GB2182488A

GB2182488A - Integrated circuit resistors

Info

Publication number: GB2182488A
Application number: GB08527048A
Authority: GB
Inventors: Roger Leslie Baker
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-11-02
Filing date: 1985-11-02
Publication date: 1987-05-13
Also published as: GB8527048D0

Abstract

High value resistors in integrated circuits are fabricated by oxygen doping intrinsic polycrystalline silicon to form semi-insulating material, and doping this material with boron, phosphorus or arsenic to provide the desired resistivity. The process provides a resistors of low temperature coefficient and low sensitivity to surface contamination. <IMAGE>

Description

SPECIFICATION Integrated circuit resistors This invention relates to integrated circuits and in particular to the provision of high value resistors for such circuits.

There are a number of applications where high value resistors are required in integrated circuits. For example, current static random access memory designs require resistors in the G-ohm range to define load currents.

These resistors are conventionally formed in intrinsic or lightly phosphorus or boron doped polycrystalline silicon. Such resistors however suffer from the disadvantage that they are extremely sensitive to surface contamination.

This results in a wide spread of resistor values and a consequent low yield of fully functional circuits. Attempts have been made to overcome this problem by increasing the doping level to reduce the sensitivity to contamination, but this reduces the resistivity of the material to an unacceptably low level.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a method of forming a polycrystalline silicon resistor in an integrated circuit, the method including providing a semi-insulating polycrystalline silicon film, and doping this semi-insulating film with phosphorus, arsenic or boron to a level corresponding to the resistor value.

The oxygen doping may be provided by in situ doping of the polycrystalline material during deposition or by oxygen ion implantation of a previously formed polycrystalline silicon film.

We have found that the use of an oxygen doping significantly reduces the sensitivity of the resistor to surface contamination. Because the effect of this doping is to increase the resistivity of the polysilicon, a relatively high p- or n-type doping level can then be employed to achieve the desired resistivity level.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows the relationship between band gap and oxygen doping levels of in-situ doped semi-insulating polycrystalline silicon (SIPOS); Figure 2 shows the relationship between phosphorus implantation dose and sheet sensitivity of SIPOS, and Figures 3 and 4 illustrate the effects of annealing on respectively phosphorus and arsenic implanted SIPOS.

In a typical process, a region of high resistivity semi-insulating polycrystalline silicon (SI POS) is formed by in-situ oxygen doping of polycrystalline material during deposition on a substrate surface, e.g. an integrated circuit.

The effect of this oxygen doping is to increase the band gap of the material as shown in Fig. 1 of the accompanying drawings. Typically we employ oxygen doping levels in the range 2% to 10% and preferably 2% to 5%, but the technique is not of course limited to these ranges.

The very high resistivity of this semi-insulating material is then reduced to a desired value by doping with boron, phosphorus or arsenic.

Typically this doping is effected by ion implantation. The reduction in resistivity achieved by this process is a function of the doping level, the relationship for phosphorus doping being illustrated in Fig. 2. For comparison purposes, Fig. 2 also includes corresponding results for polysilicon containing no oxygen. A similar relationship is obtained from arsenic and boron dopants.

Where doping is effected by ion implantation it is of course necessary to anneal the implanted material to repair damage. The effect of this anneal is to cause a further small reduction in resistivity. Figs. 3 and 4 show the effect of annealing in any nitrogen of phosphorus and arsenic implanted SIPOS.

Again, comparative results for polysilicon are included.

In an alternative embodiment the polysilicon is rendered semi-insulating by implanting previously deposited material with oxygen ions.

Typically we provide an oxygen implant in the range 10'6cm-2 to 1018cm-2 resulting in an intrinsic resistivity of 108 to 1012 ohm cm2.

The implanted material can then be doped with arsenic, boron or phosphorus to provide a controlled resistivity between 105 ohm cm and 10-' ohm cm.

We have found that resistors formed in this way have a low temperature coefficient. The incorporation of oxygen into polysilicon increases the bandgap of the material thus reducing the number of thermally generated carriers.

The technique is suitable for the fabrication of high value load resistors in a static random access memory where their low temperature coefficient and selective circuitry to surface contamination are particularly desirable features.

Claims

1. A method of forming a polycrystalline silicon resistor in an integrated circuit, the method including providing a semi-insulating polycrystalline silicon film, and doping the semi-insulating film with phosphorus, arsenic or boron to a level corresponding to the resistor value.

2. A method as claimed in claim 1, wherein the intrinsic polycrystalline silicon is oxygen doped in-situ.

3. A method as claimed in claim 2, wherein the silicon is oxygen doped to a level in the range 2% to 10%.

4. A method as claimed in claim 1, wherein the polycrystalline silicon is oxygen doped by implementation with oxygen ions.

5. A method as claimed in claim 4, wherein oxygen is implanted to a level of 1016 to 1018 cm-2.

6. A method as claimed in claim 5, wherein the resistivity of the implanted material is 108 to 1012 ohm cm2.

7. A method as claimed in any one of claims 1 to 6, wherein said phosphorus, arsenic or boron doping is effected by ion implantation.

8. A method of forming a polysilicon resistor substantially as described herein.

9. An integrated circuit incorporating a plurality of resistors as claimed in any one of claims 1 to 8.

10. An integrated circuit as claimed in claim 9 and comprising a static random access memory.