Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1998-02-27
2003-06-10
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C355S053000, C700S121000, C250S234000
Reexamination Certificate
active
06576919
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to control of movement of a wafer stage in one-shot exposure type exposure apparatus (including step-and-repeat type steppers), control of movement of the wafer stage and a reticle stage in exposure apparatus for performing the exposure operation in the reticle scanning method (including step-and-scan type steppers), and control of movement of the wafer stage and a sensor stage in wafer inspecting apparatus for performing inspection in the sensor scanning method and, more particularly, to a method of minimizing the overall turnaround time of stage movement for sequential exposure processes or sequential inspection processes in these apparatus.
2. Related Background Art
For carrying out processes such as exposures or inspections in a wafer provided with plural chip areas, higher production efficiency or inspection efficiency thereof is preferred. Particularly, focusing attention on movement of the stage upon change of chip areas being objects in the predetermined process such as exposure or inspection, the overall turnaround time necessary for the movement of stage through the all chip areas on the wafer needs to be shortened as much as possible. For that, the order of exposures or inspections or the like in the n (n>1) chip areas (i.e., a visit order of the chip areas) needs to be optimized so as to minimize the overall turnaround time necessary for the movement of stage.
For example, supposing that in a one-shot exposure type stepper there are n chip areas to be exposed on the wafer, a conceivable number of movements of the stage between the chip areas is at most
n
P
2
=n(n−1) (even under the condition that the turnaround time differs depending upon whether the direction of movement of the stage is positive or negative). Accordingly, once the exposure order or the inspection order of the all chip areas is determined, the overall turnaround time necessary for the movement of stage can be obtained uniquely and in short time. However, since there exists n
1
ways as to the orders for carrying out the exposures, inspections, or the like of the n chip areas, inordinate time is required for producing and checking the all conceivable orders and for computing all applicable solutions. Particularly, when n>13, it is practically impossible (“Practical course: Invitation to traveling-salesman problems I, II, III,” Operations Research 39 (1994), No. 1: pp 25-31, No. 2: pp 91-96, No. 3: pp 156-162).
Further, in the case of the apparatus for carrying out scanning exposure or scanning inspection, typified by the scan exposure type steppers, the reticle stage (or a sensor stage) and the wafer stage need to be controlled in synchronism upon carrying out the exposure, inspection, or the like for each shot area (equivalent to the chip area) on the wafer, and there are degrees of freedom as to scan directions in each shot area. Therefore, the exposure order (exposure sequence) for the all shot areas and the scan directions of local areas (for example, areas successively becoming exposure objects) in the respective shot areas must be optimized simultaneously. The number of conceivable exposure sequences or inspection sequences (either of which is included in the movement sequence) is given by the product of the number of combinations with the scan directions of local areas in the respective n shot areas (if the degrees of freedom of the scan directions are m, the number of combinations is re) and the number of permutation of exposures (or inspections) of the n shot areas (n!), i.e., m
n
×n!. It is thus more difficult to obtain an optimum solution of movement sequence than in the case of the one-shot exposure type steppers.
In the conventional apparatus described above, therefore, optimum simultaneous control sequences of the wafer stage and reticle stage to minimize the turnaround time of exposure sequence under specific conditions anticipated are preliminarily set in order to shorten the time for successive exposures of plural areas on the wafer within practical computation time. When an actual operation condition does not suit the above specific conditions, only an unfit portion of the optimum simultaneous control sequences preset is modified so as to fit the above specific conditions. Accordingly, recomputation of optimum movement sequences is not carried out each time in the practical operations.
SUMMARY OF THE INVENTION
The inventor examined the conventional technology described above and found the following problems.
First, the conventional apparatus such as the one-shot exposure type steppers cannot obtain an optimum or near-optimum solution to a permutational optimization problem within short time. An ideally preferred way is such that movement sequences to indicate orders of exposures, inspections, or the like in the chip areas are generated and examined and among them a movement sequence having the shortest time is determined as a solution of movement sequence to be found. However, studying what order should be employed for the exposures, inspections, or the like in the n chip areas provided on the wafer, even the one-shot exposure type steppers require examination of n! movement sequences for only exposure orders or inspection orders of n chip areas. In particular, when n>13, inordinate time is consumed for the computation of solution so that it practically seems impossible to obtain the solution. Therefore, for increasing the throughput, effective generation of optimum or near-optimum solution is necessary as to the exposure order or inspection order.
Second, the conventional apparatus such as the steppers of the scan exposure type requires more considerable time for obtaining the optimum or near-optimum solution to the composite problem of the permutational optimization problem with the combinatorial optimization problem. Since the apparatus for carrying out the scanning exposure or scanning inspection, typified by the scan exposure type steppers, has the degrees of freedom as to the exposure order or the inspection order of each chip area on the wafer and as to the scan directions of local areas in the respective chip areas, simultaneous optimization of these must also be considered. Since the optimization of exposure sequence and the optimization of scan direction on the wafer are correlated with each other, they cannot be so managed that the optimization of scan direction is carried out after the optimum solution of exposure sequence has been obtained. Conversely, they cannot be so managed that the optimization of exposure sequence is carried out after the optimum solution of scan direction has been obtained, either. In this case, the number of combinations of permissible scan directions for the all n chip areas is the n-th power of the degrees of freedom of scan directions: m (e.g., m=2 in the scan exposure type steppers); concerning movement sequences of two stages taking account of both movement of the wafer stage and movement of the reticle stage or the sensor stage (scanning of local area in each chip area), the number of conceivable movement sequences is m
n
×n!. It is thus practically impossible to perform generation of the all movement sequences or to perform all probable inspections. In order to increase the throughput, efficient generation is also necessary for the optimum or near-optimum solution of exposure sequence.
Third, the above problems must be solved even if the user gives an instruction of an arbitrary constraint in practical operations. Examples of the constraint given by the user include such constraints that no exposure should be effected in one shot area or two or more shot areas (process object regions) selected by the user, that the scan direction in one shot area or two or more shot areas selected by the user should follow the instruction given by the user, and so on. In practical operations the above problems must be solved even under the operation conditions to which the arbitrary constraint is added, as des
Le Que T.
Luu Thanh X.
Nikon Corporation
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