Multiple masking techniques use lateral etching to reduce the total area of the high etch-rate oxide exposed to the chemical etchant. One method uses a short-term etch to remove the top layer from the silicon oxide surface, another acts before the top layer is grown

Cannon, D. L.

English

NASA Technical Reports Server

August 1969	 Brief 69-10221 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Multiple-Mask Chemical Etching 
Exposed and Developed 
Photoresist Layer
dercutting 
--IT High Etch- Rate Oxide 
Stabilization Layer 
SlcoW±j lco Wafer 
a. Ideal Oxide Cat b. Actual Cat
Fig. 1. Standard Semiconductor Photoetching Process
ml	 m2 
a. Short-Term Etch to Remove 
Only High Etch-Rate Layer	 b, Longer Etch After 2nd Mask 
Fig. 2. Multiple Masking. Technique 
a Mask and Etch before Growing 
Stabilization Layer.
Grow High 
Etch- Rate 
Layer T! :1 
b Stabilization Layer Grown, 
2nd Mask Applied. And 
Final Etch. 
Fig. 3. Multiple Masking. Technique 2 
The problem: 
Standard chemical etching techniques, which re-
move semiconductor oxides during the fabrication of 
integrated circuits, sometimes result in excessive 
lateral etching. An ideal etched profile is shown in 
Figure 1; however, in practice, the etchant removes 
a larger area at the top. If a high etch-rate oxide (ti) 
is located in the structure, an area will be etched 
laterally for t distance equal to many oxide thicknesses 
resulting in the exposure of critical areas of the device.
The solution: 
The lateral etching has been contained by using 
multiple masking techniques which effectively reduce 
the total area of the high etch-rate oxide exposed to 
the chemical etchant. 
How it's done: 
One method utilizes a short-term etch which re-
moves only the high etch-rate top layer from the silicon 
oxide surface. The etchant is neutralized and a second 
(coutiuued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19690000221 2020-03-16T17:41:07+00:00Z
mask is applied as shown in Figure 2. A second etch 
is then initiated to cut through the slow etch-rate 
silicon oxide layer (t2). The second mask preserves 
the high etch-rate stabilization layer and reduces 
lateral undercutting because of the reduced opening 
size (m2) of the second mask. 
A second technique, multiple contacts, consists of 
first masking and etching before the high etch-rate 
stabilizing oxide layer is grown. Figure 3a shows the 
first photoresist mask and resulting etch; the first 
mask is removed and the top oxide then grown. A 
second photoresist mask with slightly smaller openings 
is then applied to the surface and a second etching 
follows. As shown in Figure 3b, the stabilizing high 
etch-rate oxide layer and the original opening are 
protected from the etchant; the total lateral etch is 
now less than ti +t2.
Note: 
No further documentation is available. Inquiries 
may be directed to: 
Technology Utilization Officer 
Manned Spacecraft Center 
Houston, Texas 77058 
Reference: B69-10221 
Patent status: 
No patent action is contemplated by NASA. 
Source: Don Lee Cannon of 
Lockheed Electronics Company 
under contract to 
Manned Spacecraft Center 
(MSC-131 14) 
KI
. 
Brief 69-10221	 Category 01


Multiple-mask chemical etching

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server