Method of making rib-structure shadow mask for ion implantation

• US 4,021,276 A
• Filed: 12/29/1975
• Issued: 05/03/1977
• Est. Priority Date: 12/29/1975
• Status: Expired due to Term

• Current Assignee: AT&T, Inc.
• Original Assignee: Western Electric Co., Inc. (Nokia Corporation)
• Inventors: Parry, Peter David, Cho, Kon Ho
• Primary Examiner(s): Powell, William A.
• Application Number:
  US05/645,146
• Time in Patent Office: 491 Days
• Field of Search: 156/3, 156/6, 156/8, 156/11, 156/17, 156/16, 29/580, 29/583, 428/131, 428/134, 428/135, 428/137, 428/156, 148/186, 148/187
• US Class Current: 216/2
• CPC Class Codes: H01L 21/266 using masks H01L21/26586 takes precedence
• CPC Class Codes: Y10S 438/924 To facilitate selective etching
• CPC Class Codes: Y10T 428/24298 Noncircular aperture [e.g., slit, diamond, rectangular, etc.]
• CPC Class Codes: Y10T 428/24322 Composite web or sheet
• CPC Class Codes: Y10T 428/24479 including variation in thickness

Abstract

A shadow mask particularly useful in ion implantation processes is disclosed. The mask is fabricated by doping a surface region of a semiconductor wafer to render that region resistant to a particular etchant, machining cavities in the surface of the wafer opposite the doped surface to a depth that does not quite reach the doped region, forming pattern openings in the webs remaining across the ends of the cavities, and thinning the webs by exposure to the particular etchant until the undoped material in the webs is removed. Thus, the ultimate thickness of the webs is controlled substantially by the doping depth.

Claims

1. A method of fabricating a shadow mask from a semiconductor wafer, which comprises:

doping a first region adjacent one surface of the semiconductor wafer to render the first region etch-resistant in an etching process that preferentially etches the undoped portions of the wafer;

machining a plurality of cavities into the other surface of the semiconductor wafer, leaving a web at the bottom of each cavity thicker than the first region, the walls of the cavities being substantially perpendicular to the surfaces of the wafer;

forming openings in the webs to define the pattern for the shadow mask; and

subjecting the wafer to the preferential etching process to reduce the webs to substantially the thickness of the first region.

2. The method of claim 1, which further comprises:

before the machining step, doping a second region adjacent the other surface of the semiconductor wafer to render the second region etch-resistant in the etching process.

3. The method of claim 2 wherein the wafer is silicon having surfaces oriented in (100) crystallographic planes, the cavities are rectangular with walls oriented in (110) crystallographic planes, the first and second regions are doped with boron to a concentration of about 2 ×

10²⁰ atoms per cubic centimeter, and the etchant in the etching process consists of potassium hydroxide, iospropyl alcohol, and water in a weight ratio of about 18;

5;

77.

4. The method of claim 3 wherein the wafer is at least 20 mils thick and the boron is diffused into the first and second regions to a depth less than 10μ

m.

5. The method of claim 1, which further comprises:

doping the walls of the machined cavities to render the walls etch-resistant in the etching process.

6. The method of claim 1 wherein the machining step comprises ultrasonic cutting.

7. A method of fabricating a shadow mask from a silicon wafer having surfaces oriented in (100) crystallographic planes, which comprises:

machining a plurality of rectangular cavities into a first surface of the wafer, leaving a web at the bottom of each cavity, the walls of the cavities being substantially perpendicular to the surfaces of the wafer and being oriented in (100) crystallographic planes;

doping a first region of the wafer adjacent the first surface, the walls of each cavity, and the surface of the web at the bottom of each cavity, and a second region of the wafer adjacent the second surface with boron to a concentration of about 2 ×

10²⁰ atoms per cubic centimeter, the depth of doping being less than half the thickness of the web;

machining the webs at the bottom of the cavities to remove the portions of the first region in the webs;

forming openings in the webs to define the pattern for the shadow mask; and

subjecting the wafer to an etchant consisting of potassium hydroxide, isopropyl alcohol and water in a weight ratio of about 18;

5;

77 to reduce the thickness of the webs to substantially the thickness of the second doped region.

8. The method of claim 7 wherein the machining steps comprise ultrasonic cutting.

9. The method of claim 7 wherein the wafer is at least 20 mils thick and the boron is diffused into the first and second regions to a depth less than 10μ

m.