The Influence of the LOCOS Processing Parameters on the Shape of the Bird's Beak Structure

Abstract

A qualitative model to explain the influence of processing parameters on the shape of the "bird's beak" structure, the transition region from field oxide to pad oxide, has been developed. The parameters included are (i) pad oxide thickness, (ii) field oxidation temperature, (iii) wafer orientation, (iv) thickness of the field oxide grown, and (v) field oxidation pressure. The strength of the influence of each parameter is also discussed. Experimental results are also presented that Verify the model. The local oxidation process (LOCOS) has been widely applied as a device isolation technique to improve the performance and to increase the packing density of integrated circuits (1). In a typical LOCOS process, there is a layer of SiO2 (the pad oxide layer) under the SisN4 oxidation mask. The pad oxide layer serves two major purposes: (i) as a buffer layer to reduce the stress from Si3N4 which causes defects in silicon during the LOCOS step, and (ii) as an etch step layer during the nitride etching step. Lateral diffusion of the oxidant through the pad oxide layer during the LOCOS step creates a region which is called the bird's beak (BB) near the edge of the masking nitride layer. The length of the BB (Lbb) is comparable to the thickness of the field oxide which is typically of the order of 1 #m. Therefore, the magnitude of Lbb makes it an important factor in VLSI technology. Not only does the BB region reduce the area of the active devices , but also the edge effect due to the BB has to be taken into account for small geometry devices. For two-dimensional device modeling, it is important to know the shape of the BB in detail. Furthermore, it is desirable to use the oxide wall as a self-aligned edge for the contact opening to improve the density of the circuit. The contact etching step then is critical so that a good contact can be made without exposing the p-n junction under the BB region. All of these require an understanding of the influence of the LOCOS processing parameters on the shape of the BB. In this paper, a qualitative model is presented to describe the relationship between the processing conditions and the shape of the BB. The processing parameters included in this paper are (i) pad oxide thickness (ii) oxidation temperature, (iii) wafer orientation , (iv) thickness of the field oxide grown, and (v) oxidation pressure. The shape of the BB is obtained by transmission electron microscope (TEM) or scanning electron microscope (SEM) cross-sectional techniques. The data agree very well with the proposed model. The growth mechanism of the BB can be illustrated by the schematic drawing of Fig. la. At a particular time during the LOCOS process, the point S (t), y ___ 0, is the end of the BB. At the same time, some amount of the field oxide, Tox(t), has grown in areas without SisN4. As the oxidation reaction continues, the oxidant diffuses through Tox(t) to increase the thickness of the field oxide and through the existing BB region to extend the length of the BB. By comparing the oxidation rate under the nitride mask and the oxidation rate under the existing field oxide, the influence of parameters on the shape of the BB can be obtained. * Electrochemical Society Active Member. The oxidation rate in the region y > 0 of Fig. lb can be obtained by solving a simple diffusion equation together with the oxidant consumption rate. The flux of the oxidant I(y,t) at any point y > 0 for time t > 0 can be expressed as dC(y, t) I(y, t) = Tpd " J(Y, t) =-TpdD [1] dy where J(y, t) is the flux density of the oxidant, Tpd is thickness of the pad oxide layer, D is the diffusivity of the oxidant in SIO2, and C(y, t) is the concentration of the oxident. Meanwhile, the consumption rate of the oxidant between y and y + dy is dI(y, t) d~C(y, t)- .-TroD = ksCi(y, t) [2] dy dy2 where Ci(y,t) is the concentration of the oxidant at the Si/SiO2 interface corresponding to position y, and ks is the surface reaction rate of oxidation. For a typical pad oxide of 200-500A, the profile of the oxidant along the z-direction can be assumed to be linear. Therefore C(y, t)-Ci(y, t) O-ksCi(Y, t) Tpd/2 or Z I ' Tox(t) ,, Lbb(t)"~ }/ I / , i/ltP Ro(t) (a) C (y, t) Tpd i "Y rl C i (y, t) (b) Fig. 1. (a) Schematic drawing for the growth mechanism of the bird's beak, (b) detail drawing of the region y > 0 in (a). 1563 Downloaded 12 Apr 2010 to 130.89.195.12. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp