CircuiTree - April 2009 - (Page 24)

i-Line UV Lithography and Sub-10μm Feature Size Package Substrate Technologies lier DIP, QFP, etc. to current advanced IC packages and to highly integrated future SOP system technology2. The feature size has decreased from millimeters to micrometers and will reach the nanometer regime in the future. transparent area in the photo mask to a photo-resist ﬁlm on substrate or PCB. From the ﬁgure, one can see that a UV beam illuminated on a photo resist is no longer collimated if there is a gap between the photo mask and the photo resist. This is due to the nature of light diffraction. The photo resist beneath the opaque area of the mask could be illuminated by the UV beam. The beam intensities on the photo resist surface for contact mode (d=0) and proximity are shown in Figure 4. For contact mode, the beam intensity on resist is uniform and outside of opening area is zero. With the presence of a gap, the beam intensity becomes non-uniform and wider. As the opening becomes narrower and the gap becomes larger, the effect becomes problematic and the adjacent lines become irresolvable. Fundamentals of Fine Line Photolithography Fabrication of semiconductors and package substrates starts with photolithography. Photolithography has been the dominant process for the formation and miniaturization of the feature sizes on both chip and package. The semiconductor industry has developed features of 30nm for high performance microprocessors. However, the feature size that the packaging industry can produce is about 15~20µm. A huge technical gap exists between semiconductor and packaging. In order to keep pace with semiconductor process development, a process for ﬁner pitch wiring substrate is necessary. In this paper, we investigate the key factors that hinder the ﬁner feature size formation on package substrates and explore the minimum lines and spaces that can be obtained based on the mercury i-line photolithography, both theoretically and practically. There are three basic UV light exposure models: proximity, contact, and projection imaging. Proximity and contact models are commonly used in the PCB and package substrate industry while the projection model is used in semiconductor IC manufacture. Figure 3 illustrates the principle of a collimated beam of UV light propagating through a Resolution of Proximity Photolithography Principles Proximity is commonly used in the printed circuit industry. In proximity exposure mode, there is a small gap existing between photo mask and photo resist. With the opening on photo mask 0→L and the range of diffractive effect , it can be seen that when reaches L/2, a comb structure having equal lines and spaces becomes closed Fig 5 Experimental Setup for UV Exposure at Different Gap Distances for Studying the Gap Effect on the Resolution of Photolithography Fig 3 Principle of Photolithography With UV Light. Light Diffraction Becomes Problematic When Transparent Area Becomes Smaller Fig 4 UV Beam Intensity on the Surface of Photo Resist. When ∆ Reaches L/2, the Comb Structure Having Equal Line and Space Will Not be Resolved 24 April 2009 • circuitree.com Fig 6 Optical Micrograph of Test Comb Structure With Equal Line Width and Space. 7 µm Line and Space Resolved on 8µm-Thick Photo Resist at a Gap of 30µm by Using i-line UV Lithography http://www.circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - April 2009

CircuiTree - April 2009 Contents Lead Wire The Wire Tech Talk Flexible Thinking New GUI Improves Time to Market for Serial RapidIO System Designs Environmentally Speaking Intelligent Design i-Line UV Lithography and Sub-10μm Feature Size Package Substrate Technologies Guest Column BPA Growth Curves Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - April 2009

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