Background of the Invention Cleaning and drying of semiconductor wafers and other similarly processed workpieces are often rate-limiting processes in processing a wafer into a plurality of semiconductor chips. The cleaning process often requires use of large volumes of cleaning liquids that are toxic. Plasma etching and dry cleaning using leaves plasma residues that must themselves be removed in a subsequent process. Each of the cleaning and drying processes may consume several minutes or tens of minutes. These characteristics often limit throughput to no more than 100 workpieces per hour, require consumption of large volumes of expensive liquids, and require disposal of similarly large volumes of the resulting toxic products of the cleaning and drying processes.

What is needed is a workpiece cleaning process and a workpiece drying process that can each be completed in less than about a minute, that consume relatively small amounts of cleaning and drying liquids, some of which can be recycled and reused, and that can be implemented in a single chamber of relatively small footprint (of the order of 900 cm2).

Summary of the Invention These needs are met by the invention, which provides a cleaning process and drying process that can be implemented in a single chamber with small footprint, that uses at most a few tens of ml of cleaning liquids, that uses a class of drying liquids that are non-toxic and can be recycled, and for which the cleaning and drying processes allow a throughput as high as about 100 workpieces per hour.

A workpiece to be cleaned is positioned between two disks of chemically inert material, such as Si, quartz, SiN, SiyC, BeO or A1203, where each disk has a parallel plane adjacent to the other disk with plane-to-plane separation that is as small as about 25-150 llm larger than the thickness of the workpiece (which may be 100-1000 um, or larger if desired). A selected cleaning liquid is injected into the remaining volume, forming a chemical film of average thickness in a range of 12.5-75 nm between each plane of a disk and the adjacent workpiece surface. The two disks and wafer are immersed in a tank and surrounded within the tank by flowing DI water or a similar liquid. Optionally, one or more crystals, which are capable of vibrations at one or more of a range of frequencies between 20 kHz and 1.5 MHz, are positioned on one or both of the disks and used to induce ultrasonic or megasonic vibrations of the chemical film that is adjacent to the workpiece surfaces. The workpiece cleaning action continues for 20-

45 sec, after which the cleaning liquid is displaced by DI water or a similar liquid, to clear the chamber of the cleaning liquid. Optionally, ultrasonic or megasonic vibrations can also be used in this stage, which may consume 5-15 sec. The DI water is then removed, and the workpiece is moved up out of the cavity formed by the two disks. As the workpiece moves up, the two workpiece surfaces are subjected to a spray or thin film of hydrofluoroether (HFE) or another drying liquid with similar chemical and physical properties. This spraying procedure requires no more than 7-15 sec, and the drying liquid volatilizes within 7-15 sec after the spray is no longer present. The entire cleaning and drying process requires between 30 and 120 sec from start to finish and produces a dried workpiece with no visible residues thereon.

Brief Description of the Drawings Figure 1 is a side view of apparatus used for practicing the invention.

Figures 2-5 are perspective views illustrating the workpiece cleaning and drying processes at various stages, according to the invention.

Figures 6A and 6B are schematic views illustrating methods of injecting a rinsing or cleaning or drying liquid into a cavity containing the workpiece.

Figure 7 is a flow chart of a procedure for practicing the invention.

Description of Best Modes of the Invention Figure 1 illustrates insertion of a wafer or other workpiece 11 to be cleaned into a cavity 13 formed by two disks, 15A and 15B, of chemically inert material, spaced apart a selected distance d (separ). Each disk has a planar face, and these two planar faces, 17A and 17B, are adjacent to each other and form the cavity 13 therebetween. If the workpiece 11 has a thickness of d (workpiece), the separation distance d (separ) for the planar faces preferably satisfies the constraint d (separ) = d (workpiece) + 25-150 gm (1) so that the separation distance between each workpiece surface and the adjacent planar face, 17A or 17B, of the corresponding disk, 15A or 15B, is about 12.5-75 hum, on average. The two disks, 15A and 15B, and the workpiece 11 are immersed in a tank 18 containing DI water 19 or a similar liquid that preferably continues to spill over the edges of the tank and is optionally replenished using a liquid source 20 connected to the tank.

A smaller distance of separation d (separ) can be used with some materials that have a relatively low surface tension parameter, but liquids such as DI water with a relatively high surface tension parameter (about 80 dynes/cm for ordinary water) may require use of a larger separation distance d (separ). Optionally, the separation distance d (separ) can be much greater than the minimum of the constraint indicated in Eq. (1), as large as several mm if desired, but the amount of liquid used with each process will increase approximately linearly with the increase in d (separ).

The cavity 13 shown in any of Figures 1-5 may also be formed as a true cavity, open toward the top, in a single disk of material. Preferably, the cavity 13 is formed by two planar surfaces of separate spaced apart disks, 15A and 15B, whose distance of separation can be increased and decreased to facilitate insertion and withdrawal of the workpiece 11.

Optionally, one or both of the disks, 15A and 15B, is provided with one or more ultrasonic or megasonic crystals 21A-1, 21A-2, 21B-1, 21B-2, that are connected to and driven by a current or voltage source 21 to induce vibrations with one or more selected frequencies in a liquid contained in the cavity 13. The number of ultrasonic or megasonic crystals on each of the disks, 15A and 15B, may be, but need not be, the same. Preferably, the ultrasonic or megasonic frequencies are chosen from a range between 20 kHz and 1.5 MHz. An ultrasonic transducer available from Ney Ultrasonics is capable of generating a sequence of ultrasonic frequencies, including 40,72,104 and 136 kHz, for example.

Each of the disks, 15A and 15B, may be Si, quartz, SiwNx, SiyCZ, BeO or A1203, or a similar solid material that is chemically inert when exposed to selected cleaning chemicals, such as NH40H, H2SO4, HC1, H202, and mixtures thereof, and to the products of reaction of one or more of these cleaning agents with the materials on the workpiece surfaces. The disks are preferably, but not necessarily, constructed of the same material. Where only one surface of the workpiece has been processed and the other surface is relatively untouched by chemicals (e. g., a surface of pure Si or pure GaAs), the two disks, 15A and 15B, might be constructed of different materials.

After the workpiece 11 is inserted into the cavity 13, the separation distance d (separ) is adjusted (requiring an estimated 5 sec) to satisfy the constraint in Eq. (1), or a similar constraint, as illustrated in Figure 3. The tank 18 is preferably always filled with a first rinse liquid 23, such as DI water, isopropyl alcohol (IPA), or a mixture of DI water and IPA, to remove some of the chemical residue from the workpiece surfaces, 16A and 16B (Figures 1 and 2). The rinse liquid 23, if present in the cavity 13, remains there for at most 5-15 sec and is then promptly removed or displaced by another liquid.

Optionally, the rinse liquid 23 is subjected to ultrasonic and/or megasonic vibrations.

The amount of rinse liquid 23 used in this (optional) step is estimated to be 2-100 milliliters (ml), depending upon d (separ) and other variables.

The rinse liquid 23, if present, diffuses from the cavity 13 and ultimately passes from the tank 18 with the overflowing liquid 19. A first cleaning liquid 25, such as H2S04, H2S04 + H202, NH40H, NH40H + H202, H2S°4 + H202, HCI, HC1 + H202, H202, or another suitable cleaning liquid, is inserted into and fills the cavity for 15-45 sec, as illustrated in Figure 3, and is optionally subjected to ultrasonic and/or

megasonic vibrations for part or all of this time interval. The amount of cleaning liquid 25 used in this step is estimated to be 2-100 ml.

The cleaning liquid 25 diffuses from the cavity 13 and ultimately passes from the tank 18 with the overflowing liquid 19. A second rinse liquid 26 is then inserted into the cavity 13, remains there for 5-15 sec, and ultimately passes from the tank 18 with the overflowing liquid 19. Optionally, the rinse liquid 26 is subjected to ultrasonic and/or megasonic vibrations. The amount of rinse liquid 23 used in this step is estimated to be 2-100 milliliters (ml).

Optionally, a second cleaning liquid 27, such as H2S04, H2S04 + H202, NH40H, NH40H + H202, H2S04 + H202, HC1, HCl + H202, H202, or mixtures thereof, or another suitable cleaning liquid, is inserted into and fills the cavity 13 for 15- 45 sec, also illustrated in Figure 4, and is optionally subjected to ultrasonic and/or megasonic vibrations for part or all of this time interval. Preferably, the diameter of the workpiece 11 is smaller than the diameter of each of the disks, 15A and 15B, so that some of any liquid filling the cavity 13 extends beyond, and forms a liquid"halo"12 surrounding, the workpiece. The amount of second cleaning liquid 27 used in this (optional) step is estimated to be 2-100 milliliters (ml). More than two cleaning liquids can be used for this process, if desired. The temperature of each of the rinse liquid 23, the first cleaning liquid 25, the second rinse liquid 26, the optional second cleaning liquid 27 and an optional third rinse liquid 28 is preferably in the range 10-90 °C but may be higher if desired.

The second cleaning liquid 25 diffuses from the cavity 13 and ultimately passes from the tank 18 with the overflowing liquid 19. A third rinse liquid 28 (optional) is then inserted into the cavity 13, remains there for 5-15 sec, and ultimately passes from the tank 18 with the overflowing liquid 19. Optionally, the rinse liquid 28 is subjected to ultrasonic and/or megasonic vibrations. The amount of rinse liquid 28 used in this step is estimated to be 2-100 milliliters (ml).

At this point in time, the workpiece 11 is slowly withdrawn from the cavity 13, preferably at a linear withdrawal rate r of 0.5-5 cm/sec, or somewhat faster if desired, as illustrated in Figure 5. Where the cavity 13 is formed by two disks of inert material, 15A and 15B, these two disks may be first separated to a greater separation distance and the workpiece 11 may be gripped or otherwise contacted and moved upward. As the workpiece 11 moves upward out of the cavity 13, two sprayer or liquid transfer arms, 29A and 29B, transfer a selected drying liquid 31 onto each surface, 16A and 16B, of the workpiece. Depending upon the size of the workpiece 11 and the withdrawal rate r, the workpiece can be withdrawn from the cavity 13 in an estimated 7-15 sec, or longer if desired. Optionally, the workpiece 11 and/or the drying liquid 31 are subjected to

ultrasonic and/or megasonic vibrations during part or all of the time interval during which the workpiece is being withdrawn from the cavity 13.

The selected drying liquid 31 is preferably stored in a nearby drying liquid source 33 and is preferably a liquid such as hydrofluoroether (HFE), also referred to as methyl nonafluorobutyl ether, which has a generic formula of C50FgH3 and a chemical composition that is primarily either CF3-CF2-CF2-CF2-O-CH3,<BR> (CF3) 2-CF-CF2-O-CH3, or a mixture of these two compositions. HFE is available from 3M Company, Minneapolis, as HFE-7100, or as a mixture of a hydrofluoroether and one or more other chemicals, such as trans-1,2-dichloroethylene, H2ClC-CClH2 (producing an HFE azeotrope) or another halogen-containing alkene. A preferred temperature range for use of HFE is T = 20-60 °C. The freezing point and boiling point of HFE are approximately T =-135 °C and T = 60 °C, respectively. HFE has density and surface tension of 1.52 gm/cm3 and 13.6 dynes/cm, respectively. Other HFE formulations have boiling points ranging from 38 °C up to about 80 °C. By contrast, isopropyl alcohol (IPA) and water have surface tensions of about 17.6 dynes/cm and 80 dynes/cm, respectively.

The drying liquid may also be an ethylated hydrofluoroether (eth-HFE; C60F9H5), available from 3M Company as HFE-7200, or as a mixture of a hydrofluoroether or ethylated hydrofluoroether and one or more other chemicals, such as trans-1,2-dichloroethylene, H2CIC-CCIH2 (producing an HFE azeotrope) or another halogen-containing alkene. The molecular weight of HFE and of eth-HFE are about 260 and 274, respectively, which are much higher than the molecular weight of water (18, with 260/18 = 14.5 » 1 and 274/18 = 15.2 >>1) or of IPA (60). Thus, HFE or eth-HFE can displace IPA, water and most other liquid substances with moderate to high surface tensions from the surface of a workpiece, as the workpiece is withdrawn from a bath or spray of this drying liquid. Another suitable drying liquid that has attractive chemical and physical properties, a cyclic or acyclic hydrofluoroether with a general formula of CaFbH2a+2-bOc (a = 2 or 3; 3 = b = 8; c = 1 or 2), is available from DuPont Chemical Company, Wilmington, Delaware. Another suitable drying liquid, also available from DuPont Chemical Company, is a hydrofluorocarbon with a general formula of CnFmH2n+2 m (1 = n = 4; 1 = m = 8). The DuPont chemicals are described in U. S.

Patent No. 5,605,882, incorporated by reference herein. Another suitable, low surface tension drying liquid is IPA, with a surface tension parameter of about 17.6 dynes/cm.

As the workpiece 11 is withdrawn, the higher density and lower surface tension of the drying liquid 31, relative to the rinse liquid, causes the drying liquid (1) to easily displace any residual liquid droplets with higher surface tension parameters from the workpiece surface (s) and (2) to move down the (partly) exposed surface (s) of the workpiece 11 into the liquid 19, and toward the bottom of the tank 18. The workpiece surfaces, 16A and 16B, are dry within 7-30 sec after the workpiece is removed from the drying liquid 31. The workpiece 11 is also cleaned by removal of some residues from the exposed workpiece surfaces, 16A and 16B, using this approach.

When the drying liquid runoff (HFE, eth-HFE, IPA, DuPont hydrofluoroether, DuPont hydrofluorocarbon or another suitable liquid) is held at a temperature of T = 20- 60 °C, at most a few milliliter/minute of this liquid is lost due to volatilization, for a chamber with a normal exposed liquid surface. This volatilization loss can be reduced by employing vapor recovery for the volatilized drying liquid 31. The drying liquid 31 can be drawn from the bottom of the tank 18, passed through a filter 37, to remove the residues and to recycle the drying liquid 31, and returned to its container 33, for reuse in drying and/or cleaning.

The estimated total time required to perform the process is as follows. insert workpiece 5 (sec) first rinse 5-15 first clean 15-45 second rinse 5-15 second clean (optional) 15-45 third rinse (optional) 5-15 withdraw/drying spray 7-15 dry 7-30 Total time 44 (minimum)-185 (maximum) sec The estimated 44-second minimum processing time corresponds to about 82 workpieces processed per hour. This number can be increased using tandem or batch workpiece processing according to the invention. In tandem processing, first and second workpieces are processed in first and second adjacent tanks, preferably with each workpiece undergoing a different processing step at any time.

Insertion of a liquid into the cavity 13 is preferably accomplished using a pressurized source of the liquid. One approach for implementing this is to use a syringe 41, containing between 5 and 100 ml of the liquid, or more, attached to a liquid source 43 and driven by a stepper motor 45, as illustrated in Figure 6A. An alternative approach uses a positive displacement metering pump 55 and delivery tube 51, connected to a liquid source 53, to insert the liquid into the cavity 13, as illustrated in

Figure 6B. Preferably, each different liquid has its own syringe or pump, and two or more syringes or pumps may share a common aperture, such as 14A-3 in Figure 6A.

In one embodiment, as each process (first rinse, first clean, second rinse, second clean, third rinse, dry) is being completed, the syringe or pump containing the liquid most recently injected is drawn back and the syringe or pump containing the next liquid to be injected is moved into place. A syringe or pump may be positioned at or near the top of the cavity 13, to introduce a liquid into the cavity from the top and to allow distribution of this liquid along the workpiece surfaces by gravitational and/or capillary forces.

In another embodiment, each of a plurality of syringes or pumps is positioned at a separate aperture (14A-1,14A-2,14A-3,14B-1 or 14B-2 in Figure 6A or 6B) in one of the disks, 15A and 15B, and the liquid contained in each syringe or pump is injected at a different time in the total process. In another embodiment, at least two separate apertures, such as 14A-3 and 14B-1 in Figure 1, are used to introduce different liquids into the cavity 13. A single, central aperture, such as 14A-1 in Figure 1, can be used to introduce liquid into the cavity 13. However, it is preferable to use one or more apertures that are offset from and positioned above a central position, such as 14A-2, 14A-3,14B-1 and 14B-2 in Figure 1, to introduce liquid into the cavity, to help compensate for the effect of gravity forces on this liquid within the cavity. In these embodiments, a liquid in the cavity 13 is displaced from the cavity by introduction of the next liquid into the cavity.

Alternatively, as each of the first rinse process, the first cleaning process, the second rinse process, the second cleaning process and the third rinse process ends, one of the disks, 15 A and 15B, may be drawn apart from the other disk by a small distance? d that is sufficient to allow the liquid in the cavity 13 to drain quickly by gravity. The two disks, 15A and 15B, are then moved together to the original separation distance d (separ), the next processing liquid is injected, and the next processing stage begins.

Figure 7 is a flow chart illustrating practice of one embodiment of the method invention. In step 61, a workpiece is inserted into the cavity. In step 63 (optional), the cavity is filled with a first rinse liquid and the workpiece and first rinse liquid are optionally subjected to ultrasonic and/or megasonic vibrations. In step 65 (optional), the first rinse liquid is removed from the cavity. In step 67, the cavity is filled with a first cleaning liquid. In step 69, the workpiece and first cleaning liquid are optionally subjected to ultrasonic and/or megasonic vibrations. In step 71, the first cleaning liquid is removed from the cavity. In step 73, the cavity is filled with a second rinse liquid and the workpiece and second rinse liquid are optionally subjected to ultrasonic and/or megasonic vibrations. In step 75, the second rinse liquid is removed from the cavity.

In step 77 (optional), the cavity is filled with a second cleaning liquid and the workpiece and second cleaning liquid are optionally subjected to ultrasonic and/or megasonic vibrations. In step 79 (optional), the second cleaning liquid is removed from the cavity. In step 81 (optional), the cavity is filled with a third rinse liquid, and the workpiece and third rinse liquid are optionally subjected to ultrasonic and/or megasonic vibrations. In step 83 (optional), the third rinse liquid is removed from the cavity.

In step 85, the workpiece is withdrawn at a selected linear withdrawal rate r from the cavity, preferably but not necessarily in an upward direction. In step 87, a selected drying liquid is sprayed onto or otherwise transferred to the workpiece surface (s) as the workpiece is being withdrawn from the cavity. The liquid transfer region on each workpiece surface may be a thin line, a conventional polygonal region (triangle, quadrilateral, etc.), or may be a polygonal region with one or more curvilinear boundaries.

The single processing chamber in which the invention is implemented has a footprint with an area as small as about 900 cm2, including the disks, 15A and 15B, the tank 18, the sources of rinse liquids, cleaning liquid (s) and drying liquid, and the liquid injection mechanism.