Amorphous Silicon As An Inorganic Resist

Amorphous silicon as an inorganic resistP. H. La Marche* and R. Levi -SettiThe Enrico Fermi Institute and Department of PhysicsThe University of ChicagoChicago, Illinois 60637AbstractNegative tone images have been produced directly in glow discharge deposited amorphoussilicon hydride by selective gallium -ion implantation. Like crystalline silicon, amorphoussilicon exhibits a greatly reduced etch rate in aqueous caustic solutions when implantedwith doses in excess of about 1013 Ga ions /cm2. The amorphous silicon behaves as a negativeresist with a threshold sensitivity of about 1 pC /cm2. This material is particularlyattractive as a possible masking material for ultra- violet light due to its high absorptionproperties in this wavelength region, low relative cost and the ease by which it can bedeposited on a wide range of materials.IntroductionIt has been known for some time now that the etch rate of crystalline silicon (c -Si) canbe greatly reduced in certain anisotropic etching solutions by heavily doping it withboron1,2, or phosphorus.3 The boron -doped silicon etch -stop effect has been exploited togreat advantage in the fabrication of miniature mechanical devices." 5 Recently, galliumimplanted silicon has been shown to produce an etch -stop effect as well,6'7 with the possi-bility of direct submicron microlithography by using finely focussed gallium ion beams pre-sently available. We present here our preliminary results of an investigation of an etch -stop effect observed for gallium implanted into amorphous silicon hydride (a -Si:H) anddiscuss the possible usefulness of this material as a negative resist. The method of de-positions and properties of a -Si:H9 are discussed in more detail elsewhere.Experimental MethodsVarious thickness films of a -Si:H were produced by glow discharge decomposition ofsilane (SiH4) and deposited onto glass (Corning 7059), nichrome and c -Si substrates.Samples deposited at the anode were maintained at 240° C during the deposition and cathodedepositions were performed at 510 to 530° C. One sample was n -type doped during depositionby adding phosphine (PH3) to the glow discharge. All samples were clear and showed nomicrostructure in the SEM. The samples were stored in vacuum for various times (from peri-ods of days to two years), with no apparent degradation in the quality of the a -Si:H films.The samples were installed directly into our prototype 60 kV scanning ion microscope(SIM)10 without the need of etching or polishing. This SIM, employing a liquid galliummetal ion source, produces a beam of gallium ions which is focussed to a 100 nm spot sizewith a beam current of 10 pA. The beam can be scanned over an area of up to 0.4 x 0.4 mm2of the specimen and an image can be formed by collecting secondary electrons or ions anddisplaying the signal on a CRT. Scans were performed by an analog scan generator or by amicrocomputer producing 256 x 256 vector arrays (100 ps minimum dwell time per position).Structures of lines, gratings and dot patterns were implanted by deflecting the ion beamacross the a -Si:H film surface at various rates, thus obtaining controlled ion doses.Several samples were etched in 40° C aqueous NaOH (50 wt. %) solutions for about 10 seach time. After etching, the samples were washed in deionized water and immersed in anhy-drous ethanol. The samples were then examined under the optical microscope in reflectionfor visible features. If no features were visible or well- defined, the sample would be re-etched and the cycle continued. After the samples were etched to our satisfaction, theywere installed in a Hitachi HFS -2 SEM for examination. It was necessary to lightly goldcoat (q, 10 nm) the films deposited on glass to reduce charging effects during imaging in the