Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Recessed oxide by localized oxidation
Reexamination Certificate
1999-09-14
2001-09-18
Tran, Andrew (Department: 2824)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Recessed oxide by localized oxidation
C438S445000
Reexamination Certificate
active
06291312
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 88113170, filed Aug. 2, 1999, the full disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device. More particularly, the present invention relates to a method for forming a pullback opening above a shallow trench isolation (STI) structure.
2. Description of the Related Art
Due to the rapid development of integrated circuit manufacturing techniques, highly miniaturized and integrated devices are now fabricated. As dimensions of each device shrink, isolating structures between device have to shrink correspondingly. Hence, the process of forming device isolation becomes harder. Conventionally, devices are isolated by forming a field oxide layer by a local oxidation (LOCOS) method. However, the field oxide layer is subject to bird's beak encroachment, which is a considerable barrier to device miniaturization.
In the meantime, other types of device isolation structures have also been developed. One widely adopted device isolation method, especially in the fabrication of sub-half micron integrated circuits, is shallow trench isolation (STI).
To form a conventional STI structure, a pad oxide layer and a silicon nitride mask layer are formed in sequence over a substrate. A photolithographic process is next performed to pattern out a trench region. Using a dry etching method, the silicon nitride layer, the pad oxide layer and the substrate are sequentially etched to form a trench in the substrate. A region surrounded by the trench becomes an active region where active devices are subsequently formed. Thermal oxidation is carried out to form a liner oxide layer over the interior surface of the trench. Silicon oxide is deposited into the trench and over the silicon nitride layer by chemical vapor deposition. Silicon oxide that rises above the silicon nitride layer is removed by chemical-mechanical polishing to form an isolating structure in the trench. Finally, the silicon nitride layer is removed using hot phosphoric acid solution and the pad oxide layer is removed using hydrofluoric acid solution.
In the fabrication of deep submicron devices, available space between neighboring active regions is very small. Hence, only narrow trenches can be formed. When a trench is very narrow, the gap-filling capability of the trench with respect to the deposition of silicon oxide deteriorates. Consequently, structural defects such as voids or seams are more likely to form inside the silicon oxide plug of an STI structure. Therefore, in the latest development, a ‘pullback’ process for widening the opening leading to the STI trench is introduced to facilitate the deposition of silicon oxide.
FIGS. 1A and 1B
are schematic cross-sectional views showing a first method of manufacturing a conventional STI structure with a pullback opening. As shown in
FIG. 1A
, a pad oxide layer
110
and a silicon nitride layer
120
are formed in sequence over a substrate
100
. Using photolithographic and etching processes, a trench
140
having an opening width of m is formed in the substrate
100
. As shown in
FIG. 1B
, a pullback process is next carried out to widen the opening at the top of the trench
140
by performing an isotropic etching operation using hot phosphoric acid. After the etching step, the silicon nitride layer
120
surrounding the trench opening is pulled back a distance of about n, roughly equivalent to about 200 Å. Therefore, the width of the opening above the trench
140
is increased to m+2n.
FIGS. 2A and 2B
are schematic cross-sectional views showing a second method of manufacturing a conventional STI structure with a pullback opening. As shown in
FIG. 2A
, a pad oxide layer
110
, a silicon nitride layer
120
and a silicon oxide layer
130
are formed in sequence over a substrate
100
. Using photolithographic and etching processes, a trench
140
having an opening width of m is formed in the substrate
100
. As shown in
FIG. 2B
, a pullback process involving the etching of both the silicon nitride layer
120
and the silicon oxide layer
130
is next carried out to widen the opening at the top of the trench
140
. The silicon nitride layer
120
and the silicon oxide layer
130
are simultaneously etched using an isotropic etching agent such as hydrofluoric acid in a glycerol or ethylene glycol (EG) solution. Ultimately, the silicon nitride layer
120
and the silicon oxide layer
130
surrounding the trench opening are pulled back a distance n, roughly equivalent to about 200 Å. Similarly, the width of the opening above the trench
140
is increased to m+2n.
Both the first and the second pullback processes depend on wet etching. However, the pullback distance n is rather difficult to control in wet etching operations because the first section of the silicon wafer clipped into the acid solution is usually the last section pulled out of the acid solution. Consequently, different areas of the wafer remain in the acid solution for different periods of time, causing the pullback distance to vary substantially across the wafer.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a method for forming a pullback opening above a shallow trench isolation (STI) structure, in which method the pullback distance of the trench opening can be precisely controlled.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for forming a pullback opening above an STI structure. A patterned mask layer is formed over a substrate. A sacrificial layer is formed on the sidewalls of the mask layer. The exposed portion of the substrate is etched to form a trench in the substrate. The sacrificial layer is removed to increase the width of the opening above the top of the trench.
In the aforementioned process of forming a pullback opening, the sacrificial layer is formed over the sidewalls of the mask layer before etching the substrate to form a trench. Therefore, the sacrificial layer must have chemical properties different from both the mask layer and the substrate so that the sacrificial layer can be selectively removed after the trench-etching operation. In addition, the mask layer can also be a composite layer of two or more separate material layers. If a composite mask layer is formed, a staircase-like opening with increasing width will form above the top of the trench.
According to this invention, the mask layer and the sacrificial layer are used as an etching mask when the substrate is etched to form a trench. Hence, a trench having a pullback profile at the top is formed immediately after the sacrificial layer is removed. Since the pullback distance is entirely determined by thickness of the sacrificial layer, width of the pullback opening can be precisely controlled.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 5789320 (1998-08-01), Andricacos et al.
patent: 6001707 (1999-12-01), Lin et al.
patent: 6008120 (1999-12-01), Lee
patent: 6114217 (2000-09-01), Park
patent: 6146970 (2000-11-01), Witek et al.
Chan Bor-Wen
Liu Yuan-Hung
Huang Jiawei
J.C. Patents
Smith Brad
Taiwan Semiconductor Manufacturing Co Ltd.
Tran Andrew
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