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Reticles Pick Up the Burden The inexorable drive of optical lithography continues to enable semiconductor manufacturers around the world to print increasingly smaller linewidths on larger and larger silicon wafers. Despite dire predictions that we have seen the “end,” optical lithography continues to push the envelope and stretch perceived limits with amazing regularity. The resulting ability for wafer fabs to delay investments in shorter wavelength exposure tools far beyond what was optimistically contemplated a decade ago has saved billions of dollars in capital. This has also enabled the semiconductor industry to shrink linewidths on an accelerated schedule, creating accelerated cash flows and staggering increases in shareholder value. The reticle has increasingly become a central player in this economic and technological windfall. The industry has transferred some of the burden of smaller wafer linewidths and critical dimension (CD) control from optical exposure tools to more precise and complex reticles. Because of bin yield, CD control is worth about $1 per chip per atom in linewidth precision. While there is much discussion regarding accelerated roadmaps for wafer processing, the dimensions that are important on a reticle have dropped at least twice as fast as those on the wafer, to compensate for the slowdown in adoption of next wavelength exposure tools. The burden, and the source of value, has shifted. Reticles have become amazingly more complex, endowed with billions of optical proximity correction (OPC) features and enhanced by phase shift mask (PSM) topography that minutely adjusts the wavefront of deep ultraviolet (DUV) light. Stretching physics in what is termed “sub-wavelength lithography,” we now print lines smaller than the wavelength of the DUV light used to expose them. 4
Autumn 2000
Yield Management Solutions
Unfortunately, in sub-wavelength lithography small errors on the reticle are multiplied by a mask error enhancement factor that magnifies the effect of defects and CD errors. In a reticle image measuring 1 million by 1 million pixels (representing a photo-mask for 0.13 micron technology from an inspection standpoint), a single 100 nm by 100 nm defective pixel can kill production at a rate of one hundred 300 mm wafers per hour. A few years ago, the area of a critical defect ten times as large on a reticle was only one-tenth as complex. Given this complexity, it is clearly evident that reticle quality is key to economically viable advanced lithography and semiconductor manufacturing. The leverage on the finding and eliminating of nano-scale defects has enormous economic value, the cost of developing and operating these systems is tremendous. It is a battle of exponentials: exponentially more expensive development efforts build equipment that deals with exponentially more pixels in exponentially larger end-user markets. Between the equipment makers and the end users are many commercial levels. Closest to the reticle equipment manufacturers are the mask makers. After years of Darwinian consolidation, the remaining captive and merchant reticle manufacturers compete to cost-effectively provide the semiconductor industry with total state-of-the-art reticle solutions. Each of these manufacturers (the out-source suppliers to the chip industry) adds value through reticle process integration. They select the best-of-class writers, inspectors, metrology equipment, etch and repair tools, and integrate them with proprietary process and manufacturing know-how to produce tailored products for their markets. Given the critical importance of the reticle, the lethal significance of reticle defects, the risks of yield crashes, and the exponential nature of the economics,