in higher concentration of reactive Cl and higher etch rate with the addition N2 and SF6 to BCl3 plasma.

Correlation of Optical Emission and Ion Flux with GaN Etch Rate in Inductively Coupled Ar/Cl 2 Plasma Etching

The etching of GaN was investigated in an Ar/Cl2 inductively coupled plasma. Optical emission spectroscopy and an ion flux probe were used to obtain insight into the etch mechanisms during processing. Langmuir probe measurements were also used to determine the basic Ar/Cl2 plasma characteristics. Etch rates of ≈500 nm/min were obtained at relatively low Cl2 fractions of ≈50%. The dominant emission species observed were Ga (at 294 nm) and up to six GaCl peaks between 320 and 345 nm. Plasma characterisation and ion flux indicate etch mechanisms that depend strongly on atomic chlorine but with increasing power and at low Cl2, the significance of ion-based processes cannot be ruled out.

A study of GaN etching characteristics using HBr-based inductively coupled plasmas

In this study, HBr-based(Cl 2/HBr, BCl 3/HBr, and HCl/HBr) inductively coupled plasmas were used to etch GaN selectively over photoresist and its etch characteristics were investigated. The inductive power and dc bias voltage to the substrate were fixed at 1600 W and −150 V, respectively. Among the gas combinations used in this experiment, BCl 3/HBr showed the highest etch selectivity over photoresist and the etch selectivity of 1.5 with the etch rate of 6400 Å/min could be obtained with 40% BCl 3/60% HBr. Also, the most anisotropic GaN etch profiles could be observed with this gas mixture. The highest etch rate obtained was about 7700 Å/min for pure Cl 2, however, the etch selectivity was lower than 0.6. Optical emission spectroscopy showed that the GaN etch rate and etch selectivity for BCl 3/HBr were related to the Cl 2 + ions.

GaN focused ion beam micromachining with gas-assisted etching

Halide gases, such as Cl2, IBr, or ICl, are common etchant species for the etching of III nitrides and other compound semiconductor materials in plasma etching processes. We have investigated the Ga+ focused ion beam milling of GaN in conjunction with gas-assisted etching (GAE) by halide gases I2 and XeF2. We have observed that I2 and XeF2 GAE with a 30 keV Ga+ ion beam leads to significantly enhanced GaN etch rates. When these gases are utilized with appropriate ion beam scan strategies (such as ion beam current, beam dwell time, and beam overlap), we have measured GaN etch rate enhancements of 6× to 9× and 2× to 3× faster for I2 and XeF2, respectively.

Inductively coupled high-density plasma-induced etch damage of GaN MESFETs

The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 μm min −1 and anisotropic etch profiles are readily achieved in inductively coupled plasma (ICP) etch systems. Unfortunately, under these conditions, plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study, we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, plasma-induced damage was more sensitive to ion bombardment energies as compared to plasma flux.

Magnetized inductively coupled plasma etching of GaN in Cl2/BCl3 plasmas

In this study, Cl2/BCl3 magnetized inductively coupled plasmas were used to etch GaN, and the effects of magnetic confinements of inductively coupled plasmas on the GaN etch characteristics were investigated as a function of Cl2/BCl3. Also, the effects of Kr addition to the magnetized Cl2/BCl3 plasmas on the GaN etch rates were investigated. The characteristics of the plasmas were estimated using a Langmuir probe and quadrupole mass spectrometry (QMS). Etched GaN profiles were observed using scanning electron microscopy. The small addition of BCl3 (10%–20%) in Cl2 increased the GaN etch rates for plasmas both with and without magnetic confinements. The application of magnetic confinements to the Cl2/BCl3 inductively coupled plasmas increased the GaN etch rates and changed the Cl2/BCl3 gas composition of the peak GaN etch rate from 10% BCl3 to 20% BCl3. It also increased the etch selectivity over the photoresist, while slightly reducing the selectivity over SiO2. The application of the magnetic field significantly increased positive BCl2+ measured by QMS and total ion saturation current measured by the Langmuir probe. Other species such as Cl, BCl, and Cl+ increased, whereas species such as BCl2 and BCl3 decreased with the application of the magnetic field. Therefore, it appears that the increase of the GaN etch rate in our experiment is related to the increased dissociative ionization of BCl3 by the application of the magnetic field. The addition of 10% Kr in an optimized Cl2/BCl3 condition (80% Cl2/20% BCl3) with the magnets increased the GaN etch rate about 60%. A more anisotropic GaN etch profile was obtained with the application of the magnetic field, and a vertical GaN etch profile was obtained with the addition of 10% Kr in an optimized Cl2/BCl3 condition with the magnets.

Selective etching of GaN over AlN using an inductively coupled plasma and an O2/Cl2/Ar chemistry

An alternative method for achieving etching selectivity between GaN and AlN has been demonstrated. The etch rate of AlN was significantly decreased by the addition of a low concentration of O2 to a Cl2–Ar mixture in an inductively coupled plasma (ICP) etching system. The etch rate of GaN in the O2-containing plasma was approximately 15% less than the plasma without the O2 for the same parameters. The pressure and the ICP power were varied to achieve a maximum selectivity of 48 at a pressure of 10 mTorr, a direct current bias of −150 V, and an ICP power of 500 W. The etch rates of GaN and AlN at these parameters were 4800 and 100 Å/min, respectively.