WO1987006862A1

WO1987006862A1 - Ultrasonic cleaning method and apparatus

Info

Publication number: WO1987006862A1
Application number: PCT/US1987/001083
Authority: WO
Inventors: Charles Reid White
Original assignee: Eastman Kodak Company
Priority date: 1986-05-16
Filing date: 1987-05-11
Publication date: 1987-11-19
Also published as: CN87103659A

Abstract

A method and apparatus for cleaning semiconductor wafers (30) in a liquid bath including means for supplying the bath with a liquid media such as ultrapure water and with an electroacoustic transducer (18) disposed therein. The transducer (18) is energized at a frequency in the range of from about 20kHz to 90kHz to form a compact, well-defined area (36) of intense cavitation in the bath (10). The workpiece (30) is moved in a first direction (21) past the transducer (18) through the area of intense cavitation (36). The ultrapure water is moved past the transducer (18) and the workpiece (30) in a direction (19) opposite to the direction of movement of the workpiece (21) past the transducer (18).

Description

ULTRASONIC CLEANING METHOD AND APPARATUS

Technical Field

The present invention is directed to an improved method and apparatus for cleaning the surface of an article with ultrasonic energy and more particularly for cleaning the surfaces of patterned and unpatterned semiconductor wafers in preparation for subsequent operations in the manufacture of semiconductor devices.

The concept of cleaning semiconductor wafer elements with ultrasonic baths has been widely promoted with many claims to successful application. However, the actual application of such cleaning baths has fallen short of the promise. Many patents have issued on variations on the concept of ultrasonic cleaning of semiconductor wafers using ultrasonic baths wherein the wafer to be cleaned is immersed in a solvent and is subjected to ultrasonic vibration to remove particles having a size down into the submicron range. Many of these devices have utilized ultrasonic energy having a frequency between 20kHz and 100kHz, while others have utilized high frequency energy having a frequency in the range of between 0.2_'and 5.0MHz. However, it has been found with these prior art attempts at ultrasonic cleaning of semiconductor wafers that, either the particles are not removed from the surface of the wafer, or they are removed and then redeposited onto a previously cleaned portion of the wafer, or damage occurs to the wafer because of the prolonged exposure to the ultrasonic field. In either event the result is less than complete cleaning of the wafer. With the advancement of the semiconductor art, more and more complex devices are being incorporated into the semiconductor chips resulting in an increasing value for each chip. Moreover, decreasing sizes for the devices has resulted in 5 increasing requirements for cleanliness since the submicron-size elements in the devices can be adversely affected by submicron-size particles. All of these factors are compounded by the industry's use of larger wafer sizes containing more and more

HO: individual chips. Thus, the ultimate cleanliness of each wafer takes on ever greater economic significance to the semiconductor manufacturer. Background Art

Ultrasonic cleaners utilizing very high

15 frequencies in the range of 0.2 to 5.0MHz are disclosed, for example, in U.S. Patent Nos. 3,893,869 and 4,326,553 and have been promoted for removing particles in the submicron size range. However, it has been found that such systems are

20 ineffective when the wafers are contaminated with particles having a range of sizes. This appears to result from the fact that the very high frequency ultrasonic vibration employed is unable to remove the larger particles and also from the fact that the

25.^" large particles shield the smaller particles from the effects of the ultrasonic cleaning waves whereby the wafer surface is still contaminated by both large and small particles.

Examples of ultrasonic cleaning apparatus

30 utilizing frequencies in the range of 20kHz to l.QOkHz. are represented by U.S. Patent Nos. 4,178,188 and 4,401,131, and employ ultrasonic energy in a liquid bath to generate cavitation which is intended to clean the semiconductor wafers disposed therein.

35 However, these devices utilize a limited amount of power; for example the *131 patent discloses apparatus in which only 400 watts is supplied to a 100 millimeter wafer resulting in a power density of a little over 30 watts per square inch. While the *188 patent does not specify the power or the power density, it does indicate that it is desirable to minimize the power requirements. Furthermore, the flow of the bath fluid is not controlled in these devices. As a result, attempts to clean wafers ulitizing this type of apparatus have been found either to not adequately remove all particles from the surface thereof because of the lack of power, or the cleaned surfaces were recontaminated by particles removed but not adequately flushed away by the bath and which were redeposited on the wafer surface.

Heretofore, it was thought that the use of higher power ultrasonic tranducer devices in a liquid media created as many problems as they solved. It was believed that the high cavitation power would heat both the horn and the wafer to undesirable temperatures where they could be easily damaged and that the increased power would increase the amount of recontamination of the wafer surface. In addition, damage to sensitive semiconductor devices on the wafer surface has been reported due to the extended length of time that the wafer was exposed to the ultrasonic energy in the fluid bath in order to remove particles with prior art devices.

SUMMARY OF THE INVENTION Accordingly, the present invention provides an ultrasonic bath in which it is possible to generate sufficient power to thoroughly clean the wafer surface without the problems of overheating or damaging the wafer or the tranducer horn, without mechanically damaging the wafer surface or the circuitry deposited thereon, and without the problems of recontamination of the surface by 5 particles propelled into the bath liquid.

In accordance with one aspect of the present invention, a method and apparatus is provided for cleaning a workpiece, such as a semiconductor wafer, in a liquid bath including

1.0 means for performing the steps of disposing the workpiece on a support at a first position in the bath whereby at least one surface of the workpiece is exposed. Means is provided for supplying the bath, with a liquid medium such as ultrapure water

15 and an electroacoustic transducer which is disposed in the medium at a second position. Means is provided for energizing the transducer with an electrical source of energy at a frequency in the range of about 20kHz to 90kHz to generate ultrasonic

20 energy which is emitted from the transducer to form a compact, well-defined area of intense cavitation in. the bath. Means is provided for providing relative movement between the workpiece and the transducer whereby the exposed surface passes

25. through the area of intense cavitation. Means is also provided for moving the liquid medium past the transducer and the workpiece in the same direction as the transducer moves with respect to the workpiece.

30 In accordance with another aspect of the present invention a method and apparatus is provided for cleaning semiconductor wafers in a liquid bath wherein the semiconductor wafer is disposed on a support at a first position in the bath with at

35 least one surface of the wafer facing the energy source. The bath is provided with a supply of ultrapure water (and possibly other chemicals) and an electroacoustic transducer is disposed in the bath at a second position with the radiating surface thereof disposed at an angle of about 10° to the surface of the wafer and facing the first position. Means is provided for energizing the transducer with a source of energy providing about 100 watts per square inch of radiating surface of the transducer at a frequency of about 20kHz to generate ultrasonic energy which is emitted as a compact, well-defined area of intense cavitation in the bath. The wafer is moved in a first direction from the first position past the transducer with the exposed surface facing the radiating surface of the transducer at a distance of about 3/8 of an inch and within the area of intense cavitation. Means is provided for moving the ultrapure water with non-turbulent flow past the transducer and the wafer in a direction opposite to the movement of the wafer past the transducer.

Various means for practicing the invention and other features and advantages thereof will be apparent from the following detailed description of illustrative preferred embodiments of the invention, reference being made to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of apparatus for carrying out the method of ultrasonic cleaning of articles in accordance with the present invention;

FIG. 2 is a perspective view, partially in section, of the cleaning apparatus; and

FIG. 3 is a cross-sectional view of an alternative embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2 there is illustrated apparatus for cleaning semiconductor wafers comprising a generally open topped tank 10 arranged to contain a liquid bath having a liquid level 12, a liquid inlet 14 and a liquid outlet 16 at opposite ends thereof. An ultrasonic transducer 18 having a radiating surface 34 is disposed within the bath, substantially midway between the inlet and the outlet. The electroacoustic transducer imparts an ultrasonic vibration through a horn which in turn ultrasonically develops a compact, well-defined intense cavitation field 36 within the bath contained within tank 10. The bath container 10 is provided with a width somewhat greater than the diameter of the largest semiconductor wafer intended for use therein. The length of the bath is at least twice the diameter of the largest wafer. The inlet , end of the bath is provided with a flow diffuser 20 arranged to generate generally non-turbulent flow throughout the bath area from fluid introduced through inlet 14. The outlet end of the tank is provided with a fluid level-controlling weir 22 which functions to establish and control the liquid level. Drain openings 37 are provided at the base of the weir to remove liquid media and settled particulates.

A workpiece transport means 24 is provided with a pair of movable arms 26 which are movable to grip, via pins 28, a workpiece 30 which, in the preferred embodiment, is a semiconductor wafer from which a plurality of semiconductor chips may.be produced. Preferably the workpiece is gripped at the edges thereof leaving both surfaces open to contact with the bath fluid and to minimize particulate generation on the surfaces. In the embodiment illustrated, the upper surface of the workpiece 30 is intended to be the primary surface operated upon by the action of the ultrasonic transducer 18, but in fact both surfaces will undergo cleaning as the wafer is passed through the influence of the horn. In addition to the movement of the arms 26 to grip the semiconductor wafer 30, the workpiece transport 24 is also arranged for vertical movement to receive the wafer from a wafer transporter above the bath and to lower the wafer below the surface of the bath and for lateral motion along the length of the bath from the first, entrance position, illustrated in full in FIGS. 1 and 2, in a first direction to transport the wafer to the right beneath the radiating surface of the transducer, located at a second position, and then to a third, exit position at the opposite end of the bath, illustrated in phantom in FIG. 1. The workpiece transport may provide a range of transport speeds to the wafer of from 1 to 3 inches per second. The workpiece transport is then arranged to lift the cleaned wafer out of the tank and to release it for transfer to other operational stations. The support is then lowered to clear the horn and returned to the first position to receive the next wafer. A wall 38 is shown on the right side of the transducer which terminates just about the surface of the liquid and is provided to illustrate that the exit end of the bath is preferably disposed in a clean-room atmosphere to minimize recontamination of the wafer while that is not necessarily so for the entrance side of the apparatus. As illustrated in FIGS. 1 and 2, the radiating surface 34 of the electroacoustic transducer is disposed at an angle of between 0 and .45 to the surface of the workpiece 30 and faces the first position where the wafer is loaded into 5 the bath. The angling of the transducer with respect to the wafer surface has been found to provide several advantages: The energy in the cavitation field propels particles dislodged from the wafer surface across the wafer in the direction

10 of the fluid flow across the wafer; and the wafer is exposed to varying intensities of the cavitation field as it moves past the angled radiating surface. The electroacoustic transducer 18 is arranged in such position to provide an output of

15 from about 70 to about 120 watts per square inch of radiating surface of the horn at a frequency in the range of about 20kHz to 90kHz, emitting ultrasonic energy into the bath to form a co. pact, well-defined area of intense cavitation 36 therein. The

20 transducer and horn is arranged to be mounted so that it may be adjustably disposed with respect to the liquid level and the wafer in the bath. The adjustment is sufficient to provide a range of distances between the radiating surface of the horn

25. 34 and the top surface of the wafer 30 of from between about 1/8 of an inch to about 3/4 inch.

While the bath may be provided with any liquid media or combination of liquid media which has been found satisfactory for cleaning workpieces

30 such as semiconductor wafers, it has been found that the use of ultrapure water provides certain advantages over other types of fluids. (Ultrapure water is intended to refer to filtered and deionized water having a resistivity of at least 18 megohms,

35 as is known in the art.) Primarily, the ultrapure water provides an excellent solvent action for most contaminants on semiconductor wafers. Secondarily, ultrapure water has the added advantage of leaving less residue which itself may contaminate the semiconductor wafer surface. As noted above, the fluid is introduced through inlet 14 and passes through a diffuser 20 which provides a uniform, laminar, and therefore non-turbulent, flow of the fluid from the inlet 14 to the outlet 16 passing the wafer and the electroacoustic transducer horn in a direction opposite to that of the movement of the wafer past the horn. The liquid is supplied to the bath in a quantity of from one to three gallons per minute to provide a flow of one to three inches per second past the ultrasonic horn. The bath is provided with a depth that is generally constant from the inlet diffuser 20 to just beyond the ultrasonic horn 18, with the depth then progressively increasing toward the outlet 16. The purpose of the increasing depth from about the location of the horn is to assure that the flow of liquid decreases as it passes the horn so that the larger contaminants displaced from the surface of the workpiece 30 may settle in the bath and not be subjected to agitation whereby they may be redeposited upon the portion of the surface of the the workpiece which has already been cleaned.

According to a preferred embodiment of the present invention, a semiconductor wafer is supplied to the workpiece transport means 24 which at that time is raised above the bath at the first (left in the illustration) position and the bath is provided with a flow of ultrapure water of approximately one inch per second from the inlet to the outlet, generally in the direction indicated by arrow 19. The electroacoustic transducer 18 is energized with a source of energy providing an output of about 100 watts per square inch of the radiating surface of the horn at a frequency of about 20kHz emitting 5 ultrasonic energy into the bath to form a compact, well-defined area of intense cavitation in the bath. Preferably, the radiating surface of the transducer means is disposed at an angle of about 10° to the surface of the wafer and facing the

ID- first position. The workpiece transport then lowers the wafer into the bath and then travels with the wafer in the direction indicated by arrow 21 from the first position, generally indicated at 40, toward and past the radiating surface of the

15 ultrasonic transducer at a second position 42, to the third, exit position 44. Preferably the wafer is disposed at a spacing of about 3/8 inch from the face of the ultrasonic transducer so that the cavitation energy in the bath dislodges any foreign

20 particles disposed thereon which are then carried away by the liquid towards the outlet of the bath.

Alternative Embodiments An alternative embodiment is illustrated in FIG. 3 wherein similar components are provided with

25 the same reference numeral with the prefix "1". In this embodiment, the bath is provided with a substantially uniform depth from the fluid inlet end to the fluid outlet end. The ultrasonic transducer is also disposed upright in this embodiment. With

3D this arrangement there is less tendency for particulates to settle in the bath so that there may be less build-up of particulates therein.

In other embodiments (not illustrated) the wafer may remain stationary within the bath while

35 the transducer is moved therepast. Such an arrangement can utilize a shorter bath since the wafer need not move, but there may be a greater risk of recontamination of the cleaned wafer surface. In such an arrangement, the flow of the liquid media would be in the same direction as the relative movement of the transducer with respect to the wafer surface.

The cavitation action of the ultrasonic horn has been found to form minute vapor bubbles which then collapse, generating localized pressures of up to 200,000 psi. These pressures release energy to dislodge the particles from the semiconductor surface. The effectiveness of this removal process is regulated by the energy level (watts per square inch of radiating surface) and the exposure time which is determined by the velocity of the wafer. It will also be appreciated that the flow of the liquid medium past the transducer and the moving workpiece, functions to move any displaced particles away from the already cleaned portion of the workpiece in a downstream direction, passing only over yet to be cleaned surfaces of the workpiece. Accordingly, the dislodged particles have less opportunity to recontaminate the already cleaned portions of the workpiece. This action is further assisted by the uniform, low-turbulence flow of the liquid which minimizes the tendency of any entrained particles to be transported upstream and be redeposited on the already cleaned portion of the workpiece. Moreover, it is noted that the flow of the liquid past the transducer and the workpiece prevent build-up of heat which might otherwise occur when operating at the high power values of the present invention. INDUSTRIAL APPLICABILITY With the use of the method and apparatus of the present invention it is possible to clean up to 160 wafers per hour. The cleanliness level possible with the present invention is less than 0.05 particles per square centimeter of a size equal to or larger than 0.2 microns. Still further, it is unnecessary to degas the liquid prior to cleaning of the workpieces and the continuous flow of the bath prevents the build-up of contaminants within the bath.

Claims

I Claim:

1. The method of cleaning a workpiece in a liquid bath comprising the steps of: disposing a workpiece on a support at a first position in said bath, whereby at least one surface of the workpiece is exposed; providing said bath with a supply of a liquid medium; disposing an electroacoustic transducer in said bath at a second position; energizing said transducer with a source of energy at a frequency in the range of about 20kHz to

90kHz to generate ultrasonic energy which is emitted into said bath in the form of a compact well-defined area of intense cavitation; generating relative movement between said transducer and said workpiece whereby said workpiece passes through said area of intense cavitation; and' moving said liquid medium past said workpiece in the same direction as the transducer moves with respect to the workpiece whereby particles dislodged from the surface of said workpiece are moved across the workpiece in the direction of the movement of the liquid medium thereacross.

2. The invention according to Claim 1 wherein said transducer is stationary and the workpiece is moved past the transducer.

3. The invention according to Claim 1 wherein said workpiece is stationary and the transducer is moved past the workpiece.

4. The invention according to Claim 1 including the step of providing said bath with ultrapure water.

5. The invention according to Claim 1 wherein the source of energy provides from 70 to 120 watts per square inch of radiating surface of said transducer.

6. The invention according to Claim 1 including the step of passing said one surface of said workpiece past the radiating surface of said transducer at a distance from about 1/8 of an inch to about 3/4 inch and within said area of intense cavitation.

7. The method of cleaning semiconductor wafers in a liquid bath comprising the steps of: disposing a semiconductor wafer on a support at a first position whereby at least.one surface of the wafer is exposed; immersing the wafer in said bath; providing said bath with a supply of ultrapure water; disposing an electroacoustic transducer in said bath at a second position; energizing said transducer with a source of energy providing from 70 to 120 watts per square inch of radiating surface of said transducer at a frequency in the range of about 20kHz to 90kHz to generate ultrasonic energy which is emitted into said bath in the form of a compact well-defined area of intense cavitation; generating relative movement between said transducer and said wafer whereby said wafer passes through said area of intense cavitation at a distance from said radiating surface of from about 1/8 of an inch to about 3/4 of an inch; moving said ultrapure water past said wafer in the same direction as the transducer moves with respect to the wafer whereby particles dislodged from the surface of said wafer are moved across the wafer in the direction of the movement of the liquid medium thereacross; and removing said wafer from said bath.

8. The method of cleaning semiconductor wafers in a liquid bath comprising the steps of: disposing a semiconductor wafer on a support at a first position whereby at least one surface of the wafer is exposed; immersing the wafer in said bath; providing said bath with a supply of ultrapure water; disposing an electroacoustic transducer in said bath at a second position with the radiating surface thereof disposed at an angle to the surface of said wafer and facing said first position; energizing said transducer with a source of energy providing from 70 to 120 watts per square inch of radiating surface of said transducer at a frequency in the range of about 20kHz to 90kHz to generate ultrasonic energy which is emitted into said bath in the form of a compact well-defined area of intense cavitation; moving said wafer in a first direction from said first position past said transducer with said one surface facing said radiating surface at a distance from about 1/8 of an inch to about 3/4 inch from the radiating surface thereof and within said area of intense cavitation; moving said ultrapure water with non-turbulent flow past said transducer and said wafer in a direction opposite to the movement of said wafer past said transducer; and removing said wafer from said bath.

9. The method of cleaning semiconductor wafers in a liquid bath comprising the steps of: disposing a semiconductor wafer on a support at a first position whereby at least one surface of the wafer is exposed; immersing the wafer in said bath; providing said bath with a supply of ultrapure water; disposing an electroacoustic transducer in said bath at a second position with the radiating surface thereof disposed at an angle of about 10° to the surface of said wafer and facing said first position; energizing said transducer with a source of energy providing about 100 watts per square inch of radiating surface of said transducer at a frequency of about 20kHz to generate ultrasonic energy which is emitted into said bath in the form of a compact well-defined area of intense cavitation; moving said wafer in a first direction from said first position past said transducer at a speed of from about one inch per second to about three inches per second with said one surface facing said radiating surface at a distance of about 3/8 of an inch from the radiating surface thereof and within said area of intense cavitation; moving said ultrapure water with non-turbulent flow past said transducer and said wafer in a direction opposite to the movement of said wafer past said transducer and then reducing the velocity of said water after it passes said transducer; and removing said wafer from said bath.

10. Apparatus for cleaning a workpiece in a liquid bath comprising: a workpiece support means; means forming a liquid bath; means for disposing a workpiece on said support means at a first position in said bath with at least one surface of the workpiece exposed; means for supplying said bath with a liquid medium; electroacoustic transducer means disposed in said bath at a second position; means for energizing said transducer with a source of energy at a frequency in the range of about 20kHz to 90kHz for emitting ultrasonic energy into said bath in the form of a compact well-defined area of intense cavitation in said bath; means for relatively moving said support means and said workpiece with respect to said transducer whereby said one surface passes through said area of intense cavitation; and means for moving said liquid medium past said workpiece in the same direction as the transducer moves with respect to the workpiece.

11. The invention according to Claim 10 wherein said workpiece is stationary and the transducer is moved past said workpiece.

12. The invention according to Claim 10 wherein said transducer is stationary and the workpiece is moved past said transducer.

13. The invention according to Claim 10 wherein said bath is provided with ultrapure water.

14. The invention according to Claim 10 wherein said source of energy provides an output from said transducer of from 70 to 120 watts per square inch of radiating surface of said transducer.

15. The invention according to Claim 10 including means for passing said one surface of said workpiece past the radiating surface of said transducer at a distance of from about 1/8 of an inch, to about 3/4 inch and within said area of intense cavitation.

16. Apparatus for cleaning a workpiece in a liquid bath comprising: means for forming a liquid bath; a workpiece support means; means for disposing a workpiece on said support means at a first position with at least one surface of the workpiece exposed; means for immersing the workpiece in said bath; means for providing said bath with a supply of ultrapure water; electroacoustic transducer means disposed in said bath at a second position; means for energizing said transducer with a source of energy providing an output from said transducer of from 70 to 120 watts per square inch of radiating surface of said transducer at a frequency in the range of about 20kHz to 90kHz for emitting ultrasonic energy into said bath to form a compact well-defined area of intense cavitation in said bath; means for moving said support means and said workpiece in a first direction from said first position past said transducer with said one surface facing said radiating surface at a distance of from about 1/8 of an inch to about 3/4 inch from the radiating surface thereof and within said area of intense cavitation; means for moving said ultrapure water past said transducer and workpiece in a direction opposite to the movement of said workpiece past said transducer; and means for removing said workpiece from said bath.

17. Apparatus for cleaning semiconductor wafers in a liquid bath comprising: means for forming a liquid bath; a wafer support means; means for disposing a wafer on said support means at a first position with at least one surface of the wafer exposed; means for immersing the wafer in said bath; means for providing said bath with a supply of ultrapure water; electroacoustic transducer means disposed in said bath at a second position, said transducer means including a radiating surface disposed at an angle to the surface of said wafer and facing said first position; means for energizing said transducer with a source of energy providing an output from said transducer of from 70 to 120 watts per square inch of radiating surface of said transducer at a frequency in the range of about 20kHz to 90kHz for emitting ultrasonic energy into said bath to form a compact well-defined area of intense cavitation in said bath; means for relatively moving said support means and said wafer with respect to said transducer whereby said one surface facing said radiating surface at a distance of from about 1/8 of an inch to about 3/4 inch from the radiating surface thereof and within said area of intense cavitation; means for moving said ultrapure water with non-turbulent flow in the same direction as said transducer moves with respect to said wafer; and means for removing said wafer from said bath.

18. Apparatus for cleaning semiconductor wafers in a liquid bath comprising: means for forming a liquid bath; a wafer support means; means for disposing a wafer on said support means at a first position with at least one surface of the wafer exposed; means for immersing the wafer in said bath; ^' means for providing said bath with a supply of ultrapure water; electroacoustic transducer means disposed in said bath at a second position, said transducer means including a radiating surface disposed at an angle of about 10° to the surface of said wafer and facing said first position; means for energizing said transducer with a source of energy providing an output from said transducer of about 100 watts per square inch of radiating surface of said transducer at a frequency of about 20kHz for emitting ultrasonic energy into said bath to form a compact well-defined area of intense cavitation in said bath; means for moving said support means and said wafer in a first direction from said first position past said transducer with said one surface facing said radiating surface at a distance of about 3/8 of an inch from the radiating surface thereof and within said area of intense cavitation; means for moving said ultrapure water with non-turbulent flow past said transducer and said wafer in a direction opposite to the movement of said wafer past said transducer; means for reducing the velocity of said water as it passes said transducer; and means for removing said wafer from said bath.