US3518134A - Gaseous etching of molybdenum - Google Patents

Description

US. Cl. 156-18 United States Patent 3,518,134 GASEOUS ETCHING OF MOLYBDENUM Ruth C. Preist, Menlo Park, Calif, assignor to Stanford Research Institute, Menlo Park, Calir., a corporation of California No Drawing. Filed Aug. 14, 1967, Ser. No. 660,241 Int. Cl. C23b 3/00; C23f 1/02 11 Claims ABSTRACT OF THE DISCLOSURE A procedure for the precision etching of molybdenum surfaces at an etch rate of 0.1 to 1 micron or more per minute with a high etch factor is disclosed by heating the surface to a temperature above 300 C. while delivering to the surface a mixture of oxygen and hydrogen chloride gases at a pressure below about one atmosphere and removing the volatile oxychloride reaction gases from the etched surface. Precision etched patterns are obtained by the use of a thin film of aluminum oxide resist film on the surface.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method of etching molybdenum and more particularly to the precision high etching of heated thin films of molybdenum with a mixture of gases.

Description of the prior art Reactions leading to a solution or volatilization of molybdenum may not be acceptable as etchants for several reasons. An etchant should provide good dimensional control and leave a smooth, undamaged finish surface. The reaction, even if sufficiently vigorous, may leave a residue on the surface or may selectively attack grain boundaries or dislocations leading to uneven etch rates and patterns. The corrosive chemicals may destroy resist layers or damage adjacent structures or underlying layers. It is further desirable that the etchant be based on lowpressure gases so that the etching steps can be accomplished in the same equipment used for fabrication of thin film devices.

SUMMARY OF THE INVENTION .The etching method of the invention utilizes gaseous reactants at low pressure which react with the surface molybdenum film to form a volatile compound which on removal leaves a clean, undamaged surface which is at least a smooth as the pre-etch surface. The gases do not attack an aluminum oxide resist film and remove molybdenum with good control of depth and degree of undercutting to provide high resolution and precision etched patterns.

Briefly, in themethod ofthe invention, a mixture of oxygen and hydrogen chloride gases at a low pressure below about atmospheric and preferably below 100 torr are delivered to the surface of a molybdenum article at a temperature above about 300 C. and the volatile react ionproducts are removed. This results in a very favorable etch: rate of 0.01 to 1 micron per minute.

"The smooth removal of molybdenum by oxygen and hydrogen chloride gases at high rate is quite unexpected sincepure HCl does not etch molybdenum at all under similar conditions and the reaction of molybdenum film with oxygen is very slow and a thick residue remains on the surface. The removal of molybdenum according to the invention is believed to be accompplished by the two- 3,518,134 Patented June 30, 1970 The temperature at the surface should be maintained above about 300 C. since this is the temperature at which the molybdenum oxychloride becomes volatile and the reactions shown above -will occur. The upper temperature of the etching reaction is only limited by decomposition of the molybdenum trioxide and the oxychloride. However, acceptable etch rates are obtained without exceeding 800 C. and less likelihood of damage to the associated structures and better control are obtainable at temperatures below 600 C.

At the lower temperatures the rate of etching appears to be limited by the rate of formation of molybdenum trioxide which is observed to be very slow at 300 C. with diatomic oxygen and quite fast at 600 C. However, it is believed that since energy of dissociation is not required and the probability of diffusion is increased, the surface temperature can be lowered to the 300 C. to 400 C. range while maintaining a reasonable reaction rate. It would also appear desirable to bombard the surface with slow electrons or radio frequency radiant energy to speed the reaction at the surface.

At temperatures below 770 C. lower oxidation states of molybdenum are formed. A dioxide layer possibly containing some intermediate oxides is found nearest the metal surface while the trioxide exists at the gas interface. The MoO is an intermediate in the overall removal of molybdenum but if the reaction is allowed to go to completion the film of M00 is also removed. The rate of removal of molybdenum is dependent on the thickness of the layers and correspondingly on the rates of formation and dissociation of the oxides as well as the stability and rate of diffusion of oxygen through the layers. All of these factors are temperature dependent.

If all the reaction products were volatile and there was no boundary layer of molybdenum oxide, it is not probable that the reaction zone would be dilfusion limited to the surface molecular layer as it is in the high temperature metal-gas reaction of concern and is believed to contribute to the even and smooth etched surfaces.

The above postulated mechanism is borne out by the fact that a thin, pale brown film of relatively hard, adherent and transparent material remains on a partially etched surface. This film has been identified as M00 a compound which is stable to about 1780 C. when heated in bulk in vacuum. However, under any fixed set of conditions of gas pressure, gas composition and substrate temperature, eventually an equilibrium will be established between the rate of growth of the oxide film and the rate of removal of M00 by formation of MoO Cl Thus, an increase of the total pressure by increasing the partial pressure of either HCl or 0 decreases the M00 film thickness by increasing the probability of molybdenum trioxide formation and its reaction with HCl, before the M00 gives up its 0 to form M00 in the underlying Mo. Thus the rate of removal of molybdenum increases. Lower temperatures also promote lower dioxide film thickness since the dissociation of the trioxide is less favored.

It has further been discovered that, for a given gas mixture, there is a certain critical temperature where the oxide film appears to have a minimum thickness and the etch rate is at a maximum. This temperature is believed to correspond to the temperature at which the reaction, dissociation and diffusion rates are balanced to give the most efficient etching. Variation of the temperature above or below increases the thickness of the film and lowers the etch rate. An increase in oxygen partial pressure was observed to move this band of maximum etch rate to a region of lower temperature and an increase in HCl rate moved the band of maximum etch rate to a region of higher temperature. The choice of conditions depends on whether a thinner oxide film and faster etch rate or thicker oxide film and better etch factor or resolution are desired.

Several alternatives exist for the removal of the dioxide film after etching. The film can be fired in hydrogen to reduce the oxide to molybdenum metal. However, the residue of molybdenum metal could be objectionable because of its texture or structure. Chlorine gas reacts with the substrate at a temperature above 600 C. to rapidly remove the oxide film but it has been found that the chlorine also attacks molybdenum extremely rapidly under these conditions, and 'would destroy all dimensions established by the original etching.

A suitable method which can be effected in the same vacuum equipment without movement of the substrate is to adjust the substrate temperature to about 300 C. to about 500 C. and the pressure to above 1 torr, preferably 25 to 100 torr so that surface reaction rates are temporarily favored over dissociation and diffusion rates. Again, the use of atomic or ionic oxygen may have a favorable effect by increasing the surface rate of formation of the oxychloride film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrative methods of etching according to the invention are offered by way of illustration in the following specific examples which are not intended in any way to be limiting since alternate or modified modes of operation are permissible without deviating from the scope of the invention.

A series of one-half inch squares of 0.030 inch thick barium borosilicate as drawn glass substrate specimens were cleaned and coated with one-half to 1 micron thick films of molybdenum by evaporation in vacuum from an electron bombardment heat source. The films were laid down at rates of about one to four per hour. Some of the molybdenum coated substrates were further coated with an A1 pattern as resist. Polystyrene balls of onequarter to 1 micron diameter were blown from a nebulizer over the molybdenum surface and the A1 0 was deposited over them by evaporation in vacuum from an electron bombardment source. The balls were easily removed by a short ultrasonic cleaning in water and detergent leaving clearly defined holes in the resist slightly larger than the diameter of the polystyrene spheres.

The coated glass squares were placed in a vacuum bell jar on top of a quartz envelope having a flat top surface and containing a carbon heating filament. The isolation of the radiant heater prevents unwanted reactions be tween the etchant gases on the filament and its supports and connections. The filament was energized to heat the substrate to a temperature from 400 C. to 700 C. by radiation through the quartz sleeve and oxygen and hydrogen chloride gases were metered into the jar from pressurized tanks and the gases were removed by means of a mechanical vacuum pump as illustrated in the following examples.

EXAMPLE 1 A sample containing a 4500 A. thick molybdenum film and a 1000 A. thick aluminum oxide resist film containing holes formed by one-quarter balls was heated to 670 C. in the reaction chamber and 3.8 cc./sec. of oxygen and 0.2 cc./sec. of HCl were delivered to the bell jar at a pressure of about 0.3 torr. The reaction products and unreacted gases were continuously removed for a period of 17 minutes. The molybdenum was etched at a rate of about 0.025 micron per minute.

The molybdenum surfaces exposed through the holes in the resist were uniformly removed and the etched surface was as smooth or smoother than the pre-etched surface. The etch factor which is defined as the ratio of the depth of etch divided by the distance etched under the edge of the resist has been at least about two. Dimensional control accurate to better than 0.1 micron in films 1 micron thick is permissible and even greater precision is attainable in films thinner than 1 micron. For a given set of etching conditions of temperature, time, pressure and gas composition the rate and condition of the etched surface can be accurately reproduced.

The minimum amount of HCl is that required to remove the MoO before it can dissociate and combine with the underlying Mo to form M00 which remains on the etched surfaces. At the temperatures under consideration and at pressures from 0.1 to 1 torr as little as 1 volume percent HCl present in the gas mixture appears to provide practical etch rates. In fact, over certain ranges of temperature and pressure the rate of etching increases with increased pressure provided by increasing the amount of either HCl or 0 admitted to the reaction chamber. This is apparently due to the suppression of dissociation of the trioxide and the increased probabilities of formation of trioxide and of the volatile oxychloride. This phenomena is illustrated below.

EXAMPLE 2 A sample identical to that of Example 1 was heated to 670 C. in the reaction chamber and 7.8 cc./sec. of O and 0.2 cc./sec. of HCl gas were delivered to the bell jar at a pressure of about 0.6 torr and the reaction products and unreacted gases were continuously removed for a period of ten minutes. The molybdenum was etched at a rate of about 0.045 micron per minute.

The 0.1 micron A1 0 resist layer is unaffected by the etchant gases and it has been found that A1 0 resist films at least 0.15 micron thick are impervious to the etchant gases at temperatures up to 800 C. The surface characteristics and shapes of the holes was about the same in each experiment. Other replicate samples were etched at substrate temperatures of 480 C. and 580 C. with flow rates of HCl from 0.1 cc./sec. to 0.2 cc./sec. with a reaction chamber pressures from about 0.4 to l torr. The molybdenum was etched at a rate of 0.01 to 0.6 micron per minute.

Each molybdenum specimen partially etched at lower pressure contained the hard, transparent and adherent film of molybdenum dioxide. This film can be removed by favoring surface reaction rates over dissociation rates by increasing pressure at lower etch temperature according to the following procedure exemplified with respect to an uncoated specimen. However, the oxide coated samples can be treated in an identical manner to remove the oxide film.

EXAMPLE 3 A molybdenum coated glass square was placed in the evacuated vacuum bell jar and heated to 400 C. A premeasured oxygen and HCl mixture in a volume ratio of -O /HC1 of 4/1 was introduced to provide a pressure of 0.1 atmosphere (76 torr). The molybdenum film was removed at the rate of one micron per minute.

The rate of formation of the molybdenum oxychloride was sufliciently high in this case to prevent formation of any residual transparent dioxide film which is usually of such thickness to give a brown interference color. The etch rate was simultaneously increased to a rate of 1 micron per minute. Since the final etched surface was still very smooth, it appears that the reaction is still limited to the surface molecular layer even at the higher pressure of this experiment. However, precision of depth of etching is better controlled by use of lower pressures and corresponding etch rates.

The presence of the oxide layer during etching contributes to the smoothing action and is important when it is desired to control the etch rate of contoured surfaces. With the oxide layer present, molybdenum articles are reduced uniformly without preferential attack on certain crystal faces or attack of grain boundaries. This has been demonstrated by the etching of a length of 1.5 mil diameter wire with a temperature gradient along its length. The wire was reduced smoothly and uniformly from a cross-section area of 38p. to about 4 Thus, the process of the invention can be utilized to reduce the size of massive molybdenum articles, for the precision etching of molybdenum surfaces or selective precision etching of resist coated thin evaporated molybdenum films. Another use of the process is the polishing and more importantly the cleaning of molybdenum film surfaces. Much diificulty is experienced in the bonding of junctions to molybdenum films or in the lamination of these films to other surfaces because of the greasy or oxidized condition of the molybdenum surface. However, by means of the gaseous etching process of the invention the surface can be prepared for bonding with a precise removal of an extremely thin surface film of molybdenum.

The lack of corrosive or selective attack of surfaces, the possibility of control of the etch factor, uniformity in hole shape and size, and relatively easy control of etching conditions are further important characteristics of the process of the invention.

It is to be understood that the foregoing only relates to preferred embodiments of the invention and that numerous substitutions, modifications, or alterations of the specifically disclosed materials or procedures are permissible all without departing from the scope of the invention as defined in the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of etching molybdenum articles comprising the steps of heating the surface of the molybdenum article to a temperature from about 300 C. to 800 C.;

etching the surface at a rate of 0.1 to 1 micron per minute by delivering to the heated surface at a pressure of from about 0.1 to 100 torr a mixture of oxygen and hydrogen chloride gases containing at least one percent by volume of HCl to form volatile by-products; and

removing from the surface the volatile by-products.

2. A method according to claim 1 in which the surface is heated to a temperature of from about 400 C. to 700 C.

3. A method according to claim 1 in which a molybdenum oxide layer is formed on the surface during the etching.

4. A method according to claim 2 in which the molybdenum surface is in the form of a thin molybdenum film deposited onto a substrate.

5. A method according to claim 4 in which the film is etched to a precision of 0.1 micron.

6. A method according to claim 3 in which only a portion of the thickness of the molybdenum article is etched and the oxide film is then removed.

7. A method according to claim 6 in which the oxide film is removed by reaction with a member selected from the group consisting of chlorine gas, hydrogen gas, and a mixture of hydrogen chloride and oxygen gases.

8. A method according to claim 7 in which the temperature of the film is adjusted to a temperature between 300 C. and 500 C. and the pressure of the oxygen and HCl gases is increased to a pressure above 1 torr to remove the molybdenum oxide film.

9 A method according to claim 1 in which the surface of the molybdenum article is coated with a layer of a resist pattern and the molybdenum is selectively etched in the discontinuities in the pattern.

10. A method according to claim 9 in which the resist layer is aluminum oxide.

11. A method according to claim 10 in which the aluminum oxide layer is at least 0.1 thick.

References Cited FOREIGN PATENTS 7/ 1965 Germany.

OTHER REFERENCES ROBERT F. BURNETT, Primary Examiner R. J. ROCHE, Assistant Examiner US. Cl. X.R. l48l.5; 156-13