GB2194556A

GB2194556A - Plasma enhanced chemical vapour deposition of films

Info

Publication number: GB2194556A
Application number: GB08719391A
Authority: GB
Inventors: Peter Michael Corser Martineau
Original assignee: SPECIAL RESEARCH SYSTEMS LIMIT
Current assignee: SPECIAL RESEARCH SYSTEMS LIMIT
Priority date: 1986-08-21
Filing date: 1987-08-17
Publication date: 1988-03-09
Anticipated expiration: 2007-08-17
Also published as: GB2194556B; GB8620346D0; GB8719391D0

Abstract

In a method of depositing a film on a workpiece by plasma enhanced chemical vapour deposition in a vacuum or low pressure chamber, the electrical supply to the electrode(s) generating the plasma is controlled to produce multiple frequencies. The frequencies may be switched, combined or selected to deposit films having any desired stress.

Description

SPECIFICATION Chemical vapour deposition of films This invention relates to methods and apparatus for chemical vapour deposition of thin films. For example thin plasma enhanced chemically vapour deposited (PECVD) films are widely used in the semiconductor/electronics industry. Examples include silicon nitride-like, silicon oxynitride-like, silicon dioxide-like and hydrogenated amorphous silicon films. Applications include final passivation of integrated circuits and interlayer dielectrics, annealing caps, thin film transistors, and transistor gates in device manufacture. Typical substrate materials are silicon, gallium arsenide, alumina, gold, aluminium and quartz.

The films are normally deposited by "cracking" silane (SiH4) with other gases in a Radio Frequency (RF) at controlled sub atmospheric pressure, resulting in generation of a gas plasma.

By varying the system parameters, the film composition, uniformity, deposition rates and film stress can be changed. The following simplified reactions indicate the chemistry involved: SiH4+NH3 < SiXNy:H SiH4+N20 H Six0y:H SiH4+NH3+N20 , SixOyNz:H SiH4 a-Si:H Some of the system variables are as follows: Reactor Pressure R.F. Input Power Substrate Temperature Gas Composition Gas Flow Rates Gas Flow Ratios R.F. Frequency Electrode Spacing Chamber Diameter Chamber Configuration The deposition steps involve decomposition of the gases, transport to the substrates and then a surface reaction.It should be noted that very often pure silane is not used due to its pyrophoric nature and a diluted mixture (ie. in N2,Ar,He) is substituted but essentially the chemistry is the same. One film parameter of great importance is the mechanical stress in the deposited layer. It has been shown by extensive research that the stress is a strong function of R.F. excitation frequency.

Changes in all system variables cause the value of the stress to change (inclusion/exclusion of hydrogen). However, due to the basic differential contraction of the deposited film relative to the substrate the stress will never be zero. Using frequencies in the 500KHz to 4MHz band is not allowed, therefore deposition has been restricted to high (13.56MHz) or lower (0 to 500KHz) frequency. Films deposited at HF possess much higher dielectric strengths but due to their inherent tensile nature they will tend to crack around sharp topographies. On the other hand, films deposited at low frequencies are invariably compressive in nature and are well suited to forming conformal films on severe device topographies with good cracking resistance.

It is an object of the invention accordingly to provide an improved chemical vapour deposition process which will at least partly eliminate some of the existing difficulties, and broadly stated the invention consists in a method of depositing a film on a workpiece by plasma enhanced chemical vapour deposition in a vacuum or low pressure chamber, in which the electrical supply to the electrode(s) generating the plasma is controlled to produce multiple frequencies.

The electrical supply tray may be switched in succession between different frequencies. For example if there are just two frequencies or frequency ranges the supply may be switched alternately between high and low frequencies. Conveniently the high frequency is above about 13MHz and the low frequency is below 500KHz. The switching sequence and timing may vary but conveniently the frequencies are switched at intervals of less than one second.

Alternatively the different frequencies may be maintained simultaneously.

In any case according to a preferred feature of the invention the frequencies are switched or combined or selected to produce a film having substantially zero stress, or any desired tensile or compressive stress.

From another aspect the invention consists in apparatus for plasma enhanced chemical vapour deposition of films on workpieces, comprising a vacuum chamber having a support for a workpiece, an electrode, and means for supply radio-frequency electrical current thereto, means for introducing a controlled low pressure gas into the chamber and means for controlling the electrical supply to the electrode to generate multiple frequency effects.

The invention may be performed in various ways and one example, with some modifications and variation, will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating a plasma chemical vapour deposition system, Figure 2 is a graph illustrating variations in stress of films deposited at different frequencies, Figure 3 is a diagram illustrating the main components of a typical CVD apparatus according to the invention, and Figure 4 is a diagram illustrating one typical switching sequence according to the invention.

In Fig. 1 the samples or workpieces 10, such as semi-conductor wafers on which films are to be deposited, are supported on a fixed or rotary table 11 within a vacuum chamber 12, heating means 13 are provided below the table to maintain the workpieces at the desired temperature and means are provided for evacuating the chamber by means of two or more low pressure pumps 15 and for supplying a selected controlled gas via a valve controlled gas supply system 16. A porous or perforated electrode 17 is located in the chamber above the work table and an electrical supply circuit 20 is connected between the electrode and the work table. As a result gas plasma 21 is generated between the electrode and the workpieces and as is well-known the plasma enhances the chemical vapour deposition on the semi-conductor wafers.

Fig. 2 illustrates the alterations in internal stress in the deposited film depending on the frequency of the applied RF field. At frequencies below 500 KHz the stress is compressive while at frequencies above 4MHz the stress is tensile. The actual changeover from tensile to compressive may be at approximately 1 MHz, but in practice it is difficult or impossible to locate this zero stress frequency.

In the apparatus illustrated in Fig. 3 for performing the invention, the basic components are similar to those shown in Fig. 1 and are indicated by the same reference numerals. In this case the electrical supply system 30 includes two (or more) separate frequency generators 31,32, one generator 31 being designed to generate a frequency of approximately 13.5 MHz and the other generator 32 a relatively low frequency of approximately 187.5 KHz. In addition, there is a switching or cycling control 33 arranged to activate or switch the two generators in sequence.

The timing of the sequence may be variable or preset and in a typical example it is contemplated that the low frequency generator 31 will be actuated for periods of three seconds and the high frequency generator for one second periods, alternately.

The resulting frequency switched supply to the RF electrode is illustrated in Fig. 4.

A number of variations in this system and procedure are possible. For example, it may be useful in some applications to run the two frequencies simultaneously thus producing in effect a third modulated or mixed frequency. It may also be of advantage to have three or more different frequency generators with corresponding switching or cycling systems. The switching speed can be varied or selected as required to produce optimum film parameters. The wave form of the applied R.F. currents may also be controlled, as required for example to give a square, sinusoidal, trapezoidal, saw toothed or any other wave shape.

The invention may be applied to all types of reactor PECVD and to a wide range of gases or mixtures, to substrates or components of every possible form and composition and to deposited films of all possible types.

Claims

1. A method of depositing a film on a workpiece by plasma enhanced chemical vapour deposition in a vacuum or low pressure chamber, in which the electrical supply to the electrode(s) generating the plasma is controlled to produce multiple frequencies.

2. A method according to Claim 1, in which the electrical supply is switched in succession between different frequencies.

3. A method according to Claim 2, in which the- supply is switched alternately between high and low frequencies.

4. A method according to Claim 3, in which the high frequency is above about 13 MHz and the low frequency is below 500 KHz.

5. A method according to any of the preceding Claims 2 to 4, in which the frequencies are switched at intervals of less than one second.

6. A method according to Claim 1, in which the different frequencies are maintained simultaneously.

7. A method according to any of the preceding claims, in which the frequencies are switched or combined or selected to produce a film having substantial zero stress.

8. Apparatus for plasma enhanced chemical vapour deposition of films on workpieces, comprising a vacuum chamber having a support for a workpiece, an electrode, and means for supplying radio-frequency electrical current thereto, means for introducing a controlled low pressure gas into the chamber and means for controlling the electrical supply to the electrode to generate multiple frequency effects.

9. Methods of depositing films on work-pieces substantially in any of the forms described herein.

10. Apparatus for depositing films substantially in any of the forms described herein.

11. Workpieces with films deposited by the methods or apparatus of any of the preceding claims.