(1) Field of the Invention
The present invention relates to a substrate transport apparatus for use with a substrate processing system for performing varied treatments (e.g. film-forming, prebaking, exposure, development and postbaking) in a photolithographic process of substrates such as semiconductor wafers or glass substrates for liquid crystal displays. Specifically, the invention relates to a substrate transport method and apparatus for transferring substrates through a buffer section having a plurality of storages for temporarily storing the substrates, between a processing unit for performing varied treatments before and after an exposing process and an exposure unit for performing the exposing process.
(2) Description of the Related Art
A conventional substrate transport apparatus of the type noted above transfers substrates between the processing unit and exposure unit through a buffer section having a plurality (e.g. 50 stages) of storages for temporarily storing the substrates. This apparatus includes a first substrate transport device for transporting the substrates between the processing unit and buffer section, and a second substrate transport device for transporting the substrates between the buffer section and exposure unit.
This apparatus operates as follows, for example.
In the buffer section, the first to 25th storages counted from the bottom act as a feed buffer, and the 26th to 50th storages as a return buffer. The feed buffer stores unexposed substrates to be transported to the exposure unit. The return buffer stores exposed substrates transported from the exposure unit.
A first unexposed substrate having a film formed thereon and prebaked in the processing unit is received by the first transport device and deposited on the first (lowermost) storage of the feed buffer. Next, a second substrate is received by the first transport device and deposited on the lowermost one of the vacant storages (i.e. the second storage) of the feed buffer. In this way, each succeeding substrate treated in the processing unit is deposited on a next lower storage of the feed buffer.
The second transport device transfers the substrates, successively in the order of deposition, from the feed buffer to the exposure unit. Of the first and second substrates deposited as above, for example, the first substrate is first transferred to the exposure unit.
The first substrate exposed in the exposure unit is received by the second transport device, and deposited in the return buffer (e.g. the 26th and lowermost storage in the return buffer) of the buffer section. Next, the second substrate is deposited on the lowermost one of the vacant storages (e.g. the 27th storage immediately above the 26th storage). In this way, each succeeding substrate exposed in the exposure unit is deposited on a next lower storage of the return buffer.
The first transport device receives the substrate deposited first in the return buffer if there are substrates therein, and transfers it to the processing unit. The substrate transferred to the processing unit is developed therein.
The above substrate transporting mode absorbs a difference in throughput between the processing unit and exposure unit.
The conventional apparatus with the above construction has the following drawback.
When one of the first and second transport devices transports a substrate to or from the buffer section, the other substrate transport device cannot transport a substrate to or from not only the same storage but the two storages adjacent that storage. This is because the storages are arranged close to one another, and the two transport devices could interfere with each other in their vertical movements. Thus, the transport operation may be retarded. A waiting time is caused with the first or second transport device to lower throughput.
To avoid the above inconvenience, it is conceivable to arrange the storages at increased intervals. Then, naturally, only a reduced number of substrates could be stored, and the buffer section would have an increased height, requiring the transport devices to be movable to an increased height.