Semiconductor device manufacturing: process – Making passive device – Resistor
Reexamination Certificate
1999-11-05
2001-05-15
Booth, Richard (Department: 2812)
Semiconductor device manufacturing: process
Making passive device
Resistor
C438S381000, C438S383000, C438S384000, C438S385000, C438S238000
Reexamination Certificate
active
06232194
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to a method of forming polysilicon resistors in the fabrication of integrated circuits, and more particularly, to a method of forming polysilicon resistors with precisely controlled resistance in the manufacture of integrated circuits.
(2) Description of the Prior Art
Polysilicon resistors have been used extensively in Very Large Scale Integrated circuits (VLSI), such as analog to digital converters and poly-load static random access memories (SRAM). An unfortunate characteristic of polysilicon resistors is their ability to absorb hydrogen to which the resistors are exposed during semiconductor processing. This results in resistance reduction and fluctuation of the resistors. Hydrogen penetration, as well as etch and chemical mechanical polishing (CMP) impact of backend processes, make it difficult to control resistance values precisely.
It has been shown that a metal shield over a polysilicon resistor will reduce the resistance variation of the resistor by avoiding hydrogen penetration. However, the extension of the metal to polysilicon affects the performance of the poly-resistor. The chip size can be enlarged to meet the tight control requirements of the poly resistor, but this is undesirable for cost-efficiency reasons. Double resistor load implantation (P+BF
2
) has been proposed to decrease the temperature coefficient of the poly load thereby improving the cell stability of the poly-load SRAM. However, the sensitivity of the poly resistance to implant dosage is too high for manufacturing.
A silicon nitride capping layer has been used to prevent hydrogen diffusion into the polysilicon resistor. For example, U.S. Pat. No. 5,461,000 to Liang and U.S. Pat. No. 5,108,945 to Matthews teach a silicon nitride layer in the range of 500 to 2500 Angstroms over the resistor. Co-pending U.S. Patent Application serial number 09/234.096 to Y. L. Hsu et al filed on Jan. 19, 1999 teaches a silicon nitride layer of 300 to 1000 Angstroms deposited by low pressure chemical vapor deposition (LPCVD). However, such a thick silicon nitride layer is incompatible with the self-aligned contact (SAC) process, which is widely used in SRAM and dynamic random access memory (DRAM) processes to reduce cell size, because the thicker (>100 Angstroms) nitride film acts as an etch stop in the highly selective SAC etch.
U.S. Pat. No. 5,500,553 to Ikegami teaches forming a metal cap over polysilicon resistors to equalize the change in resistance caused by hydrogen atoms diffusing into the polysilicon. U.S. Pat. No. 5,834,815 to Cheng et al also discloses a metal cap over the poly resistor. U.S. Pat. No. 5,384,278 to Singlevich and U.S. Pat. No. 4,604,789 to Bourassa disclose oxide capping layers over polysilicon. U.S. Pat. No. 4,592,128 to Bourassa employs nitride as an implant mask. U.S. Pat. No. 5,728,615 to Cheng et al teaches a thermal treatment in N
2
with a H
2
ambient to equalize the hydrogen concentration in a polysilicon resistor. No capping layer is used, but hydrogen is allowed to penetrate the resistor. U.S. Pat. No. 4,641,173 to Malhi et al show silicon nitride tunneling layers surrounding a vertical poly resistor. The resistance of a vertical resistor is not easy to control because load implantation cannot be used. In addition, while a silicon nitride cap is used to protect the poly resistor from the interpoly oxide, hydrogen atoms can still diffuse into the resistor.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide an effective and very manufacturable method of fabricating a polysilicon resistor in the manufacture of integrated circuits.
A further object of the invention is to provide a method for forming a polysilicon resistor having precisely controlled resistance.
A still further object is to provide a method for forming a polysilicon resistor having precisely controlled resistance by using a thin silicon nitride cap over the polysilicon resistor.
Another object of the invention is to block the diffusion of hydrogen atoms into a polysilicon resistor by means of a thin silicon nitride capping layer.
Yet another object of the invention is to integrate the thin silicon nitride capped resistor with a self-aligned contact process.
In accordance with the objects of this invention a new method of forming a polysilicon resistor having precisely controlled resistance by using a thin silicon nitride cap over the polysilicon resistor is achieved. A dielectric layer is provided on a semiconductor substrate. A polysilicon layer is deposited overlying the dielectric layer and patterned to form a polysilicon resistor. A silicon nitride capping layer having a thickness of not more than 100 Angstroms is deposited overlying the polysilicon resistor and dielectric layer. An interlevel dielectric layer is deposited overlying the silicon nitride capping layer. The substrate is annealed thereby densifying the silicon nitride capping layer. A self-aligned contact opening may be made through the interlevel dielectric layer, the silicon nitride capping layer, and the dielectric layer to underlying device structures. The capping silicon nitride layer is thin enough not to act as an etch stop in the self-aligned contact etching. The contact opening is filled with a conducting layer. A passivation layer is deposited overlying the conducting layer wherein the passivation layer contains hydrogen atoms and wherein the silicon nitride capping layer prevents the hydrogen atoms from penetrating the polysilicon resistor.
REFERENCES:
patent: 4408385 (1983-10-01), Mahan Rao et al.
patent: 4592128 (1986-06-01), Bourassa
patent: 4604789 (1986-08-01), Bourassa
patent: 4641173 (1987-02-01), Malhi et al.
patent: 5108945 (1992-04-01), Matthews
patent: 5384278 (1995-01-01), Singlevich
patent: 5461000 (1995-10-01), Liang
patent: 5500553 (1996-03-01), Ikegami
patent: 5728615 (1998-03-01), Cheng et al.
patent: 5824579 (1998-10-01), Subramanian et al.
patent: 5834815 (1998-11-01), Cheng et al.
patent: 6069063 (2000-05-01), Chang et al.
patent: 6100154 (2000-08-01), Hsu et al.
patent: B1 6184103 (2001-02-01), Li et al.
Wuu Shou-Gwo
Yaung Dun-Nian
Ackerman Stephen B.
Booth Richard
Kennedy Jennifer M.
Pike Rosemary L. S.
Saile George O.
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