Laser Microchemistry For Direct Writing Of Microstructures

AbstractMicroscopic laser-initiated chemical reactions can be used to write micrometer-scale features on materials of interest for solid-state electronics. In this paper, we describe the nature of direct-write-type fabrication and summarize the laser techniques which have been developed.IntroductionIt has recently become clear that a, nontraditional form of microfabrication would be useful for making devices and device components. In particular for many situations, it is desirable to limit processing to small selected regions on a wafer, and to accomplish the alteration in a single, discretionary step. This form of processing, which we term direct-writing, is important for operations such as repair, customizing or device opti­ mization. Unlike the situation in planar processing, the techniques which had previously been developed for direct writing are relatively unsophisticated and very limited in capability. For example, to remove a selected microregion on a chip today, it is necessary to use either a selective multistep photolithographic process followed by etching, or to use laser ablation. The first technique is time consuming and unreliable; the second has undesirable side effects such as debris blow off or unwanted doping. For instances requiring direct addition of a new material, such as doping atoms, the existing microtools are limited or nonexistant.In this paper, we will first summarize the necessary requirements for direct-write pro­ cessing. We will then show how the variety of laser photochemical procedures, which we and others have recently developed, are ideally suited for this form of processing. In particular, we will discuss the use of laser chemistry to write conducting lines, to etch surface relief patterns, and to dope semiconductor wafers.RequirementsDirect writing requires several distinctive process characteristics. The necessity for instantaneous changes means that a single-step approach is required. Subsequent processing of the entire device or wafer is not only undesirable from a cost point of view but it can unnecessarily subject the entire device or chip array to an extra chemical environment, etc. In the situation in which processing and testing must occur simultaneously, a single-step process is essential.For applications involving the micrometer-design-rule devices of the future, micro­ meter-scale resolution is clearly desirable. However useful and unique applications can be found for much lower resolution writing on current devices. Direct-writing should be capable of performing at least four basic functions: material removal, deposition, doping, and insulator growth. With the possible exception of removal which can be done with abla­ tion, these functions require some form of locally controllable chemistry. For a one-step process, the chemistry should be activated in a gas or liquid medium above the device sur­ face, since then the active medium can be introduced or compositionally altered at will.Laser SourcesLaser beams of visible or ultraviolet light can similarly be used to drive the micro­ scopic chemical reactions either through localized photochemical or thermal effects.1 Both types of reactions result in negligible heating of the bulk of the substrate. Photochemi­ cal reactions are particularly energy efficient since the light photon directly and selec­ tively severs a specific molecular bond. Unlike charged-particle beams, collisional*This work was supported by the Defense Advanced Research Projects Agency, the Department of the Air Force, in part under a specific program sponsored by the Air Force Office of Scientific Research, and the Department of the Army.112