John W. Lewellen

Effect of PEB temperature profile on CD for DUV resists

The effect of varying time and temperature profile at the PEB step on a 250 nm isolated line is studied for an Acetal and an ESCAP type Deep UV (DUV) resist. Experimental studies on the Acetal resist resulted in very non-linear Critical Dimension (CD) sensitivities with CD variation largest for low Post Expose Bake (PEB) temperature and short PEB time. A global CD model was created by fitting experimental data to a first order kinetics equation. An effective activation energy of 50 Kcal/mole was obtained for the Acetal resist whereas PROLITH simulations for an ESCAP type resist gave 69 Kcal/mole. These results are consistent with the well-documented diffusion- controlled deprotection reaction taking place during the PEB process. The global CD model was then used to investigate the impact of transient and steady-state temperature profiles on CD control. In order to achieve this goal, actual 2 dimensional wafer temperature profiles were input to the global CD model and PEB-induced CD variation was calculated during each sampling period of the 17 temperature readings across the wafer as a function of time. The resultant time- evolution of the PEB-induced CD variation was used to infer the relative importance of the transient and steady-state component of temperature profiles. At the low PEB temperature of 90 degrees Celsius for the Acetal resist, transient effects dominate for a nominal 90 second process. Slower deprotection reaction yields large CDs at 356 nm and predicted PEB-induced CD variation of 6.9 nm. At 100 degrees Celsius, transient effects are less prevalent but still present. Tight steady- state and transient temperature uniformity along with wafer- wafer temperature profile repeatability and stringent control of process timing and delays are important. At 110 degrees Celsius, transient effects are dominant early in the process and they are completed within about 50 seconds in this model. However as photoacid is lost to competing reactions and the time necessary for deprotection is extended, even in this case transients could still play a part in final CD results. In general, both mean CD and CD variation results improve with increasing PEB temperature for a 90 second PEB time. PROLITH simulations using ESCAP resist parameters and actual measured CD profiles both support these model predictions. Improved resist formulations along with advanced photoresist processing tool layout, wafer-handler scheduler and PEB module designs are all necessary ingredients for minimum PEB-induced CD variation.

Novel spin-coating technology for 248-nm/193-nm DUV lithography and low-k spin on dielectrics of 200-mm/300-mm wafers

An alternative coating technology was developed for 248 nm/193 nm DUV lithography and low-k spin on dielectric (SOD) materials used in the interconnect area. This is a 300 mm enabling technology which overcomes turbulent flow limitations above 2000 rpm and it prevents 40 - 60% reduction on the process latitudes of evaporation-related variables, common to 300 mm conventional coaters. Our new coating technology is fully enclosed and it is capable of controlling the solvent concentration above the resist film dynamically in the gas phase. This feature allows a direct control of the evaporation mass transfer which determines the quality of the final resist profiles. Following process advantages are reported in this paper: (1) Demonstrated that final resist film thickness can be routinely varied by 4000 angstrom at a fixed drying spin speed, thus minimizing the impact of turbulence wall for 300 mm wafers. (2) Evaporation control allows wider range of useful thickness from a fixed viscosity material. (3) Latitudes of evaporation-related process variables is about 40% larger than that of a conventional coater. (4) Highly uniform films of 0.05% were obtained for 8800 angstrom target thickness with tighter wafer-wafer profile control because of the enclosed nature of the technology. (5) Dynamic evaporation control facilitates resist consumption minimization. Preliminary results indicate feasibility of a 0.4 cc process of record (POR) for a 200 mm substrate. (6) Lower COO due to demonstrated relative insensitivity to environmental variables, robust resist consumption minimization and superior process capabilities. (7) Improved planarization and gap fill properties for the new generation photoresist/low-k SOD materials deposited using this enclosed coating technology.

Characterization of advanced DUV photoresists

The first objective of this project was to characterize the lithographic performance of an advanced Acetal-based DUV resists. This resist is targeted for 0.25-0.18 micron geometries. The sensitivity of film thickness and uniformity to critical process parameters such as final dry spin time in the coater, soft bake time, soft bake temperature, method of soft bake, exhaust and vacuum levels in the soft bake were investigated. The second objective was to compare the material and lithographic performance properties of the advanced Acetal-based DUV resist, an advanced ESCAP-based DUV resists, and a third generation i-line resist. An advanced SVG track system that was clustered with an advanced Micrascan was used in these experiments. This lithography cluster tool represents a typical high volume production tool for device features of 0.25-micron and below.

Novel wafer track-based resolution enhancement technology for 248-nm DUV lithography

248 nm DUV lithography has become a mainstream production technology for sub-0.25 micrometer feature sizes due to its superior process technology and improved cost of ownership (COO). As the semiconductor industry moves to sub-0.18 micron critical layer feature sizes, there is an enormous economic incentive to extend 248 nm technology towards smaller geometries. Traditionally, resolution enhancement technologies such as various illumination types (off-axis, annular ring and quadrapole) and phase shifted masks are based on the optimization of the diffracted aerial image wave front and they concentrate on the exposure tools and masks. In this paper we report a new novel resolution enhancement technology based on the wafer track. We have demonstrated that this new technology, along with the scanner-based resolution enhancement techniques, can substantially improve resolution capabilities and process latitudes of the 248 nm technology. Consequently, semiconductor manufacturers will be able to extend 248 nm DUV technology for smaller feature sizes than was possible before. Our new resolution enhancement technology allowed us to increase the contrast of an acetal-based DUV resist from about 5 to 13 by carefully controlling the environment during the post exposure bake process. This technology provided a continuous contrast knob that could be controlled and set based on the application. Increased photoresist contrast in turn made it possible to pattern 0.15 micrometer isolated lines and 0.2 micrometer dense lines using a 248 nm mercury lamp-based scanner with projection optics designed for 0.35 micrometer features. A DOF of 0.8 micrometer for 0.25 micrometer dense lines was achieved using this PEB- based resolution enhancement technology whereas conventional technology could not resolve 0.25 micrometer dense lines at all. Detailed lithographic characterization identified a 30% improvement in process latitude. Furthermore, cross sectional SEM studies revealed high quality CD profiles and measured high side wall angles with resolution-enhanced patterns. The cross sectional SEM studies also suggested that the new resolution enhancement technology also reduces the iso-dense bias. Detailed Prolith simulations support these experimental results. Mechanistic understanding along with the experimental data and simulation results will be presented in this paper.