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Doped Polysilicon Diffusion Source and Its Application in Single Electrode Random Access Memories

IP.com Disclosure Number: IPCOM000088427D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 61K

Publishing Venue

IBM

Related People

Ho, IT: AUTHOR [+2]

Abstract

A layer of doped polysilicon can be deposited on top of a crystalline silicon wafer at a rather low temperature, for example, around 700 Degrees C. The doping can be either P impurity, such as boron, or N impurity, such as arsenic or phosphorus, or the combination of P and N. At such temperature, no appreciable diffusion into the crystalline silicon will occur. After a conventional polysilicon etching process, the polysilicon which remains may be used as a diffusion source for the crystalline silicon at an elevated temperature, for example, around 1000 Degrees C.

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Doped Polysilicon Diffusion Source and Its Application in Single Electrode Random Access Memories

A layer of doped polysilicon can be deposited on top of a crystalline silicon wafer at a rather low temperature, for example, around 700 Degrees C. The doping can be either P impurity, such as boron, or N impurity, such as arsenic or phosphorus, or the combination of P and N. At such temperature, no appreciable diffusion into the crystalline silicon will occur. After a conventional polysilicon etching process, the polysilicon which remains may be used as a diffusion source for the crystalline silicon at an elevated temperature, for example, around 1000 Degrees C.

Alternatively, an oxide layer may first be formed on top of the crystalline silicon wafer. Windows may be opened by etching away the oxide, and then a layer of doped polysilicon may be deposited to be used as a diffusion source.

During the diffusion process, or before it, a thin layer of oxide is preferably formed on the polysilicon layer to keep the impurities from evaporating into the environment.

The polysilicon diffusion source approaches may be used in many different ways, in either bipolar or MOS technologies. One of these applications is shown in producing a single electrode random-access memory with charge-coupled device (CCD)-type storage elements. Such random-access memory cells are characterized with a single electrode which serves both as the holding plate and the word line. Underneath this single electrode, in each cell, there are two distinguishable regions different in their threshold voltages. One region is, of course, the storage area and the other, the control gate or the transfer area.

The single electrode CCD-type random-access memory can be made with three masks utilizing the polysilicon diffusion source technique. In Fig. 1, the recessed silicon oxide grids 10 may first be formed. The cross-sectional view of Fig. 1 is shown in Fig. 2. P+ diffusion grid 12 may be used under the silicon dioxide 10. A layer of polysilicon is deposited and etched to form bit lines 14 which wer...