Papers

AVS1997 Session PS1-TuA: Compound Semiconductor Etching

Tuesday, October 21, 1997 2:00 PM in Room A5/6

Tuesday Afternoon

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2:00 PM		PS1-TuA-1 Recent Advances in Light-Emitting Diode Technology R.M. Fletcher (Hewlett-Packard Company) Light-emitting diodes (LEDs) have now been in use for about 30 years. This year, in fact, marks the 25th anniversary of the HP-35, the first scientific handheld calculator, which used a GaAsP red LED numeric display. The pocket calculator was one of the first high-volume markets for LEDs, and this is why Hewlett-Packard first entered the optoelectronics business. A lot has changed in those 25-plus years, and advances in LED technology beyond those dim slivers of red light have opened up a wide variety of new applications and markets. The development of bright AlGaAs and AlInGaP red, orange, and amber chips has placed LEDs in locations where incandescent light bulbs dominate, such as next to highways in variable message signs, overhead in red traffic signals, and at the tail end of automobiles in the center mount brake lights. The clear advantages of LEDs over incandescent filament lamps are lower power consumption, better reliability, and longer life. More recent has been the development of bright blue and green LEDs using GaInN technology. With this development, the stage is set for increasing the number of areas where LEDs can replace incandescent lights in everyday applications. But beyond this are some new ideas for applications which make the future for LEDs very exciting. We will present an overview of the recent developments in LED technology and follow with a description of some of the new applications where LEDs will begin to appear over the next months and years. Also, some of the challenges encountered in the manufacture of LEDs and, in particular, the need for plasma processing and dry etching techniques will be discussed.
2:40 PM		PS1-TuA-3 Response Surface Study of Inductively Coupled Plasma (ICP) Etching of GaAs/AlGaAs in BCl₃/Cl₂ S. Agarwala, O. King, S. Horst, D. Stone (Laboratory for Physical Sciences); M. Dagenais (University of Maryland, College Park); Y.J. Chen (University of Maryland, Baltimore County) The advantage of using high-density plasma etching systems, such as inductively coupled plasma (ICP) and electron cyclotron resonance (ECR), over conventional parallel plate reactive-ion etching (RIE) is well documented. Although there have been numerous reports on ICP etching of Si-based materials and devices, very little work on the etching characteristics of compound semiconductors in ICP systems has been reported. Furthermore, these few studies were based on the traditional compilation of results using the instinctive one-factor-at-a-time strategy. In the current work, we will present the results of our investigation using a response surface method which is a much more efficient and comprehensive way of evaluating etching characteristics in a given parameter space. A central composite design was used to study etching characteristics of GaAs/AlGaAs in BCl₃/Cl₂-based plasma as a function of process parameters such as inductive power, substrate bias power, pressure, and gas composition. From analysis of the results of the designed experiments, parameter space regions are identified for applications including feature, mirror, and via-hole etching. Etch rates of GaAs/AlGaAs increased with inductive power and rates up to a few microns per minute were obtained. Bias power had little effect on the GaAs/AlGaAs etch rate. However, etch selectivity (GaAs to photoresist mask) increased by two orders of magnitude with decreasing bias power. Etched profiles examined by scanning electron microscopy (SEM) ranged from isotropic to highly anisotropic depending not only on the substrate bias but also on the gas composition. Atomic force microscopy (AFM) revealed smooth etched surfaces comparable to the as-grown surface under optimum etching conditions. Based on these results, proposed mechanisms of GaAs/AlGaAs etching in high density chorine-based plasmas will also be presented.
3:00 PM		PS1-TuA-4 The Application of High Density Plasma Sources for Optoelectronic Device Fabrication B. Humphreys, M.T. Govett, A.L. Goodyear (Oxford Instruments Plasma Technology, United Kingdom) The growing demand for increasingly sophisticated III-V optoelectronic devices has resulted in greater scrutiny being placed on the role of plasma-induced damage in the degradation of the optical and electrical properties of the material and device. This paper reports on the use of high density plasmas (HDPs) for low-damage etching (<100eV) of GaN, InP and GaAs based structures for optoelectronic applications. Etch rates in excess of 1 µm/min were achieved under low bias conditions using a variety of chemistries based on chlorine and chlorine / hydrocarbon mixtures. Surface morphology was seen to be a function of chlorine / hydrocarbon ratio especially at low ion energies whereby removal of polymer was less efficient. The addition of a polymer inhibitor resulted in much greater process flexibility allowing the use of lower bias levels without a drastic reduction in process etch rate. Processing was carried out on samples masked with SiO₂, Ni and photoresist, each of which proved suitable for producing vertically etched sidewalls.
3:20 PM		PS1-TuA-5 High Resolution, Low Damage Patterning of AlGaN/GaN Heterostructures on SiC and Sapphire Substrates by Low Energy Electron Enhanced Etching (LE4) in a DC Plasma H.P. Gillis (University of California, Los Angeles); D.A. Choutov, K.P. Martin (Georgia Institute of Technology) Fabricating III-N heterostructure devices requires dry etching processes for the formation of laser diode mirror facets, mesa isolation, and recessed gate and ohmic contacts. The etching process must produce straight sidewalls, leave smooth and stoichiometric surfaces, be selective, and cause minimum damage to the remaining material. To meet these fabrication needs, we are extending our Low Energy Electron Enhanced Etching (LE4) process, which has already etched GaN single layers grown on SiC and sapphire producing stoichiometrically unchanged surfaces with 2.5 Å RMS roughness, at etch rates exceeding 150 nm/min.¹ The etched features showed straight sidewalls with no trenching or overcut. LE4 results obtained in chlorine/hydrogen plasma will be presented for AlGaN/GaN double heterostructures on SiC and for multi-layer AlGaN/GaN heterostructures on sapphire for power FET applications. Special emphasis will be placed on conditions for equi-rate etching, for selectivity, and for identifying etch stops as gas composition and temperature are varied. ¹H.P. Gillis, D.A. Choutov, K.P. Martin, M.D. Bremser, and R.F. Davis, J. Electr. Mat., vol. 26, 301 (1977).
3:40 PM		PS1-TuA-6 High-Rate Etching of Ga-Containing Compounds for High-Brightness LEDs using ECR-Etching and CCP-RIE G.F. Franz (Siemens, Germany) GaAs and its related compounds like AlGaAs and GaN exhibit high etch rates in chlorine containing plasmas. This is due to the volatility of the formed chlorides and the chemical reactivity of the chlorine molecule itself and its positively charged ions, Cl₂⁺ and Cl⁺. Since Cl easily forms negatively charged ions, not only the electron density is reduced by more than one order of magnitude compared to Ar discharges but also the electron temperature is significantly lower. To enhance the density of positively charged ions, high density plasmas (ICP and ECR) are in common use. Adding an electron-deficient species to the gas is the chemical approach. Best suited is BCl₃ whereas F-containing molecules like BF₃ lead to an etch stop in AlGaAs layers. An enhancement is expected by the bimolecular reaction of BCl₃ with Cl₂ to form BCl₄^- and Cl⁺ which happens besides the heterolytic dissociation of chlorine. Although at constant total gas flow the amount of Cl₂ is reduced if BCl₃ is added, the actinometrically corrected signal of Cl⁺ remains constant over a vast range of gas composition. Moreover, the growth of a band system is observed which we assign to the anti-bonding state of Cl₂. Normally, the concentration of a species cannot be simply deduced from its spectral intensity. However, this coherence can be shown for the volatile Ga-containing compounds formed in these plasma reactions. This drastic enlargement of active species is reflected in the very high etch rates obtained in capacitively-coupled and ECR plasmas. Etch rates of more than 1 µm/min in GaAs are easily obtained associated with very high radial uniformity across several inches. In GaN, the etch rates are lower but remain also some hundred nm/min in both systems. Additionally, the anisotropy is remarkably high. Vertical edges some 50 or 70 microns deep can be etched. Therefore, this process is also suited to etch via-holes into semi-GaAs.
4:00 PM		PS1-TuA-7 Surface Chemistry and Damage in the High Density Plasma Etching of III-V Materials D. Leonhardt, C.R. Eddy, Jr., V.A. Shamamian, R.T. Holm, O.J. Glembocki (Naval Research Laboratory); B.D. Thoms (Georgia State University); J.E. Butler (Naval Research Laboratory) Anisotropic pattern transfer in compound semiconductor dry etching requires an in depth understanding of the chemical processes that occur at the plasma/semiconductor interface which promote the removal of volatile product species. In-situ mass spectrometry has been used to study product evolution during high density plasma etching of III-V semiconductors in a Cl2/Ar chemistry. Through definitive surface temperature control and configuration of the mass spectrometer to sample through the substrate platen, a comprehensive picture of the etch process is obtained. Validity and caveats of these techniques will be discussed. Evolution of etch products of GaAs (AsCl_x and GaCl_x) is monitored as neutral flux (pressure), ion flux (microwave power) and ion energy (substrate bias) are varied to pinpoint conditions where ion-driven surface chemistry is dominant. Observations show that sufficient fluxes of atomic chlorine neutrals and ions are required at the substrate to maximize etch product formation. These conditions are optimally met at low microwave powers (≤ 300W) and pressures (≤ 1.0 mtorr) in our system. The ion energy dependence of product formation shows regions of thermal/chemical etching for energies less than 50 eV, ion-assisted chemical etching for energies between 50 and 200 eV, and sputtering for energies greater than 200 eV. Ideal processing conditions are ascertained from the trade off between maximum etch rate and minimal electronic damage. Results from similar experiments on GaN etching will also be presented.
4:20 PM		PS1-TuA-8 In-Situ Photoreflectance Studies of Dry Etch Damage and Passivation of GaAs in a Chlorine/Ar Electron Cyclotron Resonance Generated Plasma O.J. Glembocki, R.T. Holm, C.R. Eddy, D. Leonhardt, D.S. Katzer (Naval Research Laboratory) Photoreflectance (PR) has been performed in-situ to study etch induced surface damage and subsequent passivation of GaAs that is etched in a chlorine/Ar plasma generated by an electron resonance cyclotron (ECR) source. Test layers consisting of 150nm of undoped GaAs were grown by molecular beam epitaxy on n+ and p+ GaAs substrates were used to enhance the sensitivity to the electronic properties of the etched and passivated surfaces. From the PR line shapes, we to obtain the position of the Fermi-level at the surface and detect the presence of surface states. Our measurements clearly show that the nature of the etch induced surface damage in the n-type is different from p-type. In the n-type, the Fermi-level position moves from midgap, toward the conduction band, while in the p-type, the Fermi-level moves from near the valence band edge toward midgap. This suggests that different defect states are formed during etching of n-type and p-type. Further evidence of this is obtained by comparing the in-phase and quadrature components (relative to the PR pump laser) of the PR signal which show that the defect states of p-type are much slower than those of n-type. In-situ chlorine passivation is shown to be effective in removing the etch induced defect states and returning the Fermi-level to pre-etch conditions. Here again, we observe very different behavior between n-type and p-type. We find that n-type is easily passivated in a chlorine environment, even in the absence of any plasma. For p-type, we find that an low energy (20 eV) chlorine plasma is required to achieve passivation of the etch damage. Our results will be discussed in terms of a model utilizing two surface defects and the interaction of these defects with light and chlorine.
4:40 PM		PS1-TuA-9 Proposal for an Etching Mechanism of InP in CH₄-H₂ Mixtures Based on Plasma Diagnostics and Surface Analysis Ch. Cardinaud, Y. Feurprier, B. Grolleau, G. Turban (CNRS Institut des Matériaux de Nantes, France) Reactive ion etching of InP in CH₄-H₂ mixtures is of particular interest when the goal is to obtain smooth surfaces, and high etch rate selectivity with respect to the mask. Well known features concern the loss of P from the surface and lattice damage due to ion bombardment. The mechanism of etching in these mixtures is generally assumed to proceed via the formation of volatile PH₃ and In(CH₃)₃. However, to our knowledge, no complete description of the mechanism has ever been given. Recently, we have shown that the combination of optical emission spectroscopy and mass spectrometry with quasi in-situ X-ray photoelectron spectroscopy gives very fine informations on the plasma - surface interaction¹. Using these elements and new results we propose now an analytical model for the etching process mechanism. The etch rate is controlled by the removal rate of In, which itself varies nearly linearily with the CH₃ flux on the surface. On another hand the formation rate of PH₃ depends mostly on the ion energy flux. XPS results indicate a less damaged and a less P-depleted surface when increasing the CH₃ flux or decreasing the ion energy flux, and are compatible with an exponential gradient of concentration of P in the top layers. The proposed mechanism can be defined as an "ion-assisted chemical etching mechanism" : the limiting step is the removal of In, the thickness and composition of the layer in the course of etching is controlled by a balance between the ion energy flux and the CH₃ flux. The model yields to a value of .7 to 1.0 In atom etched per incident CH₃. ¹Y. Feurprier, et al, Plasma Source Sci. & Technol. (accepted), Y. Feurprier, et al, J. Vac. Sci. Technol. B (under revision)
5:00 PM		PS1-TuA-10 Surface ModificationN of n-GaAs by Hydrogen and Methane/Hydrogen RF Plasmas J.L. Sullivan, S.O. Saied, R. Layberry, M.J. Cardwell (Aston University, United Kingdom) Hydrogen/methane or hydrogen-based plasmas are frequently used in low damage etching and other processing applications for III-V semiconductor surfaces. This paper reports on an XPS and SIMS study of the stoichiometric changes induced in GaAs(100) surfaces due to exposure to radio frequency hydrogen and hydrogen methane plasmas. Plasma power was varied from 50 to 150 W, pressures varied from 10 to 100 mbar in steps of 10 mbar and in the case of ad-mixtures methane to hydrogen ratios varied from 1:30 to 1:3. For each plasma condition, plasma and sample potentials were measured using electrostatic probes. Plasma species and species energies were monitored using a Hiden energy resolved mass spectrometric probe. Surface stoichiometric changes were then measured for each sample surface using X-ray photoelectron Spectroscopy (XPS) and angle resolved XPS. SIMS was used to gain hydrogen depth profile data. Synthesis of the Ga, As and O photoelectron peaks then allowed the changes in chemical state of these elements to be measured as functions of plasma pressure, potential and power. Large changes in stoichiometry of the GaAs were found to occur with Ga to As ratios varying from about 4 : 1 to 0.7 : 1 depending on conditions. This may be partly explained by preferential chemical reduction of the native oxides, but not wholly so. Relatively large changes were also observed in the Ga to As ratios in the GaAs substrate and these are explained in terms of thermodynamic and chemically driven segregation mechanisms, which were in turn related to calculated penetration profiles of the bombarding species.