WO1997005650A1

WO1997005650A1 - ETCHING OF FILMS OF CdSe

Info

Publication number: WO1997005650A1
Application number: PCT/CA1995/000453
Authority: WO
Inventors: Chengbin Qiu
Original assignee: Litton Systems Canada Limited
Priority date: 1995-07-31
Filing date: 1995-07-31
Publication date: 1997-02-13

Abstract

A process for the etching of a film of CdSe on a substrate is disclosed. The process comprises the steps of (a) placing the film of CdSe between electrodes in a chamber of a plasma apparatus at a pressure of 10-500 mTorr; (b) introducing gaseous ammonia into said chamber; and (c) at a temperature of at least 60 °C, effecting etching of the film of CdSe by applying a radio frequency (RF) between the electrodes to form a plasma. The preferred temperature is in the range of 100-160 °C, and the preferred frequency is 13.6 MHz. The process is particularly useful in the patterning of small featured device structures and for structures with requirements for vertical sidewalls.

Description

ETCHING OF FILMS OF CdSe

The present invention relates to a process for the etching of films of CdSe, also referred to herein as layers or coatings of CdSe, on another substrate. In particular, the process relates to the etching of films of CdSe

using a dry etching process, specifically a process using an ammonia plasma.

The invention will be particularly described herein with reference to films of CdSe.

The process described herein is a dry etching process. In particular

embodiments, it may be considered to be of the type of etching that is referred

to in literature e.g. "Plasma Etching and Reactive Ion Etching" by J.W. Coburn, AVS Monograph Series (1982), as a plasma etching.

CdSe films have been used extensively in optoelectronic e.g. solar cells, and microelectronic (TFT or thin film transistor) device applications. The latter

are used widely in active matrix liquid crystal display and X-ray imaging

applications.

Conventionally, CdSe films have been etched using wet solutions. As an example, CdSe films have been etched using solutions of bromine in water. Such solutions tend to be very corrosive, toxic and to require the use of special waste water treatment processes in order to comply with governmental, municipal or other environmental regulations, as well as such other regulations as those applying to occupational health. Moreover, such wet processes cannot be used to obtain very fine geometry in the etched product. For instance, it is difficult to obtain a square etching of the substrate i.e. to obtain etching with a vertical sidewall, as opposed to negatively sloped or undercut sidewalls, which is important for good step coverage of passivation layers and also in structures intended for use in some applications e.g. in ridge-wave-guide semiconductor

laser structures.

Dry etching offers the capability of etching a substrate to obtain vertical

sidewalls, and as such offer advantages over wet etching processes. Moreover, dry etching also offers important manufacturing advantages by elimination of the handling of corrosive solutions and of the need for disposal of relatively large quantities of acids and solvents that are utilized in wet etching processes. Dry etching uses gases. Gases are substantially easier to handle in a manner that is acceptable to the pertinent authorities, than are liquids.

In dry etching processes, a reactive species is generated in a plasma, and diffuses to the surface of the material to be etched. The species are absorbed on the surface, at which time a chemical reaction occurs. The resultant product must be volatile, to the extent that it will be desorbed from the surface under the reaction conditions, especially temperature, being used. Each of this sequence of steps is important to the operation of a dry etching process, and any step in the sequence can become rate-limiting to the extent that the overall etching process ceases to operate in an acceptable manner. Thus, the nature of the material to be etched, the gas used and the operating conditions of the process are all important parameters in the overall operation of the dry etching

process.

A process for the dry etching of films of CdSe, especially CdSe films on a substrate, has now been found. Accordingly the present invention provides a process for the etching of a film of CdSe on a substrate comprising the steps of:

(a) placing the substrate with the film of CdSe thereon between electrodes in a chamber of an apparatus capable of generating a plasma between said electrodes, with the layer of CdSe being exposed to said plasma, said chamber being at a pressure of 10-500 mTorr;

(b) introducing gaseous ammonia into said chamber; and

(c) with the film at a temperature of at least 60°C, effecting etching of the film of CdSe by applying a radio frequency between the electrodes so as to form a plasma therebetween.

In a preferred embodiment of the process of the present invention, a mask is placed on the film of CdSe prior to the etching process.

In a further embodiment, the radio frequency is applied for a period of time sufficient to effect etching to a predetermined amount. In yet another embodiment, the temperature is in the range of 100-

160°C.

In a still further embodiment, the plasma is a RF plasma having a frequency of 13.6 MHz.

Aspects of the present invention are illustrated by the drawings in which: Figure 1 is a schematic representation of apparatus for a dry etching process; and

Figure 2 shows results obtained in an embodiment of a process of the invention, as illustrated by Example I. The process of the present invention relates to a dry etching process.

Apparatus and procedures for operation of dry etching processes are known in

the art. The present invention particularly relates to the dry etching of CdSe with ammonia, using a plasma, and will be particularly described herein with reference to films of CdSe.

The CdSe subjected to the process will normally be in the form of a layer,

coating or film of CdSe on a substrate, the substrate normally being a substrate that is inert under the conditions of the etching process. Examples of such substrates include glass and silicon wafers. The thickness of the CdSe film should be such that after etching the layer of CdSe on the substrate at the position of the etching has been fully removed or only partially removed, depending on the particular proposed end-use. In addition, the amount of etching may be controlled by adjusting the etching time.

Apparatus of the type used in a dry etching process is illustrated in Fig. 1. The apparatus has a chamber 1 fitted with an inlet for gas 2 viz. ammonia in the present invention, and a pump 3. Chamber 1 also has two electrodes, 4 and 5, between which a plasma is formed. Electrode 4 is connected through capacitor 6 to RF generator 7. Wafer 8 is placed on the other electrode, 5.

In the process, the substrate with the film of CdSe that is to be etched, referred to above as wafer 8, is placed within the chamber and between a pair

of electrodes in apparatus adapted for plasma etching, as illustrated in Fig. 1 and as is known in plasma etching processes i.e. the substrate is placed on electrode 5 in chamber 1 . The chamber is then evacuated, using pump 3, especially to a pressure that is in the range of 10-500 mTorr, more preferably in the range of 25-100 mTorr.

The layer of CdSe will normally have a mask such that the etching of the layer of CdSe occurs in a controlled manner. The mask needs to be inert with

respect to the ammonia plasma at the temperature of operation of the process.

As examples, the mask may be silicon oxide or silicon nitride. The nature of the mask may have an affect on the temperature at which the process may be operated, as discussed below.

Gaseous ammonia is then introduced into the chamber, through inlet 2. It is preferred that the gaseous ammonia be in the form of a flow of ammonia through the chamber.

The process is operated at a temperature of at least 60°C and more preferably at a temperature of at least 100°C. As is exemplified hereinafter, it has been found that the rate of etching is dependent on the temperature at which the etching takes place. The upper limit of the temperature that may be used is primarily dependent on the nature of the mask that is used and other process parameters. For instance, if the mask is silicon oxide, the process may be operated at temperatures of up to at least 160°C whereas with other masks e.g. photoresists, the mask is not structurally stable at temperatures above about 130°C. Thus the temperature of operation of the process is dependent

on the particular mask that has been applied to the CdSe film.

The plasma is formed by applying a radio frequency discharge between the electrodes i.e. between electrodes 4 and 5 of Fig. 1 . The frequency of the

RF system may be varied over a wide range, but in practice because of governmental, industry and other regulations, the process is normally operated at 13.6 MHz. Other frequencies may however be used.

After the CdSe layer has been etched to a predetermined amount, the etched substrate is removed from the chamber. It may subsequently be

prepared for use.

The process of the present invention is believed to be useful in the

etching of CdSe films. It is believed that the process will be particularly useful in the patterning of small featured device structures and for structures with requirements for vertical sidewalls. The latter include ridge-wave-guide semiconductor laser structures. Other uses include CdSe IC drivers, smaller size

CdSe TFT for high resolution, high performance active matrix liquid-crystal display and X-ray imager applications.

The present invention is illustrated by the following examples.

EXAMPLE I A substrate with a coating (film) of CdSe thereon was placed on the grounded electrode of apparatus for dry etching in the presence of a plasma.

The electrodes were located within chamber 1 of the plasma apparatus, which was capable of being evacuated and of having gaseous ammonia fed into it, as illustrated in Fig. 1 . The chamber was evacuated to a pressure of 50 mTorr. Ammonia was then fed to the chamber at a gas flow rate of 40 seem. The frequency of the discharge used to form the plasma was an RF frequency of

13.6 MHz, which was applied at a power of 165 w.

In a series of experiments, the temperature of the substrate was varied

between 60°C and 160°C. The plasma was applied for a period of time, which was measured.

Subsequently, the substrate with the etched CdSe layer was removed from the chamber and the amount of etching was measured using an ellipsometer. The etch rate, in A/min, was determined. The results obtained are shown in Fig. 2. It was found that the film of CdSe was etched over the full temperature range that was tested. The rate of etching was slower at temperatures below about 100°C but at higher temperatures the etching rate increased steadily with increasing temperature.

EXAMPLE II

The procedure of example I was repeated, except that the plasma was not applied to the substrate. It was found that there was no etching of the

CdSe layer.

This example shows that the ammonia plasma is essential to the etching of the CdSe layer. Ammonia does not cause etching of CdSe in the absence of the plasma.

Claims

CLAIMS:

1 . A process for the etching of a film of CdSe on a substrate

comprising the steps of:

(b) introducing gaseous ammonia into said chamber; and

2. The process of Claim 1 or Claim 2 in which a mask is placed on the CdSe film prior to the etching process.

3. The process of Claim 1 or Claim 2 in which the radio frequency is applied for a period of time sufficient to effect etching to a predetermined amount.

4. The process of any one of Claims 1-3 in which the temperature is in the range of 100-160°C.

5. The process of any one of Claims 1 -4 in which the plasma is a RF plasma having a frequency of 13.6 MHz.

6. The process of any one of Claims 1 -5 in which the process is a plasma etching process.