Wafer cassette equipped with piezoelectric sensors

Details Wafer cassette equipped with piezoelectric sensors Wafer cassette equipped with piezoelectric sensors

2001-10-12

2004-06-08

Yu, Mickey (Department: 3728)

Special receptacle or package

Holder for a removable electrical component

For a semiconductor wafer

C206S454000, C206S832000, C211S041180

Reexamination Certificate

active

06745901

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a container for holding and transporting semiconductor wafers and more particularly, relates to a wafer cassette for transporting wafers in a semiconductor fabrication facility wherein the cassette is equipped with piezoelectric sensors that are mounted in the dividers for sensing the presence or absence of wafers on the dividers.
BACKGROUND OF THE INVENTION
In the fabrication process for semiconductor devices, wafers formed of a semi-conducting material pass through various processes in which a plurality of wafers are frequently processed simultaneously. For instance, in a low pressure chemical vapor deposition (LPCVD) reactor, a large number of wafers can be processed by stacking them side by side with only a few millimeters apart in a quartz reaction tube. The quartz reaction tube, sometimes called wafer boats, can hold up to 200 wafers. When wafers are held vertically and separated from each other by a narrow space, a maximum wafer capacity can be achieved in a reaction chamber. For instance, wafers can be positioned in a diffusion or oxidation furnace wherein the wafers are placed perpendicular to a gas flow in a circular cross-sectioned quartz tube.
When a reactor is designed for a specific process, the geometry of the reactor is dictated by the pressure source and the energy source utilized in the reactor. The geometry of the reactor is also an important factor in the through-put rate of the reactor. In general, a reactor should be designed such that an equal flow of reactants can be delivered uniformly to each wafer. It is therefor desirable to stack the wafers horizontally by laying them flat on a horizontal surface instead of stacking vertically at close spacing, even though the horizontal layout is more susceptible to contamination by falling particles.
A vertical furnace operates essentially in the same manner as a horizontal furnace, except for the orientation of the wafers. In a vertical furnace, the wafers are loaded into a horizontal position by a wafer transporting tool from a wafer cassette in which the wafers are stored or transported between processing stations. The wafer blade, or the wafer paddle that is utilized for transporting wafers into process machines from a wafer cassette is normally constructed of a metal or a ceramic material that is capable of withstanding high temperature and corrosive environments. The wafer cassette, on the other hand, is normally constructed of a plastic material that is basically a container which has an open front and corrugated sidewalls formed by dividers for separating the wafers stored therein. The wafer cassette can be advantageously injection molded of a plastic material for accommodating a specifically sized substrate, i.e., an eight inch or a twelve inch wafer. A typical wafer cassette is shown in FIG.
1
.
The wafer cassette
10
, as shown in
FIG. 1
, is constructed by a top wall
12
, a bottom wall
14
, a back wall
16
, and sidewalls
18
and
22
. A cavity
24
is defined by the walls which also provides a front opening
26
. On the interior surfaces
28
and
32
of the sidewalls
18
and
22
, a corrugated configuration is formed by a plurality of dividers
34
and
36
. The dividers
34
,
36
are formed in ridge-shape that are oriented parallel with the bottom wall
14
and perpendicular to the rear wall
16
. Slot-shaped openings, or receptacles
38
,
42
are formed by the dividers
34
and
36
. The dividers
34
and
36
are formed in a suitable length and thickness such that an electronic substrate, e.g., a wafer can be easily slid therein.
Since the wafer cassette
10
is used to store wafers between various fabrication processes and to transport wafers between these processing stations, the wafers contained therein are frequently inspected by an operator either for quality reasons or for identification of the wafer lots. When a wafer
20
is taken out of the cassette
10
and then manually put back in, human error frequently occurs resulting in a wafer misplacement, i.e., a cross-slot or double placement of the wafer
20
as shown in FIG.
2
. The cross-slot misplacement of a wafer into a wafer cassette occurs due to the large number of dividers on the sidewalls of the wafer cassette and the difficulty of identifying corresponding pairs of dividers on the opposite sidewalls of the cassette. The likelihood of making such mistakes is also further enhanced by the fact that the cassette is normally molded of a black plastic material.
When a cross-slot or double misplacement of wafers by an operator occurs, serious consequences can result when an automated wafer loader is used to unload the wafer from the cassette. For instance, when a wafer blade or paddle is used to unload wafers from a SMIF (standard mechanical interface machine) apparatus, the wafer blade or paddle may collide with the wafer that is misplaced or double placed resulting either in the breakage of a wafer, a damage to the wafer blade or both. In either event, the result can be catastrophic in lost wafers or machine down time.
In the wet bench processing of wafers, the wet bench equipment is normally provided with internal buffers for wafer cassette storage and for wafer mapping. For instance, an internal buffer for a wet bench can normally store up to 12 FOUPs (Front Opened Unified Pod). When a mapping error is discovered in the internal buffer, i.e., when wafer misplacement in the wafer cassettes is discovered, it is difficult and time consuming to unload the wafer cassettes from the internal buffer. It is always difficult to count wafers stored in the FOUPs by naked eyes.
It is therefore an object of the present invention to provide a wafer cassette for storing wafers that does not have the drawbacks or shortcomings of the conventional wafer cassettes.
It is another object of the present invention to provide a wafer cassette for storing wafers that is equipped with an automatic sensing device for detecting the placement of wafers in the cassette.
It is a further object of the present invention to provide a wafer cassette for storing wafers that is equipped with sensors in the top surface of the dividers for sensing the absence or presence of wafers.
It is another further object of the present invention to provide a wafer cassette for storing wafers that is equipped with piezoelectric thin film sensors imbedded in the top surface of dividers for sensing the presence or absence of wafers.
It is still another object of the present invention to provide a wafer cassette for storing wafers that is equipped with capacitance sensors for sensing the presence or absence of wafers on the dividers.
It is yet another object of the present invention to provide a wafer cassette for storing wafers that is equipped with piezoelectric sensors and an alarm panel for alerting an operator when a misplacement of wafers is detected in the cassette.
SUMMARY OF THE INVENTION
In accordance with the present invention, a wafer cassette for the storage and transporting of wafers that is equipped with sensors imbedded in the top surface of the dividers for sensing the presence or absence of wafers on top is provided.
In a preferred embodiment, a wafer cassette that is equipped with piezoelectric sensors is provided which includes a cassette body that has a top wall, a bottom wall, a rear wall and two sidewalls forming an open-front enclosure and defining a cavity therein, a left sidewall and a right sidewall, each has a corrugated interior surface, a first plurality of ridge-shaped dividers forming the corrugated interior surface, each of the sidewalls has a base portion integral with the sidewall and a tip portion extending outwardly from the sidewall toward the cavity in the body, the ridge-shaped dividers are oriented parallel to the bottom wall and perpendicular to the front opening defining slot-shaped receptacles therein between, a second plurality of ridge-shaped dividers forming the corrugated interior surface in the right sidewall. Each divider has a base portion integral with the sidewa

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