Vacuum/volume 42lnumber 18lpages 1219 to 122811991 0042-207x/91 $3.00+.00 Printed in Great Britain @ 1991 Pergamon Press plc An overview of X-ray lithography for use in semiconductor device preparation U S Tandon, B D Pant and Ashok Kumar, Microelectronics Area, Central Electronics Engineering Research Institute, Pilani 333 031, India received 9 March 199 1 Different aspects of X-ray lithography with a proven capability for the fabrication of 0.1 pm lines and 0.5 pm devices such as ULSI and multi-megabit memory are discussed. The technical, dimensional and economic features of modern X-ray sources such as synchrotron with classical, normal and superconducting storage rings, have been compared. Materials fully transparent or opaque to X-rays do not exist and so the choice of X-ray mask substrate and patterning of absorber on it are rather critical. EBL, FIBL, RIE, XRL and other techniques used for preparing submicron masks have been dealt with. The sensitivity and resolution of 76 positive and negative X-ray resists vis-2-vis specific sources and characteristic features of 7 XRL systems are compared. Alignment schemes using laser controlled stage and visible lights are discussed. R&D as well as commercial systems and results like quarter micron patterns, 0.5 pm CMOS, SAW BPF and large aspect ratio grooves are demonstrated. zyxwvutsrqponmlkjihgfedcbaZYXW 1. Introduction The use of soft X-rays as a practical means of replicating patterns in the fabrication of electronic and optical microdevices was suggested by Spears and Smith’,’ in 1972. Typically X-ray wave- lengths from 0.4 to 8 nm are used to proximity print Au mask patterns supported by thin transparent substrates with 0.1 pm resolution. The X-ray flux on the mask and its energy spectrum determine the throughput and contrast available in the process. The characteristics of X-ray lithography (XRL) include’ : (1) freedom from particulate defects ; (2) unlimited depth of focus ; (3) resolution independent of (practical) field size; (4) superior image fidelity ; and (5) negligible reflection from metal surfaces. These characteristics translate to : (I) resolution potential into the 0.2 pm region; (2) CD control to better than 200 A; (3) availability of large working area for CCDs and multi-megabit memory chips ; (4) excellent step coverage ; (5) use of single (vs multi-) layer resists; (6) good control of circuit performance consistency from chip to chip on a wafer; and (7) high yield of submicrometre feature circuits. Realization of the above benefits in XRL depends upon proper integration of following operational parameters : (1) source brightness, lifetime and dimension ; (2) mask fabrication, inspection, repair and durability ; and (3) resist sensitivity, resolution, contrast, throughput, amena- bility to dry etch and alignment/overlay tolerance. The typical XRL set up shown in Figure 1 is very similar to the optical proximity system. However, two important physical facts render XRL relatively considerably difficult: (I) for X-ray wavelengths, there is no material which would be fully transparent in thick dimensions (such as glass in the optical case) or fully opaque in very thin layers (such as chrome in the optical case) ; and (2) efficient imaging optics for X-rays requires a condenser for homogeneous illumination of the wafer which is not realizable. Thus only a light element with low atomic number and low absorption could be used as an X-ray mask substrate. The X-ray wavelengths useful for XRL are severely restricted-in the high region by the high absorption in the thin mask substrate and in the short region by the decreasing absorption in the opaque layer for harder radiation. Focusing of X-rays is being attempted at various institutions, however, the realization of a stable high efficiency condenser is still a problem. Figure 2 is an array diagram demonstrating the basic principle of the XRL proximity printing technique. The blur ‘s’ appears in the shadow because of the finite dimensions of the source ‘S’. Its typical value for a mask to substrate proximity gap of 20 pm, source dia of 3 mm and D = 400 mm, would be about 0. I5 pm. 2. X-ray sources The lack of useful optics in soft X-rays compels the use of radi- ation in the same form in which it is emitted from the source. (e.g. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJI bour n) ’ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP SILICON WAFLR zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED N 10.2 to zyxwvutsrqponmlkjihg 2nml RESIST ITYP lum THICK) -c EXPOSURE CHAMBER Figure 1. The exposure arrangement for X-ray lithography using synchro- tron radiation. 1219