A Review of Dry Etching of GaN and Related Materials
1. Introduction
GaN and related alloys are finding application for fabrication of blue/green/UV emitters (light-emitting diodes and lasers) and high temperature, high power electronic devices [1][2][3][4]. The emitter technology is relatively mature, with light-emitting diodes being commercially available since 1994 and blue laser diodes also available from Nichia Chemical Industries. Electronic devices such as heterostructure field effect transistors (FETs), heterojunction bipolar transistors (HBTs), metal oxide semiconductor field effect transistors (MOSFETs) and diode rectifiers have all been realized in the AlGaInN system, with very promising high temperature (>300°C) and high voltage performance. The applications for the emitter devices lies in full color displays, optical data storage, white-light sources and covert communications, while the electronic devices are suited for high power switches and microwave power generation.
Due to limited wet chemical etch results for the group-III nitrides, a significant amount of effort has been devoted to the development of dry etch processing [5][6]. Dry etch development was initially focused on mesa structures where high etch rates, anisotropic profiles, smooth sidewalls and equirate etching of dis
similar materials were required. For example, commercially available LEDs and laser facets for GaN-based laser diodes were patterned using reactive ion etch (RIE). However, as interest in high power, high temperature electronics increased, etch requirements expanded to include smooth surface morphology, low plasma-induced damage and selective etching of one layer of another occurred. Dry etch development is further complicated by the inert chemical nature and strong bond energies of the group-III nitrides as compared to other compound semiconductors. GaN has a bond energy of 8.92 eV/atom, InN 7.72 eV/atom and AlN 11.52 eV/atom.
2. Plasma Reactors
Dry plasma etching has become the dominant patterning technique for the group-III nitrides, due to the shortcomings in wet chemical etching. Plasma etching proceeds by either physical sputtering, chemical reaction, or a combination of the two often referred to as ion-assisted plasma etching, Physical sputtering is dominated by the acceleration of energetic ions formed in the plasma to the substrate
surface at relatively high energies, typically >200 eV. Due to the transfer of energy and momentum to the substrate, material is ejected from the surface. This sputter mechanism tends to yield anisotropic p
rofiles; however, it can result in significant damage, rough surface morphology, trenching, poor selectivity and nonstoichiometric surfaces thus minimizing device performance. Pearton and co-workers measured sputter rates for GaN, InN, AlN and InGaN as a function of Ar+ion energy [1][2]. The sputter rates increased with ion energy but were quite slow, <600 Å/min, due to the high bond energies of the group III-N bond.
Chemically dominated etch mechanisms rely on the formation of reactive species in the plasma which absorb to the surface, form volatile etch products and then desorb from the surface. Since ion energies are relatively low, etch rates in the vertical and lateral direction are often similar thus resulting in isotropic etch profiles and loss of critical dimensions. However, due to the low ion energies used, plasma-induced damage is minimized. Alternatively, ion-assisted plasma etching relies on both chemical reactions and physical sputtering to yield anisotropic profiles at reasonably high etch rates. Provided the chemical and physical component of the etch mechanism are balanced, high resolution features with minimal damage can be realized and optimum device performance can be obtained.
2.1. Reactive ion etching.
RIE utilizes both the chemical and physical components of an etch mechanism to achieve anisotropic
profiles, fast etch rates and dimensional control. RIE plasma are typically generated by applying radio frequency (rf) power of 13.56 MHz between two parallel electrodes in a reactive gas [see Figure 1(a)]. The substrate is placed on the powered electrode where a potential is induced and ion energies, defined as they cross the plasma sheath, are typically a few hundred eV. RIE is operated at low pressures, ranging from a few mTorr up to 200 mTorr, which promotes anisotropic etching due to increased mean free paths and reduced collisional scattering of ions during acceleration in the sheath. Adesida et al. were the first to report RIE of GaN in SiCl
4
-based plasmas [6]. Etch rates increased with increasing dc bias, and were >500 Å/min at -400 V. Lin et al.,
reported similar results for GaN in BCl
3 and SiCl
4
plasmas with etch
rates of 1050 Å/min in BCl
3
at 150 W cathode (area 250 in.2) rf power [1][2]. Additional RIE results have been reported for HBr- [1][2],
CHF
3- and CCl
2
F
2
-based [1][2] plasmas with etch rates typically <600
Å/min. The best RIE results for the group-III nitrides have been obtained in chorine-based plasmas under high ion energy conditions
where the III-N bond breaking and the sputter desorption of etch products from the surface are most efficient. Under these conditions, plasma damage can occur and degrade both electrical and optical device performance. Lowering the ion energy or increasing the chemical activity in the plasma to minimize the damage often results in slower etch rates or less anisotropic profiles which significantly limits critical dimension. Therefore, it is necessary to pursue alternative etch platforms which combine high quality etch characteristics with low damage.
2.2. High-density plasmas.
The use of high-density plasma etch systems including electron cyclotron resonance (ECR), inductively coupled plasma (ICP) and magnetron RIE (MRIE), has resulted in improved etch characteristics for the group-III nitrides as compared to RIE. This observation is attributed to plasma densities which are 2 to 4 orders of magnitude higher than RIE thus improving the III-N bond breaking efficiency and the sputter desorption of etch products formed on the surface. Additionally, since ion energy and ion density can be more effectively decoupled as compared to RIE, plasma-induced damage is more readily controlled. Figure 1 (b) shows a schematic diagram of a typical low profile ECR etch system. High-density ECR plasmas are formed at low pressures with low plasma potentials and ion energies due to magnetic confinement of electrons in the source region. The sample is located
downstream from the source to minimize exposure to the plasma and to reduce the physical component of the etch mechanism. Anisotropic etching can be achieved by superimposing an rf bias (13.56 MHz) on the sample and operating at low pressure (<5 mTorr) to minimize ion scattering and lateral etching. However, as the rf biasing is increased the potential for damage to the surface increases. Figure 2shows a schematic of the plasma parameters and sample position in a typical high-density plasma reactor. Pearton and co-workers were the first to report ECR etching of group-III nitride films []]. Etch rates for GaN, InN and AlN increased as either the ion energy (dc bias) or ion flux (ECR source power) increased. Etch rates of 1100 Å/min for AlN and 700 Å/min for GaN at -150 V dc bias
in a Cl
2/H
2
plasma and 350 Å/min for InN in a CH
4
/H
2
/Ar plasma at -250
V dc bias were reported. The etched features were anisotropic and the surface remained stoichiometric over    a wide range of plasma conditions. GaN ECR etch data has been reported by several authors with etch rates as high as 1.3 µm/min [1][2][5][6].
ICP offers another high-density plasma etch platform to pattern group-III nitrides. ICP plasmas are formed in a dielectric vessel
encircled by an inductive coil into which rf power is applied [see Figure 1(c)]. The alternating electric field between the coils induces a strong alternating magnetic field trapping electrons in the center of the chamber and generating a high-density plasma. Since ion energy and plasma density can be effectively decoupled, uniform density and energy distributions are transferred to the sample while keeping ion and electron energy low. Thus, ICP etching can produce low damage while maintaining fast etch rates. Anisotropy is achieved by superimposing of rf bias on the sample. ICP etching is generally believed to have several advantages over ECR including easier scale-up for production, improved plasma uniformity over a wider area and lower cost-of-operation. The first ICP etch results fo
r GaN were
reported in a Cl
2/H
2
/Ar ICP-generated plasma with etch rates as high
as ~6875 Å/min [7] . Etch rates increased with increasing dc bias and etch profiles were highly anisotropic with smooth etch morphologies over a wide range of plasma conditions. GaN etching has also been reported in a variety of halogen- and methane-based ICP plasmas [8].
Use of a Cl
reactive materials studies
2/Ar/O
2
chemistry produced good selectivity for GaN and
InGaN over AlGaN (up to ~50), due to formation of an oxide on the AlGaN [8].
MRIE is another high-density etch platform which is comparable to RIE. In MRIE, a magnetic field is used to confine electrons close to the sample and minimize electron loss to the wall. Under these conditions, ionization efficiencies are increased and high plasma densities and fast etch rates are achieved at much lower dc biases (less damage) as compared to RIE. GaN etch rates of ~3500 Å/min were reported in BCl
3
-based plasmas at dc biases <-100 V.The etch was fairly smooth and anisotropic.
2.3. Chemically-Assisted Ion Beam Etching.
Chemically assisted ion beam etching (CAIBE) and reactive ion beam etching (RIBE) have also been used to etch group-III nitride films [6][7]. In these processes, ions are generated in a high-density plasma source and accelerated by one or more grids to the substrate. In CAIBE, reactive gases are added to the plasma downstream of the acceleration grids thus enhancing the chemical component of the etch mechanism, whereas in RIBE, reactive gases are introduced in the ion source. Both etch platf
orms rely on relatively energetic ions (200-2000 eV) and low chamber pressures (<5 mTorr) to achieve anisotropic etch profiles. However, with such high ion energies, the potential for plasma-induced damage exists. Adesida and co-workers reported CAIBE etch rates for GaN as high as 2100 Å/min with 500 eV Ar+ ions
and Cl
2
or HCl ambients [6]. Rates increased with beam current,
reactive gas flow rate and substrate temperature. Anisotropic profiles with smooth etch morphologies were observed.
2.4. Reactive Ion Beam Etching.
The RIBE removal rates for GaN, AlN and InN are shown in Figure 3 as
a function of Cl
2percentage in Cl
2
/Ar beams at 400 eV and 100 mA
current. The trend in removal rates basically follows the bond
energies of these materials. At fixed Cl
2
/Ar ratio, the rates increased with beam energy. At very high voltages, one would expect the rates to saturate or even decrease due to ion-assisted desorption of the reactive chlorine from the surface of the nitride sample before it can react to form the chloride etch products.
There was relatively little effect of either beam current or sample temperature on the RIBE removal rates of the nitride. The etch profiles are anisotropic with light trenching at the base of the features. This is generally ascribed to ion deflection from the sidewalls causing an increased ion flux at the base of the etched features.
2.5. Low Energy Electron Enhanced Etching.
Low energy electron enhanced etching (LE4) of GaN has been reported by Gilllis and co-workers [9]. LE4 is an etch technique which depends on the interaction of low energy electrons (<15 eV) and reactive species at the substrate surface. The etch process results in minimal surface damage since there is negligible momentum transferred from
the electrons to the substrate. GaN etch rates of ~500 Å/min in a H
2
-
based LE4 plasma and ~2500 Å/min in a pure Cl
2
LE4 plasma have been reported [9]. GaN has also been etched using photoassisted dry etch processes where the substrate is exposed to a reactive gas and ultraviolet laser radiation simultaneously. Vibrational and electronic excitations lead to improved bond breaking and desorption of reactant products. GaN etch rates are compared in Figure 4 for RIE,
ECR and ICP Cl
2/H
2
/CH
4
/Ar plasmas as well as a RIBE Cl
2
/Ar plasma. CH
4
and H
2
were removed from the plasma chemistry to eliminate polymer deposition in the RIBE chamber. Etch r
ates increased as a function of dc bias independent of etch technique. GaN etch rates obtained in the ICP and ECR plasmas were much faster than those obtained in RIE and RIBE. This was attributed to higher plasma densities (1-4 orders of magnitude higher) which resulted in more efficient breaking of the III-N bond and sputter desorption of the etch products. Slower rates observed in the RIBE may also be due to lower operational pressures (0.3 mTorr compared to 2 mTorr for the ICP and ECR) and/or lower ion

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