WO1996009787A1

WO1996009787A1 - Semiconductor wafer cassette

Info

Publication number: WO1996009787A1
Application number: PCT/US1995/012073
Authority: WO
Inventors: Anthony C. Bonora; Mark R. Davis; N. Kedarnath; Joshua T. Oen; Frederick T. Rosenquist; Robert P. Wartenbergh
Original assignee: Asyst Technologies, Inc.
Priority date: 1994-09-26
Filing date: 1995-09-20
Publication date: 1996-04-04
Also published as: IL115433A0

Abstract

A cassette (114) for supporting a plurality of semiconductor wafers (25) for storage and transport through a plurality of semiconductor fabrication processes. The cassette includes top (156) and bottom plates (158), and support columns (160, 162, 164 and 166) extending between and attached to the top and bottom plates. Each of the plates and support columns is formed from a rigid material to resist warpage. A lower surface of the bottom plate includes three mounts (170a, 170b, 170c) spaced from each other around the periphery of the bottom plate. Each mount includes a trough (171) having angled side walls. Each surface on which the cassette is to be supported includes three pins (174a, 174b, 174c), oriented so as to align with the mounts. The cassette (114) is positioned on a support surface such that each of the pins rests against the lower surface of the bottom plate (158) within the troughs of each mount. Dut to the rigidity of the cassette and the pin/mount registration system, a cassette (114) is precisely and repeatably positioned on all supporting surfaces and a high degree of dimensional control is obtained.

Description

SEMICONDUCTOR WAFER CASSETTE

BACKGROUND

Field of the Invention

The present invention relates to a cassette for holding a plurality of semiconductor wafers, and more particularly to a cassette in which the semiconductor wafers may be precisely and repeatably positioned with a high degree of dimensional control during storage and transport through a plurality of semiconductor fabrication processes.

Description of the Related Art

A standardized mechanical interface (SMIF) system proposed by the Hewlett-Packard Company is disclosed in U.S. Patents Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transportation the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.

The SMIF system provides a clean environment for articles by using a smal volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING, " by Mihir

Parikh and Ulrich Kaempf, Solid State Technology. July 1984, pp. 111-115.

Systems of the above type are concerned with particle sizes which range from below 0.02μm to above 200_/tm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one half micron and under. Unwanted contamination particles which have geometries measuring greater than O. lμm substantially interfere with lμm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach

0.2μm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.

A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes which hold the semiconductor wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.

The wafer-carrying cassettes are stored and transported between processing stations in the pods. First, a pod is placed at the interface port within a canopy on top of a processing station. Each pod includes a baseplate door on which the cassette is supported, which door is designed to mate with doors on the interface ports of the processing equipment enclosures. The cover of the pod is lifted and the cassette is supported independently of the baseplate door. Latches release the baseplate door and the enclosure port door simultaneously; the baseplate door and the interface port door are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the baseplate and interface port doors. A mechanical elevator lowers the two doors together, with the cassette riding on the baseplate door, into the enclosure covered space. After processing, the reverse operation takes place.

The SMIF system has proven effective, both inside and outside a clean room, through use in semiconductor fabrication facilities and experiments. A SMIF system provides at least a ten-fold improvement over the conventional handling of open cassettes inside the clean room.

With the demand for ever-increasing output and production yields, it is anticipated that the standard size for semiconductor wafers is to change from 200mm to 300mm. Such an increase requires a redesign of conventional wafer- carrying cassettes beyond a mere proportional increase in size to accommodate the wafers. Conventional cassettes used with 200mm wafers have several problems, which problems would be compounded to unacceptable levels if the size of the cassette were merely increased. These problems primarily arise because it is extremely important to precisely position wafers within a cassette, and to precisely position a cassette on the pod baseplate and the process station indexing plates. Moreover, and it is also important that the precise positioning be repeated each time the cassette is placed on these support surfaces. The reason for this is that automated robotic means are generally used to transfer the cassette, as well to access individual wafers within the cassette. A cassette and the wafers therein must be located in a fixed position with a high degree of dimensional control each time a robot attempts to access the cassette or wafers. Any misalignment may result in the wafers becoming damaged and/or ruined. An example of a conventional cassette 22 is shown in Fig. IA. The cassette 22 generally forms a housing substantially closed on 3 sides, and includes a plurality of semicircular shelves or tracks 24 extending around the enclosure in substantially parallel relationship to each other. The tracks are provided to support the plurality of semiconductor wafers. Conventional cassettes are generally formed from a unitary piece of injection molded plastic. During the injection molding process, stresses form within the plastic while hardening, which stresses inevitably result in some degree of warping of the cassette. Similarly, the plastic cassettes tend to warp at higher temperatures. Warping of the cassette adversely effects performance of the cassette in two ways. First, if the cassette is twisted or slanted, the wafers within the cassette will be misaligned, and it is possible that they will be missed or damaged during mechanical accessing of the wafers. Second, even if the wafers are perfectly aligned with each other within the cassette, warping of the cassette prevents the bottom surface of the cassette from repeatably and precisely registering against its supporting surface, Le_^., the pod baseplate and the indexing plates. Figs. IA and IB illustrate a typical method attempting to properly position a wafer-carrying cassette in the x-y-z planes. Such a cassette may include an "H-bar" section 10 on a bottom of the cassette which rests on the supporting plate 11 between a pair of generally V-shaped mounds 12, 14 extending from the support plate. Registration of the H-bar section against the plate locates the cassette vertically, and proper seating of the H-bar section between the pair of mounds restrains the cassette in the x-y direction. However, warpage of the cassette and/or the surface of the support plate (also conventionally formed from plastic) generally causes misalignment of the cassette with respect to the plate. This misalignment produces a different and unpredictable orientation of the wafers and cassette at each support surface, and may cause damage to the wafers within the cassette during attempted access of the wafers. Any such damage may be significant as a single cassette may carry as much as $20,000 to $30,000 worth of 200mm wafers. A further disadvantage to conventional cassettes is that the footprint of the cassette is significantly larger than the size of the wafer stored therein. This has been true for two reasons. First, conventional designs for wafer-carrying cassettes evolved from a labor intensive industry that was not very adaptive to automation. Therefore, conventional cassettes include several protruding features that are provided to allow manual handling of the cassette. Second, conventional cassettes include features for improving the generally poor rigidity of the cassette. As shown in Figs. 2A and 2B, a conventional cassette typically includes sidewalls having elongated front portions 16a and 16b; rear flanges 18a and 18b to allow the cassette to be laid on its back side; and mechanical gripping flanges 20a and 20b that allow the cassette to be gripped during transfer. Each of these structures directly increases the footprint of a conventional cassette beyond that necessary for storing the wafers therein. The footprint of a cassette is important as every process chamber must be made large enough to accommodate the cassettes. It is desirable to minimize the size of these process chambers, because each is provided with a clean room environment which is expensive to maintain.

Another disadvantage to conventional cassettes is that they are only lifted and transported by gripping the cassette from the top. As there is no space intentionally provided between the bottom of the cassette and the supporting surface, it is impossible to provide a mechanism between the cassette and supporting surface to accomplish lifting of a cassette from the bottom.

Another disadvantage to conventional cassettes is that they can only be used one at a time, and no provision exists for allowing a number of cassettes to be stored or transported through the fabrication processes as a single unit.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a semiconductor wafer cassette in which the wafers are precisely and repeatably positioned within a cassette with a high degree of dimensional control. It is another object of the present invention to provide a semiconductor wafer cassette that easily, precisely and repeatably registers against a support surface on which the cassette rests between or during semiconductor fabrication processes.

It is a further object of the present invention to provide a semiconductor wafer cassette having a minimum footprint for a particular size semiconductor wafer.

It is a still further object of the present invention to provide a semiconductor wafer cassette such that a plurality of cassettes may be provided in a stacked arrangement. It is another object of the present invention to provide a semiconductor wafer cassette including a universal coupling that may used to transfer a cassette by both manual and automatic means.

It is further object of the present invention to provide a semiconductor wafer cassette that may be gripped and transported from the top, bottom or sides of the cassette.

It is a still further object of the present invention to provide a semiconductor wafer cassette including structure for securing and retaining the semiconductor wafers within the cassette. These and other objects are accomplished by the present invention which relates to a cassette for storing and transporting a plurality of semiconductor wafers before, during and after the wafer fabrication process. In a preferred embodiment, the cassette may hold a plurality of circular semiconductor wafers each having a diameter of approximately 300mm. The cassette includes substantially circular top and bottom plates, and support columns extending between and connecting the top and bottom plates. Both the plates and support columns are formed from rigid materials which resist warping or deformation. The columns are provided with a plurality of shelves, with two opposing shelves defining a plane in which a single semiconductor wafer is securely supported. The plurality of shelves together support the plurality of wafers in a parallel and coaxial relation, each wafer being separated from the next by a distance of approximately 7.5mm in a preferred embodiment.

During storage, the cassette generally rests on a baseplate within an environmentally sealed pod. When a process of semiconductor fabrication is to be performed on wafers within particular cassette, the cassette is manually or automatically removed from the pod and baseplate and transferred to an indexing plate within the process chamber.

As explained in the Background of the Invention section, it is important to maintain a high degree of dimensional control by precisely and repeatably positioning a cassette on all support surfaces such as the pod baseplate and indexing plates. Moreover, the convention adapted for positioning the cassette should be simple so that the cassette may be quickly and easily located on the support surfaces. Therefore, each support surface on which a cassette may be located includes three pins extending a small distance above the surface. The pins are located 120° apart and have spherical heads that define a plane against which the bottom surface of a cassette registers. The lower surface of the bottom plate of each cassette includes three mounts provided 120° apart at an outer radius of the plate. Each mount includes a trough having angled side walls. The cassette is positioned on a support surface such that each of the pin heads rests against the lower surface of the bottom plate within the trough of each mount. Due to the rigidity of the cassette and the pin/mount registration system, a cassette is precisely and repeatably positioned on all supporting surfaces and a high degree of dimensional control is obtained.

In addition to each support surface, the upper surface of the top plate of a cassette similarly includes three pins arranged as on the pod baseplate and indexing plates. The pins on the top plate of a cassette mate with the mount on the bottom plate of an above adjacent cassette. As such, cassettes may be securely stacked in a precise and repeatable relation.

Because the pins extend above a support surface, there is a space between the support surface and the bottom surface of the cassette except for the registration points between the pins and mounts. This space may be utilized in a "handshaking" transfer operation whereby a transfer mechanism of generally flat configuration may be inserted into the space between the support surface and the underside of a cassette to lift and transport the cassette from the bottom. This is advantageous, for example, when cassettes are stacked with respect to each other as the entire stack of cassettes may be fork-lift transferred to the desired location as a single unit.

In addition to providing for bottom lifting, the cassette of the present invention further includes a universal coupling hub on the upper surface of the top plate of a cassette. The coupling hub comprises a small knob concentric about the radial center of the cassette. The hub may be securely gripped by an automated robot to thereby allow transfer of a cassette. Alternatively, a detachable handle may fit over the hub and secured thereto so as to allow manual transfer of a cassette. The detachable handle may be provided with a dual security feature, such as requiring both depression of a release button and rotation of the handle, in order to prevent accidental release of the cassette during transport.

Another feature of the present invention is the reduced size of the cassette relative to the size of semiconductor wafers therein. Largely in view of the increased stability of the cassette due to the use of rigid materials, several sections of a conventional cassette included for support have been omitted in the cassette of the present invention. For example, the front sections of both side walls and a number of support flanges of a conventional cassette have been omitted. This allows the cassette according to the present invention to occupy a relatively small space in a clear room environment, where space is critical.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings in which: FIGURE 1 A is an isometric representation of a conventional cassette and its relationship to a conventional baseplate;

FIGURE IB is a cross-sectional side view of a conventional cassette showing a base section seated on a baseplate;

FIGURE 2A is an isometric view of a conventional cassette showing various structures on the cassette;

FIGURE 2B is a top view of a cassette shown in Fig. 2A;

FIGURE 3A is an isometric view of a processing station having a canopy for receiving a SMIF pod;

FIGURE 3B is a cutaway side view of the processing station of Fig. 3 A; FIGURE 3C is a cutaway side view of a pod and cassette with the cassette being transferred from the pod and port doors to an indexing plate within a process chamber;

FIGURE 3D is a cutaway side view of a cassette seated on an indexing plate;

FIGURE 4 is an isometric view of a cassette according to the present invention;

FIGURE 5A is prior art representation of a shelf within cassette for supporting a wafer; FIGURE 5B is a shelf within cassette for supporting a wafer according to the present invention;

FIGURE 6A is a bottom view of a bottom plate of a cassette according to the present invention;

FIGURE 6B is a cross-sectional view through line 6B-6B of Fig. 6A; FIGURE 7A is a top view of a support surface on which a cassette according to the present invention rests;

FIGURE 7B is a cross-sectional view through line 7B-7B of Fig. 7A;

FIGURE 8 is a cross-sectional view showing a pin seated within a trough of a mount on an underside of a cassette according to the present invention;

FIGURE 9 is a side view showing cassettes according to the present invention in a stacked configuration;

FIGURE 10 is an isometric view of a handshaking mechanism according to the present invention; FIGURE 11 is a bottom view of the handshaking mechanism shown in

Fig. 10;

FIGURE 12 is a bottom view of an alternative embodiment of a handshaking device according to the present invention;

FIGURE 13 is a side view of a cassette and a portion of a robotic gripping arm for transporting the cassette; FIGURE 14 is a side view of a cassette and a detachable handle for transporting the cassette;

FIGURE 15 is an isometric view of a cassette according to the present invention showing the side-grip feature; FIGURE 16A is a side view of a crib for supporting a cassette on its side;

FIGURE 16B is a front view of a crib for supporting a cassette on its side;

FIGURE 17 is a partial isometric view of a cassette according to the present invention showing the wafer restraining structure; and

FIGURES 18A, 18B, 19A and 19B illustrate alternative embodiments of wafer restraining structures according to the present invention.

DETAILED DESCRIPTION The present invention will now be described with reference to Figs. 1-

19B which in general relate to a cassette for holding, storing and transferring a plurality of 300mm semiconductor wafers. It is understood, however, that in alternative embodiments, the cassette may be formed to different sizes to accommodate larger of smaller sized wafers. The cassette is transferred between various semiconductor fabrication processes using a SMIF system. One example of a SMIF system is illustrated in Figs. 3A through 3D. Figs. 3A and 3B show a processing station 108 having a canopy 110 which is an easily removable shield that covers the wafer handling mechanism of processing equipment 112. Equipment 112 may be, for example, a photoresist applicator, mask aligner, inspection station or any similar processing equipment. Canopy 110 is preferably constructed of transparent plastic such as acrylic or lexan to facilitate visual inspection and/or maintenance within canopy 110. The canopy encloses the handling mechanism for processing equipment 112 and a holder 114, such as a wafer-carrying cassette according to the present invention. The environment within the processing station is separately maintained and separately cleaned. Therefore, equipment 112 need not be installed in a clean room.

A sealable transportable pod (or container) 118 including a box 120 having interior region 121 and a baseplate door 132 is mounted on horizontal surface 122 of a canopy 110 on a port assembly 124. Port assembly 124 includes a port 126, a port door 128, and an elevator assembly 130. Elevator assembly 130 transports a cassette 114, containing integrated circuit wafers 116 from interior region 121 of box 120 onto the region beneath canopy 110.

In Fig. 3B, port 128 and baseplate door 132 are shown in the closed position by the dotted lines. The elevator assembly 130 includes a platform

136, a shaft engagement device 138, and a drive motor 140. Platform 136 carries port door 128, baseplate door 132 and cassette 114 in a vertical direction. Platform 136 is attached by engagement devices 138 to a vertical guide 142 of elevator assembly 130. Typically, guide 142 includes a lead screw (not shown) and drive motor

140 drives a gear (not shown) which engages the lead screw for driving platform 136 up or down. When platform 136 is driven to the closed position, port door 128 closes the port opening in canopy 110.

In a similar manner, a manipulator assembly shown generally by numeral 144 includes a platform 146 which has an engagement means 148 for engaging vertical guide 142. Manipulator assembly 144 includes a robotic gripping arm 150 and engagement head 152 adapted to engage cassette 114. By vertical operation of platforms 136 and 146, and by operation of manipulator assembly 144, cassette 114 is moved to an internal chamber of processing equipment 112. As shown in detail in Figs. 3C and 3D, when brought into the internal chamber, the baseplate door 132 and port door 128 are moved away, and an indexing plate 154 is raised up to meet the cassette. Once the cassette 114 is positioned on the indexing plate, the plate transports the cassette through the particular fabrication process. Fig. 4 shows a cassette according to the present invention. The cassette is provided to store and transport a plurality of 300mm semiconductor wafers, 25 in a preferred embodiment. It is understood that the height of the cassette may increased or decreased to hold more or less wafers in alternative embodiments. The cassette 114 includes top and bottom plates 156 and 158, respectively, of substantially circular shape, both having a diameter of approximately 12.75 inches. The thickness of the top and bottom plates is preferably about 3.17mm and 4.77mm, respectively. Bottom plate 158 preferably includes a circular cutout section 159, the purpose of which is explained hereinafter. Attached to and extending between the top and bottom plates are four columns preferably provided as shown; two side columns 160 and 162, and two rear columns 164 and 166 (the top plate is shown removed from the columns in Fig. 4 for clarity). The columns may be securely attached to the top and bottom plates by conventional means such as a plurality of screws fit through the plates and into threads in the columns. The height of each of the columns is approximately 209mm. The above-described dimensions of the top and bottom plates and columns may vary in alternative embodiments.

The side columns are all provided with a plurality of shelves 168 for defining a plane in which a single semiconductor wafer is securely supported. In an alternative embodiment, the shelves of one side column may include two raised points and the shelves of the other side column may include one raised point, the three raised points together defining a support plane in which a wafer may rest. The plurality of shelves together support the plurality of wafers in a parallel and coaxial relation, each wafer being separated from the next by a distance of approximately 7.5mm in a preferred embodiment. This is a slightly greater spacing than provided in conventional cassettes holding 200mm wafers due to the fact that the 300mm wafers sag toward their centers when supported in the cassette. Therefore, when the cassettes are lifted and removed from the cassette by a robotic access arm, this sag must be accounted for. However, the 7.5mm spacing is less than what would be predicted from a mere upscaling of conventional cassettes to hold 300mm wafers. The spacing of the wafers within the cassette is largely dictated by the robotic access arm, which had to be provided with a relatively large tolerance in conventional cassettes due to the relative lack of dimensional control. Because of the increased dimensional control of the wafers achievable with the present invention, the position of a wafer may be predicted with greater accuracy than previously known, and the tolerances programmed into the robotic access arm may be decreased. Thus, the spacing between wafers may be decreased. It is understood that the shelves may be closer or farther from each other than 7.5mm in alternative embodiments.

As described in the Background of the Invention section, it is important to precisely and repeatably align both the wafers within a cassette, and a cassette against the support surfaces such as the pod baseplate and the indexing plates within the various processes. Precise and repeatable alignment of both the wafers within a cassette 114, and the cassette 114 against the respective support surfaces is in part accomplished by forming the cassette of rigid materials that resist warping. The top and bottom plates 156 and 158 are preferably formed from a stable, rigid and light weight substance. A preferred material for the top and bottom plates is aluminum, but the plates may also be formed from other materials including teflon^®, graphite, stainless steel, polyetheretherkeytone ("PEEK"), or any of various metallic compositions. The columns are preferably rigid, conductive (to dissipate static charge within the wafers), and exhibit low wear and low particle generation. Preferred materials for the columns include PEEK, aluminum, teflon^®, graphite, stainless steel, or any of various metallic compositions. All of the materials chosen are preferably temperature stable and exhibit low outgassing.

An advantage to using the above described materials is that the problem of warping with conventional plastic cassettes is substantially alleviated. The rigidity exhibited by the cassettes according to the present invention provides a high degree of dimensional control by supporting the wafers in a precisely determinable and repeatable orientation within the cassette.

The present invention is formed from a plurality of members, instead of a unitary design of conventional cassettes. It is difficult to injection mold large objects, either with plastic or metal, because it is difficult to inject molten material to areas that are far remote from the injection source. Thus, injection molding a large unitary structure would limit the choice of materials to only those which can be easily injection molded. By using a six piece construction according to the present invention, each of the pieces may be easily and uniformly manufactured by injection molding, and therefore the choice of materials is not limited by the injection molding process. Moreover, if it is desired to alter the cassette, as for example by decreasing or increasing the height of the columns, the cassette may be disassembled, and the new parts inserted. This is not possible with a unitary construction. Further still, forming the cassette from a multi-piece construction allows the top plate, bottom plate and support columns to be formed from different materials.

A further advantage of using rigid materials is that the wafers are more securely supported on the shelves of the cassette. With the columns and shelves formed from plastic in conventional cassettes, each shelf had to made wider at an apex and tapered toward the end of the shelf as shown in Fig. 5a. This is because, if a conventional plastic shelf were uniformly thick along its length, it could easily tear or break off upon removal from the mold during formation. A tapered slope shelf reduces the stability with which the wafers are held in conventional cassettes because only the outer diameter of the wafers is in contact with the shelf. However, by using sturdy materials such as aluminum or PEEK, the shelves 168 according to the present invention may be made uniformly thick along their length as shown in Fig. 5B, thus allowing a sturdy, substantially planar contact between the wafer and a particular shelf.

Another feature of the present invention allowing a high degree of dimensional control of a cassette 114 on the support surfaces relates to the contact points between the cassette and the support surfaces. As shown in Figs. 6A-6B, a bottom surface of bottom plate 158 of every cassette 114 includes three mounts 170_a, 170_b, and 170_c. Each mount is substantially U-shaped with a cross section having a trough 171 with sloped center walls 172. The width of the trough 171 at its narrowest point (i.e.. adjacent the bottom surface of the cassette) is approximately 5mm wide, and each mount extends approximately 3mm from the surface of the bottom plate. The mounts 170 are preferably located approximately 120° from each other, and are located at an outer radius of the plate 158. It is understood that the angular relation and position of the mounts with respect to each other on the plate may vary in alternative embodiments.

As shown in Figs. 7A-7B, every surface on which a cassette is located, namely each pod baseplate 132 and each indexing plate 154, includes three pins 174,, 174_b, and 174_c. Each pin 174 on each support surface extends approximately 0.25 inches from the support surface, and has a spherical head

175 with a radius of approximately 5mm. Each of the pins 174 is located approximately 120° from each other and is located approximately 5 inches from the center of a circle defined by the three pins. Again, the angular and positional relationship of the pins with respect to each other may vary in alternative embodiments, with the limitation that they align with the mounts as explained below.

When a cassette is placed on a support surface such as the pod baseplate 132 or an indexing plate 154, the three pins 174 define a plane against which the bottom surface of the cassette 114 registers. When properly positioned, the head 175 of each pin lies within a trough 171 of each of the mounts as shown in Fig. 8. As it is most important to precisely and repeatably provide a known height of a cassette and the wafers therein, each head 175 lies in contact with lower surface of the bottom plate 158. The side walls 172 of each of the mounts together act to restrain the cassette 114 in the correct x-y orientation. In this way, a cassette may be quickly, easily, precisely and repeatably provided on each of the support surfaces within the semiconductor fabrication process. In addition to being provided on each of the support surfaces, pins may additionally be provided on an upper surface of top plate 156 of the cassette. As shown in Fig. 9, pins 176_a, 176_b, and 176_c may be provided on top plate

156 in the same manner and orientation as pins 174,, 174_b, and 174_c on the support surfaces. The pins 176 allow cassettes 114 to be provided in a stacked configuration whereby the pins 176 rest in troughs 171 of the above- located cassette. The pins 176 extend 8mm above the top plate 156 surface. This height is chosen such that wafers in a stacked cassette lie in the same horizontal planes as wafers of a single high-height cassette. Thus, the precise location of wafers in a stacked cassette are known, and no specialized instructions or procedures are necessary to access the wafers in the stacked cassette (other than an instruction to skip a certain number of wafer spaces between the top wafer of a first cassette and the bottom wafer of a stacked cassette).

As shown in Figs. 9, 10 and 11, by extending pins 174 above the surface of the support plate, a space is formed between the support plate and cassette resting thereon at all locations other than the contact points of the pins against the mounts 170. Therefore, through use of a "handshaking" transfer mechanism 178 which may be slid between a support surface and a bottom of the cassette 114, the cassette may be lifted from the bottom and transported. This feature is especially useful when cassettes are stacked, as a plurality of cassettes may be fork-lifted together and transported as a single unit.

The handshaking mechanism 178 may be part of a transfer assembly within processing equipment 112, and may be controlled in a known manner through a central processing unit and an input/output device (not shown) of known construction. Handshaking mechanism 178 is preferably of a flat configuration, preferably about 10mm thick, and formed of a rigid and low wear material such as aluminum or stainless steel. In a preferred embodiment, the mechanism 178 enters the space between the support surface and a cassette from the front, 1^, the side opposite rear columns 164 and 166. In this embodiment, the mechanism 178 includes three fingers 180_a, 180_b, and 180_c, each having spherical protrusions 182,, 182_b, and 182_c at an end thereof. The protrusions 182 preferably extend approximately 5mm above the surface of the handshaking mechanism 178. As best seen in Fig. 11 , the protrusions 182 are provided so that when the mechanism is fully inserted between a support surface and the bottom of the cassette, the protrusions align within troughs 171 of the mounts 170. The troughs 171 are of sufficient length to accommodate both the pins 174 of the support surface and the protrusions 182 of the handshaking mechanism.

The combined height of the handshaking mechanism 178 and protrusions 182 is such that the fingers can slide into the space defined between the mounts 170 and the support surface. Once positioned, the mechanism 178 is raised up to seat the protrusions within the troughs 171. With protrusions 182 seated in the troughs, the cassette is securely supported on the handshaking mechanism

178 and may thereafter be lifted off of the support plate and transported. As described above, the support plane defined by providing the protrusions within the troughs is preferred because it is particularly stable. However, it is understood that the handshaking mechanism may be configured differently, and may contact the bottom surface of the cassette at other points so as to support and transfer the cassette.

An alternative embodiment of the handshaking device is shown in Fig. 12. Here the handshaking mechanism 178 enters the space between the support surface and cassette from the rear of the cassette. The mechanism 178 operates substantially the same as described above with regard to Fig. 11. Protrusions

182_b and 182_c slide under the mounts 170_b and 170_c, respectively, and then are raised into troughs 171. However, as pin 174_a wound obstruct a protrusion 182, from sliding into the trough as described with regard to Fig. 11 , the mechanism in this embodiment instead includes protrusions 182_al and 182,₂. When mechanism 178 is slid into position and raised, protrusions 182_al and 182,₂ seat on the outside edges of mount 170_a, thus avoiding interference with the pin 174,.

As shown in Figs. 4, 13 and 14, the present invention further includes a universal coupling hub 184 that allows a cassette to be gripped and transported from a top of the cassette. In a preferred embodiment, the hub 184 comprises a circular knob that is coaxial with a center axis of the cassette 114. As shown in Figs. 3B and 13, the hub 184 may be gripped by an automated robotic gripping arm 150 and an engagement head 152, both of known construction. Once secured about hub 184, the robotic arm 150 and head 152 can lift and transport a cassette 114 as desired. Engagement and disengagement of the arm 150 and head 152 may be controlled in a known manner by a central processing unit and an input/output device (not shown) of known construction.

In addition to automated transport of the cassette 114 as described above, the universal coupling hub 184 alternatively provides for manual transport of the cassette. As shown in Fig. 14, a detachable handle 186 carried by an operator may be affixed over the hub 184. The cassette 114 may then be manually lifted and transported by the handle 186. Handle 186 may be of a known construction and operation. For example, handle 186 may have a central button 188 which retracts a plurality of axially provided claws (not shown) when depressed. The handle is attached by depressing the button and locating the handle over the hub. Once in place, the button is released and the claws engage to grip the hub. The handle is removed by once again depressing the button to detach the claws. In order to avoid accidental disengagement of the handle 186 during transport, an action in addition to depressing the button 188 may preferably be required to disengage the handle. For example, the button 188 may have a locked position in which it is not possible to depress the button or disengage the handle. The button may be switched between the locked and unlocked positions by rotation of the handle.

As previously described, the cassettes 114 may be stacked on top of each other. In order to prevent a universal coupling hub 184 of a first cassette from interfering with proper stacking of a second cassette placed thereon, the bottom plate 158 preferably includes a circular cutout section 159 (Fig. 4). When stacked, the hub 184 extends slightly through the cutout section 159 of the above adjacent cassette. A robotic gripping arm 150 and an engagement head 152 have been described above for accomplishing automated, top-grip transport of the cassette. However, there are mechanisms of other known configurations that may be used for automated gripping and transporting of a cassette. Therefore, the present invention further contemplates the use of an adaptor removably attached to the universal coupling hub 184 in the same way as handle 186 described above. The adaptor has a configuration that is customized to mate and operate with any particular automated top-grip transport system. Thus, the cassette according to the present invention may be used with a wide variety of automated transport systems. As described above, the cassette 114 may be transported by gripping or supporting the cassette from the top or bottom of the cassette. As shown in Fig. 15, the cassette 114 may alternatively be gripped from the sides by means of gripping pawls 190 fitting into slots 192 formed in both side columns 160 and 162. The pawls 190 may be controlled by a known central processing unit and input/output device as described above. The slots 192 may be provided in the side columns at various locations along their length to allow side gripping of the cassette at different locations. Similarly, the side columns may include various numbers of slots along their length to allow more than one gripping location option. It is further understood that the slots 192 may be formed in the rear columns 164, 166 instead of, or in addition to side columns 160, 162.

As is further shown in Fig. 15, a cassette 114 may additionally include cutout sections 194 and 196 in the top and bottom plates 156 and 158, respectively. These cutout sections allow clearance for a sensing apparatus (not shown) to sense information provided on the wafers stored in cassette. The sensing apparatus is of a known configuration and may be, for example, an optical sensor for sensing the presence and/or orientation of a wafer, or for sensing identification information encoded onto a wafer.

As shown on Figs. 15, 16A and 16B, the top and bottom plates 156 and 158 further include notched sections 198 (shown in top plate 158 only). The cassette 114 may be laid on its side, for example when it is desired to allow drainage of process liquids from within the cassette. In such instances, the notched sections rest against a pair of restraining bars 200 within a crib 202 to stably support the cassette and to properly orient the cassette with the open end of the cassette facing upwards. As discussed in the Background of the Invention section, conventional cassettes have a footprint that is substantially larger than the footprint of the wafers support therein. This large footprint is largely due to extended front side walls, and various support flanges protruding from the outer surfaces of the cassette (Figs. 2a and 2b). However, due in great part to the increased structural rigidity of the cassette according to the present invention, the footprint of the cassette has been reduced relative to the size of the wafers contained therein. The absolute footprint of cassette 114 has increased slightly relative to conventional cassettes, as cassette 114 is configured to hold 300mm wafers instead of conventional 200mm wafers. However, the overall footprint is less than what would be predicted from a mere upscaling of a conventional cassette to hold 300mm wafers. Moreover, conventional cassettes had a non circular footprint. Thus, the conventional cassettes could not be rotated within the process chambers. The circular footprint of the cassette 114 allow such rotation. One reason the footprint has proportionally decreased relative to the size of the wafer is that the conventional elongated side walls have been truncated in the present invention into side columns 162 and 164. As previously described, wafer support shelves in conventional cassettes were thickest at their apex and tapered inward. Therefore, conventional shelves were not very wide, , the shelves provided minimal support at the periphery of the wafer. A wide shelf would require a large base, which in turn would require that the shelves be spaced farther apart. However, because the shelves 168 of the present invention are of uniform thickness, they may extend relatively far from the side wall. The increased contact area between a wafer and shelf due to the increased shelf width allows the length of a shelf 168 in contact with a wafer to be shortened without sacrificing overall support stability of the wafer within the cassette. In a preferred embodiment the shelves 168 in side columns 162 and 164 may be approximately 130mm long and 12mm wide. Alternatively, as previously described, the wafer may be supported on three raised points spaced apart on shelves 168 in opposite side columns 160 and 162. The shortened, or truncated, length of the shelves allows the side column 162 and 164 to follow a substantially circular footprint that is only slightly larger than the footprint of the wafers.

Moreover, as stated in the Background of the Invention section, conventional cassettes include a pair of flanges along the backside of the cassette to support the cassette on its side. However, with the provision of notched sections 198 and restraining bars 200, these flanges have been omitted in the present invention. Similarly, as discussed above, the present invention includes structure for lifting a cassette 114 from underneath, from the top, and from the sides, all without the need for support flanges included as part of conventional cassettes. The omission of the conventional flanges from the present invention allowed a further proportional decrease in the footprint of the cassette 114 relative to the size of the wafer.

Figs. 17-19B show structures included as part of the cassette 114 for restrairiing the wafers within the cassette. Fig. 17 illustrates a plurality of projections 204, are located at the front end of each shelf on one or both side columns 162, 164. The projections physically abut against a wafer located within the cassette 114 to prevent the wafer from falling out of the cassette, as when it is tilted or jarred. In a preferred embodiment, the projections 204 extend approximately 1mm above the surface of a shelf 168. Figs. 18A and 18B show a cassette 114 including wafer restraining gates 206, each gate having a first end mounted in the top plate and a second end mounted in the bottom plate along a common axis of rotation. The cassette according to this embodiment may include one (Fig. 18B) or two (Fig. 18 A) gates 206. Once the wafers are located within a cassette, the gates 206 are either manually or automatically pivoted from a retracted position into a restraining position across the front of the cassette. In the restraining position, the gates keep the cassettes properly positioned against a rear of the cassette.

It is advantageous to prevent scraping of a gate against the outer radius of the wafers as a gate moves from a retracted position to a restraining position, because such scraping may generate harmful paniculate matter. Scraping may be prevented a number a ways. In one embodiment, a free rolling sheath 208 including a low friction surface may be provided around the gate 206 in the area that contacts the wafers. Another method of preventing scraping used instead of or in addition to sheath 208 is to have the gate spaced away from the wafers as it pivots from a retracted position to the restraining position so that the gate contacts the wafers only upon reaching the restraining position. Such motion may be accomplished by selective location of the axis of rotation of the gate in the top and bottom plates. As shown in Figs. 19A and 19B, scraping may additionally be prevented by controlling the pivoting of gate 206 by a cam assembly 210. Figs. 19A and 19B show a top view of a cassette top plate including a top member 212 of the gate 206, a slot 214, and guide pins 216 and 218. An end 220 of the top member 212 provided within slot 214 is moved in a direction indicated by arrow A, either by manual or automated means. As end 220 moves within the slot, guide pin 216 causes the gate 206 to pivot along a path generally shown by a dotted line 222. Eventually, a curved section 224 of top member 212 engages the guide pin 216, thereby drawing the gate 206 into a restraining position against guide pin 218 as shown in Fig. 19B. It is understood that scraping of the gate 206 against the wafers may be prevented by other known methods. As previously explained, the height of a cassette may vary in alternative embodiments. However, in general the cassettes will be provided within a pod 118 (Fig. IA) which is sized to fit the largest cassette. Moreover, in general, it is desirable to restrain a cassette within a pod during transport by having a portion of the cassette in contact with an upper surface of the pod. Therefore, it is another feature of the present invention to include a spacer to raise the top surface of a cassette 114 up to the upper surface of the pod. Alternatively, the spacer may be placed on top of a cassette 114 so that the cassette is restrained by the spacer, which is in turn in contact with the upper surface of the pod. Although the invention has been described in detail herein, it should be understood that the invention is not limited to the embodiments herein disclosed. Various changes, substitutions and modifications may be made thereto by those skilled in the art without departing from the spirit or scope of the invention as described and defined by the appended claims.

Claims

We Claim:

1. A cassette for supporting a semiconductor wafer with a high degree of dimensional stability, comprising: a rigid, warp-resistant, substantially circular bottom plate; a rigid, warp-resistant, substantially circular top plate; and a plurality of rigid, warp- resistant support columns extending between and attached to said top and bottom plates, each support column of said plurality of support columns including a plurality of shelves, with a group of shelves, one from each support column, supporting the semiconductor wafer.

2. A cassette for supporting a plurality of semiconductor wafers as recited in claim 1 , wherein said top and bottom plates are comprised of aluminum.

3. A cassette for supporting a plurality of semiconductor wafers as recited in claim 1 , wherein said plurality of support columns are comprised of polyetheretherkeytone .

4. A cassette for supporting a plurality of semiconductor wafers as recited in claim 1 , further comprising a plurality of protrusions on an upper surface of said top plate for aligning with and being received within a plurality of mounts on a bottom plate of a second cassette stacked on top of the cassette.