Henry T. Gaw

Printability of pellicle defects in DUV 0.5-um lithography

An image simulation program was developed and the corresponding experiment was performed to study the printability of particle on pellicle or pellicle defect for a DUV (X = 248 nm) stepper exposure. As an application of the program, we calculated the printability of pellicle defects for pellicles on both sides of the mask as a function of light source wavelength, numerical aperture, pellicle stand off distance, partial coherence, mask pattern feature size, defocus, etc. The results showed that at 248 nm wavelength exposure the change of aerial image at the wafer plane induced by the chrome side pellicle defect was larger than that of g-line (X=436 nm) or Mine exposure (X=365 nm). Simulation for a given pellicle defect size, placed at different side of mask indicated that glass side defect gave a larger change on the aerial image in terms of loss of image energy, and smaller change in terms of decrease in modulation transfer function (MTF) than that of chrome side. Experimental results performed on our DUV stepper substantiated the simulation results.

An analytical model for self-limiting behavior of hot-carrier degradation in 0.25 mu m n-MOSFET's

Hot-carrier degradation of short-channel n-MOSFETs becomes saturated after reaching a certain threshold value. The physical mechanism for this self-limiting behavior is investigated. It is proposed that the hot-carrier-induced oxide trapped charge and interface states form a potential barrier that repels subsequent hot carriers from causing further damage and can lead to the saturation of device degradation. A physical model is developed on the basis of the analysis. The model is verified by experimental results and can be used for more accurate device reliability projection.<<ETX>>

Experimental characterization and modeling of electron saturation velocity in MOSFETs inversion layer from 90 to 350 K

From saturation transconductance of devices of 0.25- mu m CMOS technology, the saturation velocity of electrons ( nu /sub sat/) in the inversion layer from 90 to 350 K has been determined. The extracted nu /sub sat/ at 300 K was 7.86*10/sup 6/ cm/s, which is significantly lower than that of bulk silicon ( nu /sub sat-blk/) and has a much weaker temperature dependence. The ratio nu /sub sat-blk// nu /sub sat/ is 1.27 at 300 K, and is increased to 1.68 at 90 K. Consistent values of nu /sub sat/ have been determined for devices of three vastly different MOS technologies, demonstrating the technology independence of nu /sub sat/. The results are useful for developing and testing theoretical carrier transport models, and are of practical importance in estimating the ultimate speed performance of surface MOSFETs. An empirical model for nu /sub sat/ as a function of temperature has also been derived for application in predictive device simulation.<<ETX>>

Optical lithography with chromeless phase-shifted masks

Chromeless phase-shifting is a novel concept that completely avoids the use of chrome for pattern formation in optical lithography. This scheme uses 180 degree(s) phase-shifters on transparent glass to define patterns. The method relies on the destructive interference between phase-shifters and clear areas at the edges of the phase-shifters to define dark or opaque areas on the mask. Gratings sufficiently small (named dark-field gratings) will produce sufficient interference to completely inhibit the transmission of light. The combination of these effects makes it possible to form a wide range of patterns, from line-space patterns to isolated bright or dark areas. The lithography simulators SPLAT and SAMPLE were used to understand the principles behind this new scheme, and to verify various pattern designs. Simulation and experimental results are presented to demonstrate the concept.

Process control with chemical amplification resists using deep ultraviolet and x‐ray radiation

Resists for optical electron beam, and x‐ray lithographies that operate on the principles of chemical amplification are entering into widespread use in laboratories. They offer an attractive alternative to conventional positive Novolac photoresists and we may soon see similar use in manufacturing environments. One class of resists within this family is based on acid‐hardening chemistry. We have characterized one such resist (Shipley ECX‐1033) for illumination by excimer laser deep ultraviolet sources and also by x‐ray radiation. A matrix of postexposure bake (PEB) conditions and development conditions was used to examine resist sensitivity and contrast. For all exposure sources we found that contrast is independent of PEB processing and that sensitivity obeys an Arrhenius dependence. Contrast increases with increasing development time while exposed resist loss is minor. A simple kinetic model was developed to explain the observed variations of apparent resist sensitivity with PEB time and temperature. The generality of this model suggests that it is widely applicable to other chemical amplification resists that require a postexposure bake. Together with aerial image calculations for different light sources, the model makes it possible to predict the dependence of linewidth on PEB conditions. In other words, we can anticipate the PEB process control required to achieve a specified critical dimension control. Electrical linewidth measurements of submicron features and their temperature dependence are compared with the predictions of the model.

Design methodology for dark-field phase-shifted masks

Simulation has been used to systematically investigate the effects of phase-shifters on dark- field patterns (openings in a dark-field mask), and to determine the phase-shifter configurations that are most effective for different mask patterns. This study has resulted in a design methodology based on the distance between the centers of clear features and the surrounding phase-shifters. A key verification is that isolated phase-shifted patterns print best when the distance from the center of the phase-shifter to the center of the feature is approximately 0.7 (lambda) /NA. At this optimal spacing, the peak image intensity, image slope and resist wall-angle of the printed pattern is maximized. Optimally-aligned phase- shifters will also have the best focus-exposure behavior of all the different dark-field phase- shifter configurations. However, dark-field phase-shifters will only provide a resolution increase on the order of 0.05 0.10 (lambda) /NA. Different dark-field mask configurations will print with different amounts of bias; the amount of print bias is dependent on the width of the phase-shifters and the distance between the phase-shifters and the feature. Response curves of resist opening as a function of feature size and phase-shifter/feature separation can be used to keep track of the amount of bias required in a given phase-shifter configuration.

Study of the relationship between exposure margin and photolithographic process latitude and mask linearity

The utility of exposure margin, defined as the ratio between the 1:1 mask reproduction exposure energy and the open frame threshold exposure energy (E0), as an indicator of process latitude, is probed using extensive computer simulations and some experimental photolithography. The correlation is shown to be excellent for latitudes which depend primarily on critical dimension such as mask linearity and exposure latitude: a high exposure margin implies a high process latitude. Qualitative physical arguments are offered to explain this. For sidewall angle constrained latitudes such as defocus, the correlation is also good if comparing photoresists with similar optical absorption characteristics. These results are potentially significant because exposure margin is easily measured and therefore provides an efficient means for process optimization (at zero mask bias). As part of this work a simulation procedure which reduces the focus: exposure: mask dimension latitude to a single process latitude volume was developed and is described.

New Energy-Dependent Soft X-Rav Damage In MOS Devices

An energy-dependent soft x-ray-induced device damage has been discovered in MOS devices fabricated using standard CMOS process. MOS devices were irradiated by monochromatic x-rays in energy range just above and below the silicon K-edge (1.84 keV). Photons below the K-edge is found to create more damage in the oxide and oxide/silicon interface than photons above the K-edge. This energy-dependent damage effect is believed to be due to charge traps generated during device fabrication. It is found that data for both n- and p-type devices lie along a universal curve if normalized threshold voltage shifts are plotted against absorbed dose in the oxide. The threshold voltage shift saturates when the absorbed dose in the oxide exceeds 1.4X105 mJ/cm3, corresponding to 6 Mrad in the oxide. Using isochronal anneals, the trapped charge damage is found to recover with an activation energy of 0.38 eV. A discrete radiation-induced damage state appears in the low frequency C-V curve in a temperature range from 1750C to 325°C.

Modeling spray/puddle dissolution processes for deep-ultraviolet acid-hardened resists

A study of the dissolution behavior of acid-hardened resists (AHR) was undertaken for spray and spray/puddle development processes. The Site Services DSM-100 end-point detection system is used to measure both spray and puddle dissolution data for a commercially available deep-ultraviolet AHR resist, Shipley SNR-248. The DSM allows in situ measurement of dissolution rate on the wafer chuck and hence allows parameter extraction for modeling spray and puddle processes. The dissolution data for spray and puddle processes was collected across a range of exposure dose and postexposure bake temperature. The development recipe was varied to decouple the contribution of the spray and puddle modes to the overall dissolution characteristics. The mechanisms involved in spray versus puddle dissolution and the impact of spray versus puddle dissolution on process performance metrics has been investigated. We used the effective-dose-modeling approach and the measurement capability of the DSM-100 and developed a lumped parameter model for acid-hardened resists that incorporates the effects of exposure, postexposure bake temperature and time, and development condition. The PARMEX photoresist-modeling program is used to determine parameters for the spray and for the puddle process. The lumped parameter AHR model developed showed good agreement with experimental data.

Modeling spray/puddle dissolution processes for DUV acid-hardened resists

An investigative study of the dissolution behavior of acid hardened resists (AHR) was undertaken for spray and spray-puddle development processes. A unique tool, the Site Services DSM-100 End-point detection system, is used to measure both spray and puddle dissolution data for a commercially available deep ultra-violet AHR resist, Shipley SNR-248. The DSM allows in- situ measurement of dissolution rate on the wafer chuck and hence allows parameter extraction for modeling spray and puddle processes. The dissolution data for spray and puddle processes was collected across a range of exposure dose and PEB temperature. The development recipe was varied to decouple the contribution of the spray and puddle modes to the overall dissolution characteristics. The mechanisms involved in spray versus puddle dissolution and their impact on process performance metrics have been investigated. The PARMEX photoresist modeling program is used to determine parameters for the spray and for the puddle process. A lumped parameter AHR model developed at Intel was used in iPHOTO for simulation studies.