CONTROL OF ELECTROSTATIC CHARGES ON RETICLES Field of the Invention: [0001] This invention relates to reticles for optically forming circuit patterns on semiconductor wafers, and more particularly to apparatus and method for reducing electrostatic charge and associated particulate contaminants and pattern damage on complex fine conductive structures on insulators such as contained in reticles, flat-panel display plates, integrated circuit patterns on wafers, and the like.

Background of the Invention: [0002] Certain known semiconductor fabrication processes rely upon forming circuit patterns upon semiconductor wafers using pattern masks, or reticles, to optically form the desired circuit patterns on prepared surfaces of the wafers. A reticle containing a desired circuit pattern is commonly formed with an optically opaque, thin conductive layer of chromium on a quartz substrate, and such thin conductive layer is vulnerable to damage from electrostatic discharges. Such discharges are commonly attributable to static charges that accumulate on nearby objects and on isolated segments of the circuit pattern to form electrostatic fields of sufficient field strength to cause ionization of ambient air and spark discharge that erodes or melts the thin chromium layer. And isolated surface charges attract and retain contaminant particles with significant static attractive force, with resultant optical reproductions on a prepared surface of a semiconductor wafer of the reticle containing defects attributable to discharge erosions and adhered particles. In addition, a protective cover, or pellicle, for the reticle is commonly provided to enclose the reticle and to protect the reticle from particulate contaminants and surface scratching. Such pellicles are commonly formed of non-conductive materials that may also support isolated surface charges which, with any charges on an enclosed reticle, may produce combined electrostatic fields sufficiently high to promote spark discharge and associated damage to the reticle.

Summary of the Invention: [0003] In accordance with embodiments of the present invention, static charges on the reticle are significantly reduced using alpha radiation to produce high density ions adjacent the reticle to make ambient air, or other gas contiguous the reticle, slightly conductive for forming conductive paths through which static charges may be neutralized or otherwise dissipated.

Alpha radiation sources produce ion pairs through collision with molecules in atmospheric gas, or air, and these ions serve as free charge carriers which can neutralize surface charges on the reticle. Seed ions are formed in the atmosphere adjacent the surface of the reticle that effectively lower the threshold voltage for static discharge of charged regions.

[0004] In one embodiment, a foil containing Polonium 210 is disposed near the reticle as a source of alpha radiation that ionizes the air, and a portion of a guard ring 15 provides a ground path for conducting static charges away from the reticle.

Brief Description of the Drawings: [0005] Figures 1A and 1B are perspective views of a reticle and pellicle assembly including an alpha radiation source in accordance with one embodiment of the present invention; and [0006] Figure 2 is a perspective view of an environment surrounding typical segments of a reticle to be protected from static charge buildup.

Detailed Description of the Invention: [0007] In accordance with one embodiment of the invention for protecting a device having segregated conductive surface patterns on an insulating sublayer, a reticle 9 as illustrated in Figures 1A and 1B, is formed of optically transparent quartz includes a desired pattern of circuits and components formed in a thin surface layer of chromium. A pellicle frame 11 surrounds the chromium pattern on the quartz substrate of the reticle 9, and supports thereon a pellicle 13, typically formed of quartz, in spaced relationship away from the surface pattern of the reticle 9. Configurations of reticles and pellicles are known in which the pellicle is supported away from the surface pattern of the reticle in a structure that encloses a certain volume 17 of air.

[0008] In the illustrated embodiment of the invention, a foil 15 containing Polonium 210 is attached to a sidewall of the pellicle frame 11 to supply alpha radiation into the ambient air within the volume of ambient air about the surface pattern of the reticle 9. Of course, other mounting schemes can be used to keep the alpha source close to the chromium pattern, preferably within a distance typically of less than 1 cm. Such source 15 of alpha radiation creates ion pairs through collisions with gas molecules in the air within the volume 17 enclosed about the reticle 9. These ions serve as free charge carriers which can neutralize static charge on reticle and pellicle surfaces over an interval typically less than about <20 seconds initially and thereafter continually to inhibit buildup of surface charge sufficient to prevent discharge or attract contaminant particles.

[00091 Additionally, the introduction of a source of alpha radiation into the volume of air surrounding the surface pattern of the reticle 9 significantly increases the number of electrons and ions available to serve as charge carriers. Thus, electrostatic discharge events caused by electric fields from external charged object can occur at such lower energy levels without creating sufficient heat to damage the chromium surface layer of the reticle 9.

The differential voltage induced on adjacent chromium features on the reticle is determined by the electric field from the external charged object and the geometry of the two features. The energy in a discharge resulting from 50% lower voltage differential is 75% lower and less damaging, and lower voltage differential is assured by charge carriers that are present in the vicinity of an air gap between isolated features or segments of the chromium surface pattern.

Charge-carrying ions thus aid in precipitating a discharge before the differential voltage becomes high enough to cause a damaging discharge.

Thus, a reticle 9 protected by a source 15 of alpha radiation in accordance with the illustrated embodiment of Figures 1A and 1B may be moved into a region of electric fields capable of inducing a voltage differential between adjacent, isolated segments of the chromium surface pattern, and electrostatic discharge may result, but at much lower voltage threshold and concomitant lower energy levels. For continual charge-inducing events, many electrostatic discharges may occur, but each occurring at lower voltage thresholds and lower energy levels, with resulting negligible damage to micron and submicron features of the chromium surface pattern of the reticle 9. And the rapidity with which discharges may recur will depend upon the rate of buildup of voltage differential between isolated segments of the surface pattern, and such rate of buildup may be significantly retarded as a result of the alpha radiation source 15 which increases the density of charge carriers in the ambient air or ionizable gas surrounding the isolated segments. Buildups of differential voltage resulting, for example, from movement of charged objects will produce multiple discharges each at lower voltage thresholds and lower energy levels significantly lowering the temperature of discharges, with cooling intervals between discharges for significant reductions in or elimination of damage to the chromium surface pattern. A source of alpha radiation is preferred over sources of beta, gamma or x-ray radiation for the benefit of confinement of the generation of charge carriers within a limited small volume of air, but other sources of such forms of radiation may also be used.

[0010] Referring now to Figure 2, there is shown a pictorial perspective representation of a volume 19 of ambient air surrounding isolated features 21,23 of a surface pattern of reticle 9. The volume 19 is shown surrounding the gap 25 approximately to the limits of force upon an ion attributable to the electric field associated with a voltage difference across the gap 25. An ion within these surroundings would likely help initiate a discharge, but if further away, would not likely influence a discharge. Thus, the minimum level of ionization required to promote an electric discharge across a gap 25 can be calculated for such volume 19 of surrounding ambient air. The source activity that produces an average one ion pair in the volume 19 immediately surrounding a gap 25 in a surface pattern of the reticle is determined in part by the dimensions of the surrounding volume. By way of example for a typical 5X reticle (i. e., 5 times larger than the desired pattern on a semiconductor wafer), then 0.2 micron features on the semiconductor wafer yield typical 1 micron gaps in the reticle. For a volume extending approximately 0.5 to 1.0 microns in directions about a gap 25, the resultant dimensions may be about 2 llm wide and 2 jum long and 1 jjm high, or about 4 nm 3 about the gap 25. Such gap 25 exists within the larger volume 17 disposed within the reticle/pellicle assembly having dimensions of about 5 inches wide by about 5 inches long by about 0.5 inches high, or about 12.5 inches3 (200 cm3 or 2 x 1014 µm3). Therefore, in order to supply one ion pair per 4 µm3 volume about gap 25 within the total volume of 2 x 1014 Zm 3 <BR> <BR> <BR> <BR> <BR> <BR> requires ########## or 5 x 1013 ion pairs (assuming uniform distribution of generated ion pairs within the total volume 17 of the reticle/pellicle assembly).

Since about 1.7 x 105 ion pairs are generated per alpha (in particle or wave <BR> <BR> <BR> <BR> <BR> 5 x 1013<BR> theory), then # 3 x 108 alphas. (One Curie is defined as 3.7 x 1010<BR> 1.7 x 105 annihilations or disintegrations per second.) Therefore, in the foregoing example, an alpha source of about 8 milliCurie disposed within the foil 15 establishes a minimum threshold of about one ion pair per 4, um 3 volume of ambient air surrounding a gap 25 in the surface pattern of reticle 9 within the assembly 9,11,13, as shown in Figures 1A and 1B. Enhanced rates of generation of ion pairs may be attained with increased alpha radiation activity from sources larger than about 8 milliCurie. Of course, similar electrostatic discharges and the attraction of contaminating particle by the electrostatic fields associated with surface charges can adversely affect reticles without an enclosing pellicle, and can similarly affect devices generally involving regions of metallization on insulating sublayers. Such devices include flat-panel, liquid crystal displays (LCD) and integrated circuits and other semiconductor devices. Accordingly, introduction of a source of alpha radiation in the operative proximity of such metallized patterns beneficially reduces or eliminates damage to such devices attributable to static discharges and static field attractions of contaminant particles surrounding such metallized patterns.

[0011] Therefore, damage to reticles attributable to the field from electrostatic charges attracting and retaining contaminant particles, and attributable to electrostatic discharges across gaps between isolated segments of reticle circuit patterns, can be significantly reduced by introducing alpha radiation to generate ion pairs within surrounding ambient air that promote conduction and neutralization of isolated surface charges.