Latent image diffraction from submicron photoresist gratings

Light scattering from latent images in photoresist is useful for lithographic tool characterization, process monitoring, and process control. In particular, closed‐loop control of lithographic processes is critical for high yield, low cost device manufacturing. In this work, we report use of pulsed laser diffraction from photoresist latent images in 0.24 μm pitch distributed feedback laser gratings. Gated detection of pulsed light scattering permits high spatial resolution probing using ultraviolet light without altering the latent image. A correlation between latent image and etched grating diffraction efficiencies is demonstrated and shows the value of "upstream" monitoring

Limits to ion energy control in high density glow discharges: Measurement of absolute metastable ion concentrations

Unprecedented demands for uniformity, throughput, anisotropy, and damage control in submicron pattern transfer are spurring development of new, low pressure, high charge density plasma reactors. Wafer biasing, independent of plasma production in these new systems is intended to provide improved ion flux and energy control so that selectivity can be optimized and damage can be minimized. However, as we show here, an inherent property of such discharges is the generation of significant densities of excited, metastable ionic states that can bombard workpiece surfaces with higher translational and internal energy. Absolute metastable ion densities are measured using the technique of self-absorption, while the corresponding velocity distributions and density scaling with pressure and electron density are measured using laser-induced fluorescence. For a low pressure, helicon-wave excited plasma, the metastable ion flux is at least 24% of the total ion flux to device surfaces. Because the metastable ion density scales roughly as the reciprocal of the pressure and as the square of the electron density, the metastable flux is largest in low pressure, high charge density plasmas. This metastable ion energy flux effectively limits ion energy and flux control in these plasma reactors, but the consequences for etching and deposition of thin films depend on the material system and remain an open question

Real-time, in situ monitoring of surface reactions during plasma passivation of GaAs

Real-time, in situ observations of surface chemistry during the remote plasma passivation of GaAs is reported herein. Using attenuated total reflection Fourier transform infrared spectroscopy, the relative concentrations of -As-O, -As-H, -H2O, and -CH2 bonds are measured as a function of exposure to the effluent from a microwave discharge through NH3, ND3, H2, and D2. The photoluminescence intensity (PL) from the GaAs substrate is monitored simultaneously and used qualitatively to estimate the extent of surface state reduction. It was found that, while the -CHx(x = 2,3) and -As-O concentrations are reduced rapidly, the rates at which the -As-H concentration and the PL intensity increase are relatively slow. The concentration of -H2O on the GaAs surface increases throughout the process as surface arsenic oxides and the silica reactor walls are reduced by atomic hydrogen. These observations suggest that removal of elemental As by reaction with H at the GaAs–oxide interface limits the passivation rate

Collisional depolarization of state selected (J,M J ) BaO A 1Σ+ measured by optical–optical double resonance

The optical–optical double resonance (OODR) technique is used to investigate the change in magnetic quantum number (M) a state selected molecule undergoes on collision with other molecules. A first linearly polarized dye laser prepares A  1Σ+BaO(v = 1) in the J = 1, M = 0 sublevel. The extent of collisional transfer to other M sublevels of both J = 1 and J = 2 is then probed by a second polarized dye laser which induces fluorescence from the C  1Σ+ state. Elastic collisions (ΔJ = 0) between BaO (A  1Σ+) and CO2 are observed to change M from 0 to ±1 leaving J unchanged. The total elasticM‐changing cross section is σΔM CO2 = 8.4±2.4 Å2. Inelastic collisions (ΔJ = +1’ which transfer molecules to j = 2 also cause M changes. with both Ar and CO2 as collision partners. M, the s p a c e‐f i x e d projection of J, is found to be neither conserved nor randomized. Quantum atom–diatom collision models with quantization axis along the relative velocity vector are considered. Transition amplitudes in this system are evaluated using the l‐dominant and CS approximations

Ammonia plasma passivation of GaAs in downstream microwave and radio-frequency parallel plate plasma reactors

The poor electronic properties of the GaAs surface and GaAs–insulator interfaces, generally resulting from large density of surface/interface states, have limited GaAs device technology. Room-temperature ammonia plasma (dry) passivation of GaAs surfaces, which reduces the surface state density, is investigated as an alternative to wet passivation techniques. Plasma passivation is more compatible with clustered-dry processing which provides better control of the processing environment, and thus, improves interface integrity. Passivation was monitored in real-time and in situ using photoluminescence (PL). In addition, the passivated surfaces are inspected using x-ray photoelectron spectroscopy. Passivation with two different plasma excitation methods, downstream microwave (2.45 GHz) and rf (13.56 MHz) parallel plate, are compared, and effects of operating parameters such as pressure, flow rate, and power are examined. In both methods plasma-generated H atoms reduce the surface state density by removing excess As and As2O3 during the first few seconds of the plasma exposure. This step is followed by formation of Ga2O3 which takes place on a longer time scale (5–10 min). While the final passivation result appears to be similar for both methods, surface damage by ion bombardment competes with passivation in the parallel plate method, reduces the PL yield and adversely affects the long term stability of the passivated surface. Although it is common to heat the sample during passivation, we show that NH3 plasma passivation is possible at room temperature without heating. Low-temperature processing is important since passivation can be done at the end of device processing when it is undesirable to expose the device to elevated temperatures. The absence of ion bombardment damage combined with efficient generation of H atoms in the downstream microwave treatment, make this scheme a preferred dry passivation process, which could be easily and inexpensively clustered with existing GaAs processes

Real-time monitoring of low-temperature hydrogen plasma passivation of GaAs

By monitoring photoluminescence (PL) in real time and in situ, hydrogen plasma operating conditions have been optimized for surface passivation of native-oxide-contaminated GaAs. PL enhancement is critically dependent on exposure time and pressure because of competition between plasma passivation and damage. Optimal exposure time and pressure are inversely related; thus, previous reports of ineffective passivation at room temperature result from overexposure at low pressure. Plasma treatment is effective in removing As to leave a Ga-rich oxide; removal of excess As increases the photoluminescence yield as the corresponding near-midgap-state density is reduced. Passivation is stable for more than a month. These results demonstrate the power of real time monitoring for optimizing plasma processing of optoelectronic materials

Microscopic and macroscopic uniformity control in plasma etching

By cooling substrates to low temperatures (–40 °C), plasma etching of AlGaAs/AlAs/GaAs structures is performed in an ion-activated, surface reaction limited regime. As a result, microscopic and macroscopic uniformity are vastly improved and etching is independent of gas flow patterns, plasma geometry, and reactor loading. Because the reactant is concentrated on the surface, etching rates remain large

Use of light scattering in characterizing reactively ion etched profiles

Currently, profile control in plasma etching of submicron structures requires inspection of cleaved samples by scanning electron microscopy. This is time consuming, destructive, and limited to a small subset of processed wafers. We show that light scattering can be used to rapidly characterize submicron differences in reactively ion etched, periodic Si structures. A similar approach has been used previously to monitor etching rates and undercutting using specular and first order diffraction peaks. Here, we measure all orders scattered over 180° as a function of incident angle and polarization and focus on the use of this technique coupled with statistical methodology to distinguish subtle changes in line profile. Although scatter from grating test patterns is examined here, this method should also be applicable to complex, submicron device structures

Atomic Resonance and Scattering

Contains reports on three research projects.U.S. Air Force - Office of Scientific Research (Grant AFOSR-76-2972)National Science Foundation (Grant CHE79-02967)National Science Foundation (Grant PHY79-09743