多晶硅刻蚀方法

时间：2023-02-06 10:59:27 浏览量：0 次

Abstract

Purpose - The purpose of this paper is to present a selective wet-etching method of boron doped ow-pressure chemical vapour deposition (LPCVDpolysilicon film for the realization of piezoresistors over the buk micromachined diaphraam of (100) silicon with improved yield and uniformity.)

Design/methodology/approach - The method introduces discretization of the LPCVD polysilicon film using prior etching for the grid thus dividinteach chip on the entire wafer. The selective etching of polysilicon for realizing of piezoresistors is imited to each chip area with individual boundaries

Findings --The method provides auniform etching on the entire silicon water irespective of its size and leads to economize the fabrication process in abatch production environment with improved yield.Research limitations/implications - he method introduces one extra process step of photolithoqraphy and subsequent etching for discretizing thepolysilicon film.

Practical implications -- The method is useful to enhance yield while defning metal lines for contat purposes on fabricated electronic structures usintmicroelectronics.Stress developed in LPCVD polysilicon can be removed using proposed approach of discretization of polysilicon film.

Originality/value - The work is an outcome of regular fabrication work using conventional approaches n an R&D environment. The proposed methodreplaces the costly reactive ion etching techniques with stable reproducibility and ease in its implementation.

Introduction

In the fabrication of polysilicon piezoresistive pressure sensor using silicon bulk micromachining, polysilicon resistors are realized on the silicon diaphragm in full or half wheatstone bridge confifiguration (Akhtar et al., 2003a). A large number of arrays of the diaphragm are accommodated on the silicon wafer in case of the batch fabrication of the sensors. The number of arrays increases with the size of the silicon wafer. In order to make the fabrication process cost effective, wet etching is used for the delineation of polysilicon resistors in a selective manner. Employing standard photolithography, pattern of the polysilicon resistors is protected with photoresist and the exposed polysilicon is etched out from the entire silicon wafer either using reactive ion etching (RIE) or wet etching. However, in the wet process, fast etching takes place on the wafer edge. The etching process starts from the wafer edges and moves towards the center of the wafer for a uniform thick polysilicon layer. This results into resistors with over-etching, particularly, near the wafer edges, and leads to low yield. Non-uniformity in the polysilicon resistor’s values is the outcome of low yield. Handling of selective etching process also becomes a tricky workmanship with practically no control over reproducibility. In order to

overcome the non-uniform wet etching of polysilicon, dry etching is used to enhance the yield by uniform etching of polysilicon. In the dry etching process selectivity of polysilicon in respect of under layers of silicon nitride and silicon dioxide is poor, owing to which a fifine control on the recipe for dry etching and etching time need to be optimized very accurately. Besides, the dry etch process is not a cost effective. The unavoidable stress developed in the low-pressure chemical vapour deposition (LPCVD) polysilicon during its deposition, the entire silicon wafer becomes oval and a non-uniform etching takes place during RIE. With the growing demand of sensors in a number of varieties of applications, a great deal of efforts is to reduce their manufacturing cost with improved reliability and reproducibility. The wet-etching method is easy to implement particularly for a larger number of wafers in a batch fabrication environment and offers minimum controlling parameters and much cheaper input infrastructure as compared with RIE.

A method of uniform wet etching of polysilicon has been devised and implemented in the present work for the realization of piezoresistors of boron doped polysilicon over the silicon diaphragm in the batch fabrication of pressure sensor. A 2 inch diameter silicon wafer has been considered as a model size in order to show the capability of the proposed method. Blanket wet etching has been carried out purposely to compare it with the proposed method. The method is economic and easy in its implementation. The methodology of the process is outlined in the next section, which is followed by the fabrication process adopted for the sensor fabrication. Relevant results of uniformly defifined polysilicon resistors with statistical data of resistors values are presented. The results are discussed with conclusion in the end.

Methodology of wet-etching process

There are two situations ofwet etching ofpolysiliconthinlayer onthesilicon nitride/silicon dioxide cappedsiliconwaferas shown inFigure 1; blanket wet etching of polysilicon film with protectedpatterns of resistors over the entire surface, and second.discretization of polysilicon layer and then wet etching of eachunit simultaneously. In the previous situation etching rates ofpolysilicon is higher on the wafer edges and anon-uniform etchedpattern of remaining polysilicon is resulted. The size of arrowsshown in Figure 1 schematically shows the rate of etching whichis higher on the corners and slow on the flat surface.

Figure 1

Thediscretization ofthe polysilicon layer into smallsegments prior to the actual delineation ofpolysilicon resistors, enhances corners ina distributed manner on the entire polysilicon surface. The eachsmall independent island of polysilicon establishes its own etchrate according to its surface area and provides a better controlover the etching mechanism. By selection a uniform area of eachisland on the entire polysilicon surface, a uniform etchingrate canbe established. The geometry influence on the wet-etching ratehas been extensively studied in the literature (Koehler, 1999).Parameters related to reaction and diffusion mechanisms of wetchemical etching are strong functions of geometries to be etchedout. Fringing of reaction rates around sharp corners in thegeometry enhances the etching rates. With the decreasingdimensions ofthe structures to be defined by wet etching, moreinvolved etching mechanisms have been encountered in therecent studies (Yamamura and Mitani 2008).

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