Patent Application
20090205677
The present invention pertains in general to wafer processing and in particular to a single wafer cleaning process.
One of the most important tasks in semiconductor industry is the cleaning and preparation of the silicon surface for further processing. The main goal is to remove contaminants such as particles from the wafer surface and to control chemically grown oxide on the wafer surface. Modern integrated electronics would not be possible without the development of technologies for cleaning and contamination control, and further reduction of the contamination level of the silicon wafer is mandatory for the further reduction of the IC element dimensions. Wafer cleaning is the most frequently repeated operation in IC manufacturing and is one of the most important segments in the semiconductor-equipment business, and it looks as if it will remain that way for some time. Each time device-feature sizes shrink or new tools and materials enter the fabrication process, the task of cleaning gets more complicated.
Today, at 0.18-micron design rules, 80 out of ˜400 total steps will be cleaning. While the number of cleans increases, the requirement levels are also increasing for impurity concentrations, particle size and quantity, water and chemical usage and the amount of surface roughness for critical gate cleans. Not only is wafer cleaning needed now before each new process sequence, but also additional steps are often required to clean up the fabrication process tools after a production run.
Traditionally, cleaning has been concentrated in the front end of the line (FEOL) where active devices are exposed and more detailed cleans required. A primary challenge in FEOL cleans is the continuous reduction in the defect levels. As a rule, a “killer defect” is less than half the size of the device line width. For example, at 0.25 μm geometries, cleans must remove particles smaller than 0.12 μm and at 0.18 μm, 0.09 μm particles.
Most cleaning methods can be loosely divided into two big groups: wet and dry methods. Liquid chemical cleaning processes are generally referred to as wet cleaning. They rely on combination of solvents, acids and water to spray, scrub, etch and dissolve contaminants from the wafer surface. Dry cleaning processes use gas phase chemistry, and rely on chemical reactions required for wafer cleaning, as well as other techniques such as laser, aerosols and ozonated chemistries. Generally, dry cleaning technologies use less chemicals and are less hazardous for the environment but usually do not perform as well as wet methods, especially for particle removal.
For wet-chemical cleaning methods, the RCA clean, developed in 1965, still forms the basis for most front-end wet cleans. A typical RCA-type cleaning sequence starts with the use of an H2SO4/H2O2 solution followed by a dip in diluted HF (hydrofluoric acid). A Standard Clean first operation (SC1) can use a solution of NH4OH/H2O2/H2O to remove particles, while a Standard Clean second operation (SC2) can use a solution of HCl/H2O2/H2O to remove metals. Despite increasingly stringent process demands and orders-of-magnitude improvements in analytical techniques, cleanliness of chemicals, and DI water, the basic cleaning recipes have remained unchanged since the first introduction of this cleaning technology. Since environmental concerns and cost-effectiveness were not a major issue 30 years ago, the RCA cleaning procedure is far from optimal in these respects.
Marangoni drying is a commonly used method to dry wafers after being processed in a wet bench. The method uses a difference in surface tension gradients of IPA and DI water to help remove water from the surface of the wafer. This surface tension phenomenon is known as the Marangoni effect. The Marangoni effect is characterized in thin liquid films and foams whereby stretching an interface causes the surface excess surfactant concentration to decrease, hence surface tension to increase; the surface tension gradient thus created causes liquid to flow toward the stretched region, thus providing both a “healing” force and also a resisting force against further thinning.
The Marangoni technique can be practiced by the slow batch withdrawal of wafers from a DI water bath to an environment of isopropyl alcohol (IPA) and nitrogen such that only the portion of the surface that is at the interface of the liquid and vapor phases is “drying” at any one time. In this way, uncontrolled evaporative drying on the wafer is prevented. IPA drying provides a great advantage in hydrophobic cleaning steps such as pre-gate, pre-silicide and pre-contact cleans.
During the rinse operation, a nozzle can flow fluid such as DI water onto the wafer. The water flowing onto the wafer can splash and create a spray. The splash back of the spray onto the wafer can bead up especially on hydrophobic surfaces. During a later drying phase, the water can evaporate to leave a watermark. Watermarks can be the result of an outline of the water bead that can contain a redeposit of the particles that were intended to be removed by the rinse operation. Alternatively, these watermarks can be the result of hydrolysis of the DI water, producing small amounts of hydroxide ion, which, in the presence of oxygen, allow the silicon substrate to oxidize, creating an oxide deposit upon final drying.
Megasonic agitation is the most widely used approach to adding energy (at about 800 kHz or greater) to the wet cleaning process. The physics behind how particles are removed, however, is not well understood. A combination of an induced flow in the cleaning solution (called acoustic streaming), cavitation, the level of dissolved gases, and oscillatory effects are all thought to contribute to particle removal performance.
The present invention provides for improved wafer cleaning in a single wafer cleaning chamber. In one embodiment, a pair of nozzles can generate a Marangoni force by flowing fluids having different surface tension characteristics onto a top surface of a rotating wafer and where the Marangoni force can act on particles remaining on the wafer surface. Such particles can be silicates that can be the product of an HF etch or a cleaning operation and where the particles can be directed by the Marangoni force to the wafer outer edge and removed from the wafer surface. The Marangoni force can be created by flowing a rinse fluid from a first nozzle that can be deionized (DI) water and by flowing a second fluid from a second nozzle that can be IPA (isopropyl alcohol) vapor in nitrogen gas (N2). The Marangoni force can be created where the force is in a direction to move the contaminants toward the outer edge (outer diameter) of the wafer.
In one embodiment of the present invention, a summation of forces can act to maintain a wafer in a wafer holding bracket. A transducer plate can be positioned beneath the wafer holding bracket in the single wafer cleaning chamber. The wafer holding bracket can translate to place the wafer in a process position above the transducer plate where a small gap can exist between the transducer plate and the wafer. The total force acting on the wafer to maintain the wafer in the wafer holding bracket can include a number of different forces.
During various process cycles that can include the rinse cycle, forces acing on the wafer can include fluids flowing from the nozzles where the force of the fluids striking the wafer top surface acts as a “down” force. Other down forces acting on the wafer can be, for example, gravity, and air flow from an air filter above. A flow of fluid through the transducer plate that can strike the wafer bottom surface can be one example of an “up” force on the wafer as can vibration of the wafer holding bracket during rotation. Capillary forces created by a fluid placed between the transducer plate and the wafer can act to restrain the wafer from movement away from the transducer plate.
During wafer drying portions of the cleaning cycle, a gas may flow from one or more nozzles to strike the wafer top surface and flow into a gap between the wafer and the transducer plate. A high wafer rotation rate can create non-symmetric air flow across the wafer top surface versus the wafer bottom surface, i.e. in the gap between the wafer and the transducer plate. The result can be a pressure differential acting on the wafer and where this differential can result in a down force onto the wafer, i.e. a Bernoulli force. As such, in the drying phase where wafer rotation rates are high, yet no capillary force exists, the Bernoulli force can act on the wafer to maintain the wafer in position in the wafer holding bracket.
In one embodiment of the present invention, UV light bulbs are placed into the single wafer cleaning chamber to flood the interior, and the wafer top surface with UV light. UV light can break down some contaminants such as any remaining organic molecules from previous operations on the wafer and where the smaller (lower molecular weight) molecules can be more easily removed by the DI water rinse operation. The UV light can break down the organic molecules by direct impingement onto the molecules during a dry cycle prior to the rinse. The UV light can further contribute to this breakdown by ozonating the DI water during the rinse phase where the ozone can also act on the organic molecules to break them down into smaller molecules. Finally, after a final rinse, UV light can be used to accelerate the oxidation of exposed bare silicon on the wafer top surface as a protective coating.
In one embodiment, a nozzle is angled so that flow of a liquid is angled incident to a rotating wafer at an angle. Liquids, such as the rinse water, striking the wafer at the incident angle can reduce the amount of splashing that occurs as opposed to fluids that are vertically incident to the wafer surface.
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
For purposes of discussing the invention, it is to be understood that various terms are used by those knowledgeable in the art to describe apparatus, techniques, and approaches. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in gross form rather than in detail in order to avoid obscuring the present invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, chemical, and other changes may be made without departing from the scope of the present invention.
The present invention is a method and apparatus for enhancing the cleaning operation on a wafer in a single wafer cleaning chamber. The method and apparatus are specifically useful for single wafer cleaning, but the method and apparatus disclosed may also be used in applications where more than one wafer is cleaned at a time. In one aspect of the present invention, a surface tension force, i.e. a Marangoni force, is created on a rotating wafer to assist in removing contaminants produced by previous cleaning and etch operations. In another aspect of the presenting invention, a number of forces can be generated onto the wafer such that a summation of these forces can result in a down force onto the wafer to maintain the wafer in position on a wafer holding bracket. It is a further aspect of the present invention to direct a UV light onto the wafer to breakup residual organics into smaller molecules that are easier to rinse away and further, where the UV light can assist in creating a thin silicon oxide protective coating on the wafer. In still another aspect of the present invention, a nozzle can be used in a rinse cycle where the nozzle is angled to flow a liquid that is incident to the wafer at an angle to reduce splash back that might contribute to watermarks on the wafer surface.
A single wafer cleaning chamber can be used to clean wafers before and after a variety of wafer processes, such as, for example, deposition of a metallized film, photoresist patterning, or Rapid Thermal Processes where RTP can be used for such process as wafer annealing, doping, and oxide growth. The wafer cleaning process can include several types of cleaning cycles as well as an hydrofluoric acid (HF) etch on the wafer to remove oxides. As a result, there are usually contaminants such as particulate matter (particles) in the rinse water that can remain on the wafer, where such particles can be, for example, silicates. It is important to remove those contaminants from the wafer surface. When applying a liquid to remove particles, a boundary layer, i.e. a thin static layer of liquid, can exist near the wafer surface that can contain these particles. Under these conditions, electrostatic repulsion forces may only exist once the particle is removed a certain distance from the wafer. As such, there may be no force strong enough to remove the particles from the wafer. Therefore, to remove the particles from the viscous boundary layer on a rotating wafer (at 1600 rpm a boundary layer of 12.5 microns can exist), a Marangoni force can be developed to act on these particles, and in particular, the particles made of silicates.
Once the wafer 210 is placed onto the bracket 206, the bracket 306 can be lowered to a process position as shown. This process position can place the wafer 310 a short distance above a circular plate 218. The circular plate 218 can contain transducers 220 that are capable of emitting sound in the megasonic frequency range. A fluid feed port 224 can be added to the transducer plate 218 to fill an approximate 3 millimeter (mm) gap 326 between the transducer plate 218 and the wafer 210 with a liquid 222 at various times during wafer 210 processing. The liquid 222 can act as a carrier for transferring megasonic energy onto the wafer bottom surface 225. The top of the single wafer cleaning chamber 200 can contain a filter 226 to clean air that is flowing 227 into the process chamber 200 and onto a wafer top surface 216.
As shown in
In one embodiment, the translating nozzles 230 and 232 can be used to create a Marangoni force for removing particles from the wafer surface. The liquid 234 used can be highly purified water, such as, for example, DI water, and can be applied onto the wafer 210 to flush away the particulate matter. A stream of the water 234 can be initially applied near the wafer center 244 by the first nozzle 230. The IPA vapor nozzle 232 can be positioned offset from the first nozzle 230, i.e. behind the first nozzle 230 relative to the direction of travel for the two nozzles 230 and 232. During a cycle, such as, for example, a rinse cycle, the nozzles 230 and 232 can translate in unison to move progressively out toward the wafer outer edge 217 (outer diameter). With the rinse water 234 dispensed onto the wafer 210, the IPA vapor nozzle 232 can apply a stream of IPA vapor 236 to contact the rinse water 234 on the inboard side of the wafer 210.
The two nozzles 230 and 232 can pivot about a common pivot point 252 in fixed relationship to each other, i.e. the two nozzles 230 and 232 can maintain a fixed position relative to each other as the two nozzles 230 and 232 are pivoted over the wafer top surface 216. Each nozzle 230 and 232 can have a radius of pivot R1 and R2 respectively from the common pivot point 252. The nozzles 230 and 232 can maintain their relationship with each other during pivot by using electronic commands to the pivot mechanisms or, alternatively, the two nozzles can be physically attached together.
In the alternate embodiment, the wafer can rotate counter-clockwise (looking top down) while the nozzles 230 and 230 can pivot clockwise (looking top down). As shown, the two nozzles 230 and 232 can pivot out (clockwise) toward the wafer edge 217. By positioning the IPA vapor nozzle 232 to lag the rinse water nozzle 230, the IPA vapor 236 will contact the inboard side 254 (i.e. closer to the center of wafer rotation 244) of the rinse water 234 that has been dispensed on the wafer 210. The counter clockwise rotation of the wafer 210 can further assist by translating the rinse water 236 on the wafer 210 into the IPA vapor 236 that is trailing the rinse water, i.e. is dispensed behind the rinse water relative to the direction of travel 256 and 258 for the two nozzles 230 and 232.
Returning to
During a rinse phase, as shown in
As illustrated in
Such Bernoulli forces have been demonstrated by experiments where in one embodiment a 300 mm wafer 410 was used in a one atmosphere environment, rotating at 1000 rpm, with a 25 mm gap above a fixed plate. With a pressure of one atmosphere or 101.3 kiloPascals (kPa) acting on the wafer top surface 416 a pressure of approximately 15 Pascals (Pa) has been found in the gap 426. The 300 mm wafer 410 rotating at 2000 rpm in the one atmosphere environment has been determined to still have one atmosphere acting on the top surface 416 but with a pressure of approximately 46 Pa in the gap 426.
As shown in
Thus a method and apparatus for removing particles that are the products of etch and cleaning operations from within a thin boundary layer existing on a rotating wafer is described. A method and apparatus to maintain a wafer in a single wafer holding bracket has been described. A method and apparatus for using UV light in wafer cleaning and wafer oxidation has been described. And finally, an apparatus for reducing watermarks from forming on a wafer by angling a nozzle has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.
The application is a continuation of U.S. patent application Ser. No. 11/496,826, currently pending, filed Jul. 31, 2006, which is a divisional of U.S. patent application Ser. No. 10/121,635, now abandoned, filed Apr. 11, 2002.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10121635 | Apr 2002 | US |
| Child | 11496826 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11496826 | Jul 2006 | US |
| Child | 12423760 | US |
