Robert L. Hsieh

Effect of central obscuration on image formation in projection lithography

Central obscuration of the pupil is a prominent feature of many high performance reflective designs being considered for sub-200nm lithography. The performance of centrally-obscured designs were investigated using computer simulations of projected image intensity and major features from simulation were experimentally confirmed. The effect of partially-coherent illumination on centrally-obscured systems was studied and an optimized annular illumination system is proposed. 1.

Markle-Dyson optics for 0.25 μm lithography and beyond

Using a modified Dyson design working at 0.7 NA with 248 nm illumination, reflective 1X masks, and conventional single‐layer resists, 0.25 μm lithographic resolution has been obtained over a semicircular field 4 mm in diameter. 0.19 μm lines and spaces have been printed using an experimental bilayer resist. To offset the small depth of focus, two precision autofocus schemes have been demonstrated and FLEX has been shown by computer simulation to triple the depth of focus for isolated features. A full size system has been designed with a 20×40 mm2 field sufficiently large for patterning 256 Mbit DRAM chips. This concept can be extended to shorter wavelengths limited only by availability of one refractive material and a suitable light source. Thus optical projection lithography appears feasible for features at least as small as 0.15 μm.

A catadioptric reduction camera for deep UV microlithography

Abstract A 5X catadioptric reduction camera for use at wavelengths below 200 nm is described. It is based on a Cassegrainian mirror configuration, and in addition, uses 2 lenses to correct for Petzval curvature and spherical aberration. An image field 2.4 mm in diameter can be covered at 0.4 NA. Larger fields upto 20 mm in diameter can be realised, but only with large optics with a diameter of about 1.5 m.

Rational argument for the impracticability of 1× reticles

Even at 0.5 micrometers design rules, the specifications on 5X reticles are set extremely tightly, on the grounds that ULSI patterns are so complex that tight specifications are essential to obtain acceptable yield. It is usually assumed that these specifications scale with the design rules, and that they should be even tighter for 1X reticles. As a consequence, it has been argued that 1X reticles for 0.25 micrometers design rules are impracticable. A statistical analysis, starting from first principles, and assuming point independent, normally distributed errors, supports the way in which mask specifications are currently set. The assumptions of spatial invariance and normal distribution are crucially important in the analysis. However, it is far from clear that they are valid. Consequently, mask specifications in general, as they are currently set, may be unnecessarily severe.

All‐reflective phase‐shifting masks for Markle–Dyson optics

Previous work using a Markle–Dyson projection system, operating at 248 nm wavelength, demonstrated 0.19 μm resolution with nonphase‐shifting masks. The use of Levenson‐type phase‐shifting masks should in principle, allow sub‐0.1 μm feature resolution using a k1 of 0.25. To investigate this possibility, a novel reflective phase mask was fabricated and used on the Markle–Dyson system to projection print 0.125 μm lines and spaces in photoresist. An analytical study has been carried out to determine the tolerance of alternating‐phase masks to errors in linewidth and phase.

Silicon on quartz reflective masks for 0.25‐μm microlithography

One approach to 0.25‐μm lithography currently being explored is the unity‐magnification Markle‐Dyson projection system. This system, operating at λ=248 nm, incorporates a reflective mask in its design. This utilizes the internal reflection at the quartz/film interface. To best meet the requirements of high reflectivity and quarter micron processing, amorphous silicon on quartz masks were chosen. Completed masks were used to print 0.19‐μm lines and spaces on the prototype Markle–Dyson system, demonstrating the feasibility of silicon reflective masks for lithography.

Lithography for 0.25 mu m and below using simple high-performance optics

A mostly reflective approach to 0.25- mu m lithography that has great simplicity (only two or three critical optical elements) and outstanding performance is described. A 1/6 scale prototype system has demonstrated 0.25- mu m resolution in a commercially available resist using a conventional mask, and 0.125- mu m resolution using a phase-shifting mask. The approach is particularly amenable to depth of focus enhancement by aperture apodization, and a fundamental trade-off inherent in this technique is described.<<ETX>>

Reflective masks for 1X deep ultraviolet lithography

Recent work has demonstrated the high resolution optical performance possible with simple IX mostly-reflective optics: using 248nm light from a mercury arc lamp, 0.25pm features were delineated across a 2mm radius semicircular field, and much larger fields are possible with a scaled up version.[1] The mask required for this system consists of a quartz substrate, a patterned thin film reflector and a non-reflective backing which also serves to protect the reflector film. The mask is reflective at the quartz/reflector interface so the substrate is part of the projection optical path and so acts as the pellicle. We have investigated chromium, silicon and aluminum for the reflector material; their reflection coefficients at 248nm at the quartz- reflector interface are 30, 55, and 90 per cent respectively. Silicon has been chosen because it has a practical combination of reflectivity and ease of deposition and etching. Moreover films as thin as 30?m provide the full (bulk-value) reflection and so precise etching is further facilitated. Among possible absorber materials, novolak photoresist is a practical choice having a quartz/film reflectivity of 1%. Features down to 0.25?m are regularly patterned for these masks with a MEBES I using Shipley SAL-601 or PMMA electron beam resist.