Jeffrey A. Gregus

Multiple steady states in electron cyclotron resonance plasma reactors

Although electron cyclotron resonance (ECR) plasma reactors are being used in the microelectronics industry for etching and deposition of thin films they are prone to instabilities that can wreak havoc with manufacturing processes. Plasma conditions are often unstable because of nonlinear coupling between wave propagation, power absorption, neutral density, and charge density profiles. We report on hysteresis, multiple steady states and abrupt transitions in an ECR plasma reactor that can alter the plasma properties drastically. Substrate platen floating potential and Langmuir probe measurements are used to identify several abrupt transitions and regions in the operating parameter space where the plasma can exist in either one of two different states under identical operating conditions (microwave power, pressure, flow rate, magnetic field, rf bias etc.) Depending on how the plasma is started either branch can be obtained. Abrupt transitions and hysteresis in plasma properties are observed both with and w...

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.

Electron cyclotron resonance plasma reactor for cryogenic etching

Electron cyclotron resonance (ECR) plasma reactors are being used for ultralarge scale integrated circuit fabrication to meet the stringent requirements on submicron feature etching. Three issues are critical for ECR reactor design: plasma uniformity, ion energy control, and wafer temperature control. Plasma uniformity is important for minimizing over etch times and reducing the probability of producing charging damage. Ion energy control is needed to optimize etching rate, anisotropy, and selectivity without compromising device yield. Wafer temperature control is important because large ion currents at low pressure can result in wafer heating and thereby alter the rates of surface chemical processes. An ECR plasma reactor is described that is designed to etch compound semiconductors and Si at low temperatures (−170 to 20 °C), where superior selectivity and linewidth control are achievable. By measuring dc bias, floating potential, and ion saturation current densities it is shown that ion energies in this...

Low-temperature plasma etching of GaAs, AlGaAs, and AlAs

Dry etching of compound semiconductors is becoming increasingly important as design ruler shrink for electronic devices. For photonic device applications, dry or plasma etching is used fin- device isolation, fine-line pattern transfer, and fabrication of optical quality interfaces. As has been well established for Si and W. plasma etching at reduced temperatures can provide superior critical dimension control and obviate the need for operating at high bias voltages that produce excessively energetic ion bombardment t. In this work, we explore low-temperature (−60 C to +60 C) etching of the compound semiconductors GaAs, AlGaAs, and AlAs, In addition to improving etch anisotropy, which provides critical dimension control, rye find thut processing at lower temperatures improves microuniformity and reduces loading effects. At high lemperaturcs, where larger samples are observed to etch more slowly than smaller pieces (loading effect), etching rates appear limited bv reactant transport to the wafer. In this regime, both microuniformity and macrouniformity arc poor. As the temperature is reduced, the etching rate becomes limited by surfitce processes us a residue containing the semiconductor elements, etchant gases, and residual background gases forms on the surface. hi this regime, the etch rare becomes independent of surface area and uniformity is improved.

Control of an unstable electron cyclotron resonance plasma

Although plasmas are used throughout the microelectronics industry for etching, deposition, and cleaning of thin films, control of plasma processes has been a long‐standing problem. Because of the nonlinear properties of plasmas, such as the coupling between wave propagation, density profile, and power absorption, plasma reactors are prone to unstable operation, multiple steady states, and hysteresis. We report observation and suppression of an abrupt transition in the operating mode of an electron cyclotron resonance reactor that alters the ion flux to device wafers by more than twofold. While the origin of this mode change is not well understood, we show here that it is strongly correlated with the neutral gas density, which slowly decreases as the reactor temperature increases during a process or from run to run. By measuring the quartz liner temperature and adjusting the pressure to maintain an approximately constant neutral gas density, the mode change can be avoided indefinitely. In a simulated manu...

Real-time latent image monitoring during holographic fabrication of submicron diffraction gratings

Real‐time monitoring of latent images in photoresist during the holographic exposure of submicron period diffraction gratings is reported. The first‐order diffraction efficiency is recorded as a function of time for different resist thicknesses and polarizations. By monitoring the diffraction efficiency in real time, the process can be controlled to compensate for variations in exposure dose, resist thickness, and the optical properties of underlying materials. It is demonstrated that latent image monitoring (LIM) can be performed in real time and that the diffraction efficiency correlates well with the linewidth. To interpret diffraction efficiencies with changes in resist thickness and polarization, rigorous coupled wave analysis combined with a commercial software package to simulate the real‐time data was used. However, the data show evidence for slow changes in the composition of the resist during exposure that are consistent with chemical intermediates reacting with water in the film. The commercial...

Soft x-ray projection lithography: experiments and practical printers

The feasibility of using X-ray projection cameras as a practical lithography tool for making integrated circuits with tenth-micron features was investigated in experiments performed with a 20-fold reduction Schwarzschild camera operating at 36 nm and 14 nm, and with a 1:1 magnification Offner ring field system at 40 nm. The paper examines the requirements on the resist, the source, the camera design and the fabrication of its mirrors, the mask, and the alignment system. The experiments proved that high-reflectance multilayer mirrors are capable of diffraction limited imaging. Some problems exposed by the experiments, such as the deposition of carbon on surfaces exposed to X-rays, are discussed.

Experiments in projection lithography using soft x-ray

Abstract We have demonstrated soft x-ray projection lithography using radiation at wavelengths of 14 nm and 37 nm with a commercially available 20X reduction Schwarzschild camera. Line widths as small as 0.05 microns have been printed. The resolution obtained was essentially diffraction limited. Iridium coated mirrors were used with 37 nm radiation and Mo/Si multilayer coated mirrors with 14 nm radiation. A 1:1 magnification Offner Ring-field system with iridium coated mirrors has been used with 42 nm radiation. This optic has imaged line widths as small as 0.2 microns, which is close to the diffraction limit for this system. Transmission masks were used for all these experiments and the radiation was obtained from an undulator in the Vacuum Ultraviolet Synchrotron Storage Ring at Brookhaven National Laboratory.