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Method to Electrically Measure the Lateral Bias of Recessed Oxide Isolation

IP.com Disclosure Number: IPCOM000059898D
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 4 page(s) / 35K

Publishing Venue

IBM

Related People

De La Moneda, FH: AUTHOR

Abstract

In this article, a method is described to measure the lateral bias of the recessed oxide (ROX) commonly used to isolate device islands containing MOSFET or bipolar transistors. Wafer-level device dimensions can then be determined by applying this bias to layout dimensions. To precisely define lateral ROX bias, the conventional fabrication steps that yield a ROX structure are reviewed with the aid of Fig. 1. Starting with a p-type substrate 1, a layout-level dimension Wo is transferred onto a stack of oxide-nitride layers 2-3 by means of masking and etching operations. This leaves an oxide-nitride mask which defines the device region 4. Its dimensions are smaller than those of the layout since the photoresist is positive and the etching of layers 2-3 is isotropic.

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Method to Electrically Measure the Lateral Bias of Recessed Oxide Isolation

In this article, a method is described to measure the lateral bias of the recessed oxide (ROX) commonly used to isolate device islands containing MOSFET or bipolar transistors. Wafer-level device dimensions can then be determined by applying this bias to layout dimensions. To precisely define lateral ROX bias, the conventional fabrication steps that yield a ROX structure are reviewed with the aid of Fig. 1. Starting with a p-type substrate 1, a layout-level dimension Wo is transferred onto a stack of oxide-nitride layers 2-3 by means of masking and etching operations. This leaves an oxide-nitride mask which defines the device region 4. Its dimensions are smaller than those of the layout since the photoresist is positive and the etching of layers 2-3 is isotropic. Next, a boron dosage of around 1013 ion/cm2 is blanket implanted in the field region 5, outside the implant-blocking oxide-nitride mask. An oxidation cycle follows to grow ROX in the field regions using the top nitride layer as an oxidation mask. However, this layer does not prevent oxygen from laterally diffusing through the oxide layer and sustaining some ROX growth thereunder. As a result, the dimensions of the device region are reduced. This lateral ROX growth 6 is commonly known as bird's beak. After etching the oxide-nitride mask, the ROX structure is complete, as shown in Fig. 1b. Oxide layer 2 is usually overetched to insure its complete removal and that of the silicon nitride that grows at the edges of the oxide- nitride mask during the steam growth of the ROX. Consequently, the edge of the bird's beak recedes and exposes a rim around the sloping substrate surface underneath. The edges of the bird's beak define the dimension W of the device region. The lateral ROX bias is given by WW = 1/4(Wo - W). Besides this geometrical WW, the device region of Fig. 1b also has an electrical or effective WWe related to the lateral diffusion of the boron dose implanted in the field region. Tails of diffused boron 7 extend past the bird's beak and thus increase the surface conductivity of the device region near its edges. Consequently, WWe may not be equal to WW. The method described below requires electrical measurements of MOSFET devices fabricated in the device region 4 of Fig. 1. Specifically, these are n-channel-polysilicon-gate MOSFETs with their threshold voltage, VT, adjusted by a boron implant. Some of the process operations used in fabricating these devices alter the doping profile around the edge of the bird's beak. These alterations are examined here since they may influence the measured electrical characteristics. MOSFET fabrication in Fig. 1b begins with the thermal growth of a thin-gate oxide 8 in the exposed regions of the substrate. This yields the cross-section of Fig. 1c. Since boron segregates into this thermal oxide, its concentration decreases thereunder while remaining unchanged un...