This invention relates to work-piece holding devices. More specifically, the invention is concerned with providing a fixture for holding a silicon wafer during fabrication processes in the semiconductor industry.

Background of the invention

Many fabrication processes in industries such as the semiconductor industry require one or more semiconductor wafer workpieces to be held in a fixture, e.g., using cageboat or wafer cassette fixtures. Some of these wafer fabrication processes (e.g., deposition processes) are accomplished under harsh conditions, e.g. furnace-like temperatures. In addition to holding workpieces, the fixture must also be cleanable for deposition process applications.

Prior art fixtures are typically composed of quartz. Quartz offers economic fabrication of fixtures (e.g., uncomplicated forming by bending and attachment by welding at elevated temperatures) , but quartz fixtures have experienced problems in semiconductor fabrication applications. Those problems include maintaining positional tolerances under elevated temperatures and difficulties in cleaning deposits from the fixtures. For example, some cleaning processes briefly immerse the quartz fixtures in an HF acid bath. If the quartz fixtures are immersed in an HF acid bath long enough to consume all deposits, damage to the fixture will typically occur. Quartz fixtures may also not be suitable for precisely positioning wafers during process steps having furnace-like temperatures. Elevated temperatures, acid erosion of fixture features (e.g., slot widths, slot depths, and fixture length features), and other harsh conditions of semiconductor fabrication applications result in unreliable fixture positioning accuracy which in turn degrades wafer positioning accuracy. Reduction in positioning accuracy degrades wafer deposition uniformity. Carried further, positioning accuracy loss will prohibit

6/17691 PCI7US95/15575

2 the use of wafer autoloaders and/or other automated equipment. When this happens, quartz fixtures are considered unusable. Other problems with prior-art quartz fixtures in the semiconductor industry include: difficult removal of welded components and repair/replacement of these components, particulate contamination, and chipping/breakage requiring replacement of the entire fixture. Similar fixture problems also occur in other industries, such as aerospace and medical apparatus manufacturing.

Other materials of construction have certain advantages for elevated temperature applications, but other difficulties have restricted applications to wafer fixtures. Some ceramic materials, such as graphite, are typically more capable of withstanding elevated temperature and acid cleaning processes, but these materials are typically difficult to fabricate, e.g., they are brittle and difficult to weld.

Attachment of elements composed of these high temperature-resistant materials can be especially difficult. For example, machining brittle ceramics to form threaded or other mechanical connector shapes may impose unacceptable stresses. Even if low-stress mechanical connectors can be fabricated, typical mechanical connectors provide traps for contaminants which may be later released to contaminate semiconductor wafers during later elevated temperature processes. Adhesives typically do not have the high temperature, purity, and strength properties needed for many semiconductor industry applications.

Summary of the Invention

By using a high strength ceramic (such as a fine grain graphite) and restricting interference dimensions to limit stresses, ceramic elements can be economically press fit to form a fixture assembly for holding semiconductor wafers. The combination avoids the problems of prior-art holding fixtures such as unreliable cleaning. A press-fit graphite structure also creates a rigid, damage-resistant fixture that maintains highly accurate dimensional

tolerances and allows full (i.e., longer exposure) acid cleaning of deposits while being able to withstand furnace¬ like temperatures. The surfaces of the ceramic element are also more easily purified to minimize the risk of contaminating wafers during subsequent semiconductor wafer processing.

The invention requires a male ceramic element and a female ceramic element to be composed of materials having a flexural strength of at least about 4,000 psi and dimensioned to provide a normalized interference fit of no more than about 1.0%. Assembly process steps for the fixture, such as purification and pressing, may also be controlled to further assure a reliable holding fixture for the semiconductor industry.

Brief Description of the Drawings

Figure 1 shows a front view of a fixture assembly; and

Figure 2 shows a side view of the fixture assembly as shown in Figure 1. In these Figures, it is to be understood that like reference numerals refer to like elements or features.

Detailed Description of the Invention

Figures 1 and 2 show a fixture assembly 2 for holding one or more workpieces, such as the three semiconductor wafers 3 shown dotted for clarity. The wafers 3 are held in place by slots 4 in a plurality of substantially cylindrical rails 5 which are in turn press fit into supports or end straps 6. The fixture assembly 2 holds the wafers 3 during wafer fabrication steps, such as depositing, chemical milling, cleaning, diffusion, and annealing. Although the fixture assembly 2 shown can be used to hold wafers 3 during several different process steps, other ceramic fixture assemblies (not shown) may be required for other fabrication steps, e.g., steps ^' requiring greater separation between wafers.

Typically, the fixture assembly 2 holds about 25 semiconductor wafers 3 which are substantially circular in

shape, but other wafer quantities and shapes can be held in alternative embodiments. Although planar wafers are shown, alternative embodiments can also hold workpieces having irregular shapes. At the start of wafer fabrication, a typical wafer is substantially composed of a silicon-based semiconductor material and is about 125 mm in diameter and about 0.55 mm thick. Deposition processes can add several layers of materials and thereby increase the thickness of the wafer, but the increased thickness is typically measured in Angstrom units.

Although four rails 5 each having twenty-five slots 4 are shown holding the wafers 3 in Figures 1 & 2, alternative embodiments may have a different number of slots and rails or other means for holding the workpieces. Because of the automated nature of typical processing equipment in the semiconductor industry (e.g., using wafer autoloaders that require relatively accurate fixture positioning) , fixtures for this application will typically hold or otherwise contact the wafer workpieces in at least three positions to achieve greater positional accuracy. In the embodiment shown in Figures 1 and 2, the planar wafers 3 rest on four rails 5 positioned to hold and precisely position the wafers in aligned slots 4. The preferred holding slots 4 easily allow automated insertion and removal of the wafers 3 from the fixture assembly 2, but alternative embodiments may include pins, flats, or other means for holding wafers or different workpieces.

The slots 4 are typically shallow, i.e., the slots do not extend as far as the centerline of the rails. The depths of slots 4 are preferably uniform in depth and spacing along the length of rails 5. For semiconductor wafers 3 and fixture elements (rails 5 and end straps 6) composed of a preferred graphite material, a nominal slot depth of 0.086 inches (2.184 mm) is preferred. Slot depths can range from about 0.04 to 0.18 inches (1 to 4.5 mm) or more for alternative embodiments.

The end straps 6 include recesses 7 and relatively thin shoulders 8. These features minimize fixture deposition and distortion effects during wafer

fabrication steps. These features also minimize costs and the amount of materials required for the fixture.

The end straps 6 and rails 5 are preferably composed of a ceramic, preferably a fine grain synthetic graphite supplied by Poco Graphite Inc. , located in

Decatur, Texas. Other ceramic materials having acceptable properties may also be used by themselves or in conjunction with the preferred graphite materials, e.g., high strength silica carbide, aluminum oxide, titanium diboride, and boron nitride.

An important feature of the preferred fine-grain graphite materials is the relatively isotropic nature of their properties, especially flexural strength. In contrast, other graphite materials have anisotropic properties, i.e., have different properties in different directions. The isotropy of the preferred graphite minimizes structural failures and avoids the need for added control of material property orientation in the fabricated elements. A key property of acceptable materials is their high flexural strength relative to some other ceramic materials. Although acceptable ceramic materials may have flexural strengths as low as about 4,000 psi (at elevated temperatures) , preferred ceramic materials have a flexural strength typically ranging from about 5,000 to 25,000 psi (352 to 1758 kg/cm ² ) . A minimum flexural strength of at least 8,000 psi (562 Kg/cm ² ) is more preferable and at least about 10,000 psi (703 Kg/cm ² ) still more preferable. These flexural strengths (and the corresponding dimensional limitations) are chosen to allow the fixture elements to be mechanically attached by the press fit without structural failure or damage.

Another important property of the required ceramic materials for these fixtures is their chemical resistance. This property allows the material to be cleaned in HF acid baths or other harsh cleaning environments typical in the semiconductor manufacturing industry. In contrast, quartz materials in prior art holding fixtures typically suffered deteriorating

properties due to acid attack.

Another important property is the ability to withstand elevated temperatures. Elevated temperatures during semiconductor manufacturing may be 500 °C or more. The preferred graphite materials maintain sufficient strength and dimensional stability even under elevated temperatures comparable to furnace temperatures.

The assembly of rails 5 and end straps 6 requires pressing the male ends 10 of the rails 5 into the female mating surfaces 9 of the end straps 6. The protruding male ends 10 for the embodiment shown are machined or otherwise formed into a surface having a representative interface dimension which requires a press fit to assemble, e.g., a cylindrical protrusion having a nominal diameter of 0.3753 inches (9.53262 cm) with tolerances of plus 0.0003 and minus 0.0000 inches (plus 0.00762 mm, minus 0.0000 mm). Similarly, the mating female surfaces 9 are machined or otherwise formed into a receptor arc shape having a nominal diameter of 0.3750 inches (9.5250 mm) with tolerances of plus 0.0003 and minus 0.0013 inches (plus 0.00762 mm, minus 0.03302 mm) .

A normalized interference fit dimension, I (in percentage terms) , can be defined as I = 100% x (D _B - D _f )/D _m where: D _m = protruding interfacing dimension of the male element; and D _f - receptor interfacing dimension of the female element. For the dimensions and tolerances of the preferred embodiment shown and described, the resulting normalized interference fit ranges from 0 (i.e., no clearance) to about 0.5%. For the elements composed of the preferred graphite or other acceptable ceramic materials, a normalized interference fit can be 1.0% or more, but the normalized interference fit would more typically be less than about 0.4%, even more typically, be less than about 0.3%. A minimum normalized fit of at least about 0.015% is also preferred to assure a reliable mechanical attachment of the press-fit elements. This limited range of

normalized interference fits avoids damage to the ceramic materials while achieving a reliable attachment and a rigid structure.

Assembly of elements having these normalized interference fits requires application of a press-fit force. The force is typically applied to the end straps in a direction generally parallel to the aligned length or other major dimension of the rail. For the preferred embodiment as shown and described, a nominal force or force component of about 20 lbs (89.0 newtons) is applied to the end straps in a direction parallel to the length of the rails or male elements. For other embodiments, the press- fit force or major force component may range from about 5 lbs to 100 lbs (22.2 to 445 newtons) or more. Pressing can be achieved using conventional pressing means such as an arbor press.

The application of the press-fit force results in compressive stresses in the ceramic rails 5 and tensile/ flexural stress in the end straps 6. The maximum force is limited by the strength of the ceramic materials. Maximum force limitations also prevent buckling and failure of the rails 5. Minimum force limitations assure a reliable press-fit attachment during wafer fabrication processing. Alternative embodiments of the fixture are possible. If wafer positioning tolerances permit a less rigid fixture structure, an alternative embodiment has a single support or strap with rails extending from each side. .Another alternative embodiment (e.g., for applications requiring a more rigid fixture without "end" straps) provides a U-shaped rail pressed into a support strap at two points. In another alternative embodiment, the end strap(s) and rail(s) are composed of different materials, e.g., materials having different coefficients of thermal expansion. In still another alternative embodiment, the rail can be composed of a rod and collar, the rod press fit into the collar and the collar press fit into a support strap. The collar and rod embodiment allows one end strap (having a plurality of similar rail-receptor sites) to support different diameter rods.

The process of assembling the fixture begins with pre-conditioning the end straps and rails. Pre¬ conditioning for the semiconductor industry typically includes purification or other cleaning steps to remove contaminants. Pre-conditioning may also include coating one or more component elements, e.g., to obtain harder interfacing surfaces.

After pre-conditioning, the rail and end strap elements are aligned or otherwise positioned and pressed together. The forces applied to the elements during pressing are typically limited such that the resulting stresses in the elements are limited to no more than about 50% of the compressive, flexural, and other strengths of the material. The pressing forces are removed when the elements are sufficiently pressed together to form a mechanical attachment and the rails are aligned to hold the workpiece wafers in the desired positions, e.g., the slots on one rail are substantially in the same plane as the slots on other rails. Post-conditioning of the press-fit assembly is then accomplished. Post-conditioning typically includes grit blasting (preferably using aluminum oxide or silicon carbide grit) and may also include other cleaning, coating, and purification steps. The surface finish of grit blasted surfaces is preferably no more than about 400 R,. Preferred post-conditioning also removes substantially all contaminants such that no more than 5 ppm (of the assembled wt.) contaminants remain.

The post-conditioned assembly is then used to hold one or more workpieces, such as semiconductor wafers, during one or more wafer fabrication processes. Wafers are typically loaded and unloaded using automated equipment. If the fabrication processes include deposition, the fixture will also typically become coated with the deposition material requiring periodic cleaning/removal. In addition, other fabrication processes may generate dust or other materials which coat the fixture and require periodic removal.

In the semiconductor industry, the typical

cleaning process is an acid bath. Because graphite and other ceramic materials have a high resistance to acid attack, the fixture is typically immersed for periods ranging from about 3 to 120 minutes, preferably at least about 10 minutes.

After repeated use and cleanings, a repair or replacement step may also be required. The repair or replacement step may require removal of an element of the fixture and substituting another (possibly pre-conditioned) element prior to repeating the pressing step. Removal is typically also accomplished by pressing, but other removal means may also be used, e.g. , drilling out.

Because of the number of wafers and fixtures needed, standardizing the dimensions of component fixture elements is expected to facilitate the repair or replacement step. The repair or replacement step could also reconfigure fixtures for use in other wafer processing steps, especially if (alternative embodiment) collars and rods are used. For example, a single type of end strap support could be reconfigured to be repeatedly used interchangeably with several different rail configurations. Alternative sequences and process steps are also possible. These include deleting the pre-conditioning step, the post conditioning step, and/or the repair/replace step. Moreover, several different types of fixtures may be required in order to hold workpieces in precise locations during wafer fabrication processes, e.g., one type holding bottom portions of wafers and another type holding top portions of wafers. Different types of fixtures would require appropriate additional fixture assembly steps, e.g., a second pressing step.

While the preferred embodiment of the invention has been shown and described, and some alternative embodiments also shown and/or described, changes and modifications may be made thereto without departing from the invention. Accordingly, it is intended to embrace within the invention all such changes, modifications and alternative embodiments as fall within the spirit and scope of the appended claims.