WO1999060614A1 - A wafer buffer station and a method for a per-wafer transfer between work stations - Google Patents

Abstract

A buffer station is disclosed, for a 'per wafer' transfer of wafers between work stations. The wafers are retrieved from the pod by a track robot at a first work station and are processed. When the processing at the first work station is completed, the track robot takes the wafer and, rather than returning it to the pod, places the wafer in the buffer station. When the second work station is ready to accept the wafer, its track robot retrieves the wafer from the buffer station and inserts it into the second work station for processing. When processing is completed at the second work station, the track robot inserts the wafer into a pod located at the second work station.

Description

A WAFER BUFFER STATION AND A METHOD FOR A

PER- WAFER TRANSFER BETWEEN WORK STATIONS

Field of the Invention

This invention relates to a method and an apparatus for improving throughput

and reducing or minimizing idle times in production of semiconductor wafers used in

the fabrication of integrated circuits and flat panel displays. In particular, the

invention relates to a method and apparatus for improving and optimizing the rate of

transfer of wafers between successive stations of a production line.

Background of the Invention

The next milestone in semiconductor wafer processing is the transformation

from 200mm wafers to 300mm wafers. While the present invention is applicable to

both standard 200mm wafers and the next generation 300mm wafers, the following

description relates mostly to the more complex 300mm wafers technology.

Semiconductor wafers are processed in processing lines which generally

comprise a number of stations. One such station is depicted in Figure 1 and generally

indicated at 10. The station 10 comprises a transfer chamber 11 with a suitable

platform (not shown). Several process chambers (four in this example) 12 are

mounted at four facets of the transfer chamber 11, which, in this example, has six

facets. Two load lock chambers 13 are mounted on two other facets of the transfer

chamber and connected to the mini-environment (also called Factory Interface, FI) 15,

hereinafter described. A robot schematically indicated at 14 operates to transfer the

wafers from the load lock chambers 13 to and between the process chambers 12. Examples of such a station are the Centura™ or Endura™, available from Applied

Materials™ of Santa Clara, CA.

The mini-environment, generally indicated at 15, serves as a clean

environment for wafer scheduling and handling. Such a mini-environment may be a

SMIF-300 Wafer Management System™ available from Asyst Technologies, Inc. of

Fremont, CA. It includes an enclosure 16 and several (four in this example) wafer

pod loaders 21, 22, 23 and 24. The enclosure 16 houses a track robot 19 for

transferring the wafers from the pods to the load lock chamber 13. A suitable track

robot is available from Equipe Technologies of Sunnyvale, CA. The robot 19 is also

used to transfer wafer to and from the wafer aligner 18.

The work stations could be differently structured and, for instance, comprise

other elements, such as a buffer chamber, pre-clean and cool-down chambers, pre¬

processing and post-processing chambers, and so on. However, the present invention

is independent of the particular structure and operation of the work stations, since it

concerns the transfer of the wafers from one work station to the next, especially via

the mini-environment.

In the present state of the art, the wafers are conveyed to each work station by

means of pods (or cassettes), are transferred from the pods by the robots 19 and 14 to

the various positions they have to occupy successively in the work stations, and when

the processing stages that take place in that station are completed, the wafers are

returned to the pods and the pods are then transferred to the next station, where they

and the wafers carried thereby are handled in the same way. This method of transfer

tends to reduce throughput because of the various transfer motions that are required.

Throughput can also be unfavorably affected by imperfect synchronization between adjacent stations. Considering two adjacent stations, A and B, station B will not be

filled with wafers until station A has completed processing all the wafers of at least

one pod and the pod is then transferred to station B. Even if station B operates at

exactly the same rate as station A, it may remain idle for the time required to process

the wafers of the whole pod. If station A is delayed for any reason, the idle time may

considerably increase. There is no way in the art for controlling the flow of wafers

through the processing line in order to eliminate, or at least to minimize, idle times.

Additionally, transferring the pods between stations can increase the number

of defects on the wafer. This issue becomes increasingly important as design rules are

reduced, thereby making even minute particle killer defects.

Summary of the Invention

It is a purpose of this invention to improve the transfer of wafers along a

processing line and between adjacent work stations so as to eliminate, or at least

minimize, said idle times.

It is another purpose of this invention to provide control means for

maintaining the flow of wafers through the processing line at an optimal rate at all

times.

It is a further purpose of this invention to provide method and apparatus means

for achieving the aforesaid purposes.

It is a still further purpose of this invention to provide means for the early

detection of any malfunction in the processing line.

Other purposes and advantages of the invention will appear as the description

proceeds. The present invention provides a method of transfer of wafers between two

adjacent work stations, hereinafter called the "preceding" or A stations and the

"following" or B, station, respectively, which method comprises individually

transferring all or some of the wafers, after they have been processed in the preceding

work stations, to a buffer station and therefrom to the following work station, instead

of returning them to a pod at the preceding work station and transferring the pod to the

following work station, as in the prior art. Thus, the present invention provides for

"per-wafer" processing line, rather than the conventional "per-pod" processing line.

Preferably, the transfer of wafers to and from the buffer station is carried on by the

robot of the mini-environment. While the buffer station can be a simple stage having

no operable functions, preferably the buffer station includes an inspection or

metrology tool.

Brief Description of the Drawings

In the drawings:

Fig. 1 is a schematic plan view of a work station according to the prior

art;

Fig. 2 is a schematic plan view of individual wafer transfer between

adjacent work station according to an embodiment of the present invention;

Fig. 3 is a schematic plan view of a second embodiment of the

present invention;

Fig. 4 is a schematic plan view of a third embodiment of the present

invention; Fig. 5 is a schematic plan view of a fourth embodiment of the present

invention;

Fig. 6 is a cut-away view of a buffer station suitable for use in the

embodiment depicted in Fig. 5.

Detailed Description of Preferred Embodiments

Fig. 2 schematically illustrates an embodiment of the invention, comprising

two adjacent work stations 30 and 40. These are indicated in this drawing in broken

lines. The mini-environments of the two work stations are indicated respectively by

numerals 31 and 41. The lock load chambers are indicated respectively by numerals

32 and 42, the pod loaders by numerals 33-36 and 43-46; and the wafer aligners are

indicated by numerals 37' and 47'. As noted above, in the prior art, wafer transfer

between the stations 30 and 40 can be performed only by moving pods. Thus, the

processing of all the wafers of a particular pod must be completed before the pod can

be transferred to the next station.

According to the present invention, single wafer transfer is made possible, so

that a "per wafer" processing can be achieved. Specifically, according to the

embodiment of Figure 2, a buffer station 50 is placed between the two mini-

environments 31 and 41 of work stations 30 and 40. As will be discussed in the

following disclosure, the buffer station can be implemented as a simple support

structure, ca be a part of the mini-environments, or can include an inspection and/or

metrology tool.

In Figure 2, the buffer station 50 includes an inspection tool 51, such as, for

example, disclosed in patent application, Attorney's Docket No. 2417. The inspection tool 51 is schematically indicated as comprising a mechanical structure 52, which

actuates, by means not shown, a rotating turntable 53 upon which is mounted a wafer

54. Said wafer is rotated by turntable 53 and is scanned by scanning means, not

shown, which can be of any suitable kind, insofar as this invention is concerned. In

particular, the scanning may be carried out by laser beams and the wafer be classified

as containing or not containing suspect pixels, depending on the reaction of pixels to

the scanning, said reaction being defined by the intensity of the light scattered by each

pixel in a number of directions. The inspection tool 51 may comprise one or more

optical heads, all described in said co-pending application, Attorney's Docket No.

2417. Moreover, it should be understood that any conventional X-Y stage can be used

instead of the turntable 53, so as to effect an X-Y scanning of the wafer, rather than an

r-theta scanning. Alternatively, the wafer may be held stationary and the optical head

can be scanned over the wafer.

The patent application Attorney Docket No. 2417, the contents of which are

herein entirely recalled by referenced, discloses a wafer inspection control apparatus

which can be used to detect suspected defects in wafers, and among them, defects

which indicate a malfunction of the processing line, possibly so sever as to require

taking a chamber off the production line for purposes of repair or maintenance.

According to the invention disclosed in said application, the wafers are individually

inspected/controlled by loading them on a device that rotates them about their centers,

scanning their surfaces with any appropriate means, e.g., by means of a laser beam,

and evaluating the response of each pixel of the wafer surface to the scanning,

whereby wafers are classified as approved or suspect wafers, according to said

response. Preferably, according to this embodiment of the invention, a wafer inspection or control apparatus, as described in the said patent application is located

between the work stations 30 and 40, and occupies and constitutes the buffer waiting

station 50. In this embodiment, therefore, the transfer of the individual wafers

between the work stations is carried out by transferring each wafer, which has

undergone all the processing stages at work station 30 and has been returned to the

mini-environment 31 , to the wafer control apparatus 51 , and effecting therein the

inspection or control of the wafer. Then, the approved wafers are transferred to the

following work station 40. Suspect wafers can be taken offline to another apparatus,

not shown, which effects further control of the suspect wafers or carries out other

operations. Transfers of the individual wafers are preferably carried out by robots 36

and 43, or may be assisted by an optional dedicated robot 55.

The amount of wafers that are so individually transferred is controlled by the

FAB controller, so as to maximize throughput, or, in other words, to eliminate or

minimize idle time. The programmed control of the individual wafers transfer will be

understood from the following considerations. Ideally, if two successive stations A

and B worked at same production rate, and no wafers from station A were found

suspect by an inspection device interposed between the two stations, the transfer of

wafers from station A to station B could occur entirely through the inspection device

and by the handling of individual wafers, and the wafer pods would be used only for

loading station A and unloading station B, or, if more than two successive stations are

so connected, only for loading the first station and unloading the last station.

However, this ideal situation will not occur often. First of all, each process

stage has its own duration and this alone would prevent complete synchronization

between different work stations. Further, the production rate in a work station is determined not only by the time required for the various process stages, but also by

the motions which the robots must make in order to transfer the wafers within the

station. By decreasing the number of wafers that are returned to the pods in station A,

the overall throughput of station A is increase; and if station B, at a particular time,

has a higher rate of work, and would be idle if it had to wait for station A to fill a pod

and for the pod to be transferred, it can be kept in operation by transferring wafers

individually, according to the invention.

Transferring the wafers individually between adjoining stations, as provided

by this embodiment, has the further advantage that they are inspected/controlled in

their transfer between stations. Not all of the wafers are controlled, or need to be

controlled. The control is generally statistical in nature. A statistical control,

however, is sufficient to reveal serious malfunctions in a work station, in this case,

station A. If a serious malfunction were not detected, defective wafers would

continue to be fed through the line and would be detected only after processing had

been completed, a considerable waste of time and material. In any case, it is a waste

of production time and operations to let wafers go down the production line to the end

of it, and only then detect defects that may have occurred in an early work station.

Many phenomena may occur in semiconductor wafers production lines that

cause deviation from a flawless operation and interfere with the regular flow of the

wafers. Providing two manners of wafer transfer - by pods and individually - affords

an elasticity of operation that results in production efficiency. A general control of

wafer transfer should receive as pertinent data inputs: a) the programmed processing

times of each work station; b) the times required for the transfers within each station;

c) the time required for pod unloading and loading; d) the statistic percentage of rejects (suspect wafers) of each station; e) detected and/or foreseen work station

malfunctions. Based on said data, a program can be formulated to determine the ratio

of processed wafers that are individually transferred and the ratio of individually

transferred wafers that are controlled and to react to any change in the input data to

vary said ratios, so as to optimize the flow of wafers through the processing line.

In operation, when processing of a wafer is completed in station 30, track

robot 36 transfers the wafer to the buffer station 50. Depending on the respective

throughput of the stations 30 and 40 and the inspection tool 51, all wafers may be

inspected, or a sampling plan may be implemented. If the wafer is to be inspected, the

inspection tool 51 inspects the wafer and, upon completion, indicated to the controller

of track robot 43 that the wafer if ready to be moved to station 40. When station 40 is

ready to accept the wafer, track robot 43 retrieves the wafer from buffer station 50 and

inserts it into one of the load lock chambers 42.

Depending on the throughput of the workstations 30 and 40, the inspection

tool can also be used by both stations synchronously. For example, when work

station 30 completes processing of a wafer, it sends it via the track robot 36 to the

inspection station 51. If the wafer has no defects, it is then delivered to work station

40 by track robot 43. When work station 40 completes processing of the wafer, it then

returns the wafer to the inspection station 51 for a second inspection. However, such

processing requires the fab controller to synchronize the operations of the track robots

36 and 43 to use the inspection station 51 one at a time.

Figure 2 also depicts (in broken lines) an optional dedicated robot 55. This

robot can be used to transfer wafers from the mini-environment 31 of station 30 to the

mini-environment 41 of station 40, or to take out suspected faulty wafers from the inspection tool 51. Thus, for instance, in cases where inspection of the wafer reveals

a potentially defective situation, and the wafer must be transferred to further

inspection apparatus (not shown), robot 55 may remove it from the fabrication line

from further processing.

According to another embodiment of the invention, depicted in figure 3,

buffer station 50 has no operative function. Rather, it merely serves as a waiting

station for the wafers. Thus, the buffer station 50 may include a table 56 supporting a

vacuum or electrostatic chuck 58 for supporting the wafer. In operation, track robot

36 places the wafer on the chuck 50, and track robot 43 retrieves the wafer from the

chuck 58.

According to a further embodiment, depicted in Figure 4, the buffer station is

eliminated. Instead, the robots 36 and 43 perform a "handoff operation between each

other whenever a wafer needs to be transferred. This embodiment, however, may

require a fine tuning of the throughput of stations 30 and 40, and a delicate tuning of

the robots 36 and 43. A more preferred embodiment is depicted in Figure 5, wherein

a buffer station 50' is inserted partially in station 30 and partially in station 40, and

includes a chuck 58' for holding the wafer in transfer. Moreover, as shown in figure

6, buffer station 50' may include several chucks 58' for holding several wafers. This

can be easily accomplished since not much of the vertical space of the mini-

environment is occupied.

Specifically, Figure 6 depicts parts of the two mini-environments 31 and 41

with the walls removed. Track robot 36 is depicted with its arm vertically retrieved,

while track robot 43 is depicted with its arm partially vertically elevated. Such a

vertical movement is standard with track robots, such as that available from Equipe Technologies. In the particular example of Figure 6, three pin chucks 58' are depicted

arranged one above the other. A wafer 59 is depicted resting on the middle chuck.

Using this arrangement, track robot 36 can place wafers on the chucks on its own

pace, while track robot 43 can retrieve the wafer on its own pace.

It should be appreciated that the arrangement of Figure 6 can be used in

conjunction with the inspection station 51 of Figure 2. Such an arrangement can be

advantageously used when both work stations 30 and 40 use the inspection station 51

for inspecting the wafers after processing as described above. Thus, the pin chucks 58

can be used to synchronize the inspection of wafers from two stations. Alternatively,

one or more wafer pods, connected to any of pod loaders 33-36 and 43-46, can be

used as a "buffer" pod for pods in a queue for the inspection station 51.

While some embodiments of the invention have been described by way of

illustration, it will be apparent that the invention can be carried out with many

modifications, variations and adaptations, without departing from its spirit or

exceeding the scope of the claims.

Claims

1. A method for transferring wafers between two adjacent work stations in a

processing line for the manufacture of semiconductor wafers, comprising:

individually transferring wafers, after they have been processed in a first

work station, to a waiting station, and therefrom to a second work station.

2. The method according to claim 1 , wherein the transfer of wafers between

two adjacent work stations comprises individually transferring wafers from

a first work station to a wafer control apparatus, effecting therein the

control of the wafers, and transferring wafers approved by the control

apparatus to a second work station.

3. The method according to claim 2, further comprising transferring the

wafers that have not been approved by the control apparatus, to other

apparatus, which effects a further control thereof.

4. A buffer station for transferring individual wafers from a first processing

station to a second processing station, comprising:

a table; and,

a chuck positioned on said table and configured to support wafers.

5. The buffer station of claim 4, further comprising an inspection tool.

6. A semiconductor processing module, comprising: a first processing station

having plurality of chambers; a first interface connected to said first

processing station and designed to receive and support wafer pods; a first

track robot situated inside said first interface; a second processing station

having plurality of chambers; a second interface connected to said second

processing station and designed to receive and support wafer pods; a

second track robot situated inside said second interface; a buffer station

having a wafer support and configured to receive wafers from said first

track robot and store them until retrieval by said second track robot.

7. The semiconductor processing module of claim 6, wherein said buffer

station comprises one of a wafer inspection and metrology tools.