旋涂、喷涂和电沉积技术比较

Abstract - This paper presents three coating methods ofphotoresist, spin, spray and electrodeposition (ED) on largetopography surfaces. Characteristics of each method aswell as its advantage and disadvantages are outlined. Acomparison is made to point out the most suitable coatingmethod in terms of complexity, costs and type of application.

I.INTRODUCTION

For several MEMS applications and 3D microstructuringpattern transfer onto silicon wafers with extensivetopography requires a uniform resist layer not only onplanar surfaces but also on the irregular ones. To date.three photoresist coating techniques have beenintroduced for the fabrication of MEMS devices. Spincoating is the most conventional coating methodgenerally applied to standard flat wafers. It can only beused for some MEMS applications with certainmodifications of the equipment or spinning method used[l] or of the coating program. Alternative methods suchas electroplating and spray coating of photoresist havebeen proposed. Electrodeposition of photoresist has beenreported as an attractive method for 3D stacks of chipsand interconnects [2,3], but it requires a conductivelayer. Recently, a new coating method, direct spraycoating of photoresist,(4] has been introduced as a resistcoating technique specifically for MEMS. Although thistechnique is still in the early stages of exploration, itappears to be a promising technique for coating irregularsurfaces as it presents some advantages over spin coatingand electrodeposition of photoresist.In this paper we report on the use of these three coatingtechniques to coat highly non-planar wafers, i.e. waferswith anisotropically etched grooves or cavities with adepth up to 400 um. Potentials and limitations arepointed out and a comparison of the three coating methods is presented in order to identify the most suitable coating technique for a specific application.

II.SPIN COATING

Spin process and photoresistSpin coating of photoresist is the standard coatingmethod for flat wafers in IC technology. Spin onphotoresist applied to irregular wafers has been reportedfor some application such as for flat panel displays [5] orwith a modified equipment to coat wafers with throughholes [l]. Spin coating can be used for wafers withdeeply etched cavities even with conventional equipmentif the spinning program is appropriately modified asrecently reported in [6]. Thick positive photoresistAZ4562 is used. First, photoresist is flooded onto thewafer in order to cover the whole surface. A pause afterthe dispense step allows additional time for the solutionto flow into the deep features. A slow acceleration andspin speed is applied in the first step. This allows timefor the solution to flow and spread prior to drying. Asecond step with a fast spin speed promotes the drying ofthe film and reduces the further flowing of photoresistthat can result in non-uniformal coating. Using thisprogram, the uniformity of photoresist ( 1%) is between10-20%.This value is reasonable for patterning somestructures at the bottom of deep cavities, but lessconvenient for patterns running across cavities.

Advantage

Spin coating is a mature technique andusescommercially available equipment and resists. Theprocess is compatible with the IC technology and can beused at all stages of processing on all types of substratelayers.There are only two parameters, ie. the resist solutionviscosity and the spinning speed, that strongly influencethe layer forming. Therefore, the process optimizationfocuses only on these two parameters.

Disadvantages

The main obstacle is caused by the centrifugal forcewhen spinning. The deeplyetchedfeatures cause aphysical obstruction to the solution flow, preventingcomplete coverage and often causing striation or resistthickness variation such as the variation on the near andfar sides of a cavity or on cavities at different positionson the wafer. Sizes and shapes of the cavities also haveinfluence on the resist uniformity and coating defects.Experiments investigating the effect of cavity's size toresist layer are performed on wafers with cavities ofrectangular and square shapes. The dimension of thecavities are between lmmx3mm and 3mmx3 mm in sizeor with the H/V ratios of 1/3 to 3 (H and V are thehorizontalandvertical dimensions, respectivelyHorizontal and vertical dimensions here are with respectto the flat of the wafer). The measured resist uniformity(1%) vs. H/V ratio is depicted in fig 1. Each set ofpoints corresponds to an area of the wafer (upper leftupper right, lower left and lower right, with respect to thewafer flat). The lowest uniformity variation can beobserved at H/V = 2/3,1, 3/2,which indicates a betterresist uniformity in the square or large rectangle cavities.