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DYNAMIC METROLOGY SCHEMES AND
SAMPLING SCHEMES FOR ADVANCED
PROCESS CONTROL IN SEMICONDUCTOR
CROSS-REFERENCES TO RELATED
This application claims the benefit of U.S. Provisional Application Ser. No. 60/322,459, filed Sep. 17,2001, which is 10 expressly incorporated herein by reference; and U.S. Provisional Application Ser. No. 60/298,878, filed Jun. 19, 2001, which is expressly incorporated herein by reference.
BACKGROUND OF THE INVENTION 15
1. Field of the Invention
The present invention concerns computer-related and/or assisted methods, systems and computer readable mediums for metrology during process control. More specifically, it 20 relates to dynamic adjustment of metrology schemes and sampling during advanced process control methods, for example during control of semiconductor technology manufacture.
2. Related Art 25 In the wafer fabrication art, measurements are made by
metrology tools on wafers as they are being manufactured by processing devices, in order to ensure that the wafers are produced according to a predefined specification. The measurements are made of physical properties such as film thick- 30 ness and uniformity, dopant concentration, gate length and critical dimension. This is known as the science of "metrology"
Measurements to be made are typically specified in a "die map". The die map indicates where the different chips (or die) 35 are located on a wafer (in the typical situation where multiple chips are formed on and eventually cut from a single wafer), as well as significant locations, such as corners, on each die. In order to measure the right hand corner on each die, for example, multiple points are measured on the wafer in accor- 40 dance with the die map. Ordinarily a die map is a digital representation of coordinate points, or "metrology coordinates," on the wafer.
The metrology coordinates are usually provided by an engineer, and vary depending on the engineer's preferences. 45 Metrology coordinates are conventionally provided as x, y coordinates.
A "sampling plan," alternatively referred to as a "metrology plan," contains metrology coordinates drawn from the die map. The sampling plan denotes a specific plan for taking 50 certain measurements. These measurements may include some or all of the possible points and/or chips in the die map.
A conventional metrology system assigns a sampling plan that predetermines which wafers are to be measured in connection with a processing device, and the measurements 55 which are to be taken of those wafers by the metrology tool. For example, the sampling plan might define that each fifth wafer should be measured at pre-designated locations. These sampling plans are not changed after being initially assigned, and hence the metrology systems are static. 60
Unfortunately, manufacturing results tend to drift away from the intended target or specification when there is a change in the manufacturing process, such as a change in recipe, preventative maintenance, consumables change, environmental change or a new lot of wafers. Conventional 65 metrology systems tend to miss some wafers which are outside specification limits, since these systems use a virtually
consistent measurement scheme, having consistently frequent measurements with consistent spatial resolution, without taking into consideration whether any changes were introduced into the manufacturing process which might change the manufacturing results.
Manufacturing systems do not typically call for a measurement of every wafer, since measuring takes time and increasing the number of measurements results in a decrease of productivity. On the other hand, measuring fewer wafers tends to lead to delayed detection of critical information for process control that may significantly impact wafer yield. While conventional sampling systems will sample wafers during and/or after production, these systems do not adjust the initially assigned sampling plan for the wafers during production.
Thus, there remains a need for dynamic metrology to improve the quality of products. For semiconductor wafers, there remains a need to better check whether each specification is met under production conditions. There also remains a need to respond to a change in parameters which may cause a variance from intended target results, such as recipe parameters, and to adjust the frequency and/or spatial resolution of measurements. Unfortunately, taking measurements takes time, and most processing devices are faster than the measurements that need to be taken by metrology tools in order to characterize the wafers using a metrology. Thus, there remains a need for a method, system and medium to react to changes potentially affecting the system results, and to appropriately adjust, increase, or decrease the measurements accordingly.
SUMMARY OF THE INVENTION
The present invention alleviates the problems of the conventional techniques described above by dynamically determining whether a wafer needs to be measured for process control based on changes in the resources, recipes, etc. In addition, for a given wafer to be measured, measuring points are also dynamically assigned to the metrology tool.
More specifically, two variations of embodiments of the present invention are contemplated and may be used independently or together. According to the first variation, the frequency at which wafers are measured ("wafer-to-wafer") is adjusted, following an event that suggests that more (or fewer) wafers should be measured. According to the second variation, the spatial resolution of the measurements of those wafers selected for measurement ("within-wafer") is increased or decreased, following an event that suggests each wafer which is measured should be measured in greater (or lesser) detail.
In one or more embodiments of the present invention, candidate coordinate measurement points are mapped in a die map, and a subset of the candidate coordinate measurement points are selected as the initial points where measurements are to be made. Subsequently, according to the within-wafer variation, the invention dynamically selects more, fewer or different points (depending on the circumstances) to be measured from among the candidate coordinate measurement points. According to the wafer-to-wafer variation, when there is a change in the manufacturing process, the number of measurements may be increased, to measure every wafer rather than just every third wafer for example. As one example, when a new recipe is implemented to significantly change the thickness at a particular region on the wafer, a greater number of within-wafer measurements can be made at that location by selecting more and/or different candidate measurement points. As another example, when a fault is
detected, the frequency of wafers selected for measurement is increased; this increases the probability of detecting defectively manufactured wafers and correcting the control parameters (such as in connection with a feed forward/feedback method). In some situations, large deviations may require less 5 frequent measurement or less spatial resolution than small deviations when the large deviations clearly identify the problem, whereas small deviations may be difficult to identify and more frequent and/or dense measurements may be necessary. The reverse may be appropriate in other situations regarding 10 the frequency and density of measurements, or it may be the case that the same number of measurements may be taken regardless of deviation.
According to one or more embodiments of the present invention, there is provided a method, system and/or com- 15 puter-implemented method for measuring at least one manufacturing characteristic for at least one product manufactured by a manufacturing process. Information is provided, representative of a set of candidate points to be measured by the manufacturing process on the at least one product. The manu- 20 factoring process executes a plan for performing measurements on the at least one product to measure the at least one manufacturing characteristic, the plan defining the measurements to be made responsive to the set of candidate points. A change in the manufacturing process is detected, the change 25 including at least one of: receiving new material in the manufacturing process, detecting a fault in the manufacturing process, detecting a change in a control parameter in the manufacturing process, and detecting a variation in a measurement of the at least one product. 30
According to one or more embodiments, the plan for performing measurements is adjusted based on the detected change and at least one additional measurement is performed responsive thereto. 35
According to one or more embodiments, the measurements of the plan are adjusted wafer-to-wafer and/or within-wafer.
According to one or more embodiments, the product is a semi-conductor wafer and the manufacturing process is an automated semi-conductor manufacturing process. 40
According to one or more embodiments, the plan further includes information representative of a metrology recipe.
According to one or more embodiments, the candidate points are included in a map corresponding to the at least one product. The plan may be a pre-determined sampling plan. 45
According to one or more embodiments, the plan defines at least one region on the product, each of the candidate points corresponding to the at least one region.
According to one or more embodiments, the adjustment 50 includes determining the at least one region corresponding to the detected change, selecting the at least one additional measurement responsive to the candidate points corresponding to the determined region, assigning the selected at least one additional measurement to be performed under the plan, 55 and revising at least one of the measurements, the additional measurement and the plan. The adjustment may include determining whether the detected change may affect a series of products, and if so, determining whether to measure at least one of the products in the series of products. The products 60 may be provided in a group, and the plan may further include first information representative of the products in the group that are available to be measured, and second information representative of the products in the group that are to be measured under the plan. 65
According to one or more embodiments, information representative of measurement results on the product is dis
carded when the measurements results indicate a variation in measurement of the product and/or when a fault is detected in the manufacturing process.
According to one or more embodiments, the sampling plan includes a plurality of splines radiating from a center of a product, and the candidate points are distributed along the splines. The distribution of the candidate points along the splines may be weighted according to a surface area of the product. According to one or more other embodiments, the sampling plan includes a plurality of radially distributed candidate points.
BRIEF DESCRIPTION OF THE FIGURES
The above mentioned and other advantages and features of the present invention will become more readily apparent from the following detailed description in the accompanying drawings, in which:
FIG. 1 is a flow chart showing one example of dynamic metrology for "wafer-to-wafer" processing in the present invention.
FIGS. 2A and 2B are an illustration of regions on a wafer, with FIG. 2A being a plan view of the wafer and FIG. 2B being a cross-section of the wafer along radius B-B of FIG. 2A.
FIG. 3 is a flow chart showing one example of dynamic metrology for "within-wafer" processing in accordance with one or more embodiments of the present invention.
FIGS. 4A and 4B are a spiral sampling plan for a wafer for use with one or more embodiments of the invention, with FIG. 4A being a plan view of the wafer and FIG. 4B being a cross-section of the wafer along a radius of FIG. 4A.
FIG. 5 is an example of another sampling plan for use with one or more embodiments of the invention.
FIG. 6 is a block diagram of a computerized process control system which may be used in connection with one or more embodiments of the present invention.
The following detailed description includes many specific details. The inclusion of such details is for the purpose of illustration only and should not be understood to limit the invention. Throughout this discussion, similar elements are referred to by similar numbers in the various figures for ease of reference. In addition, features in one embodiment may be combined with features in other embodiments of the invention.
In one or more embodiments of the present invention, static metrology means there is a pre-determined sampling plan in connection with a wafer (or other device) to be measured, specifying substantially the same points for each wafer (or the other device). In contrast, a dynamic metrology plan utilizes an initial sampling plan and adjusts the sampling responsive to certain events or non-events. As an example of an adjustment due to a non-event, if the last ten wafers measured are all the same, and if the processing device did not change, and if the recipe on the processing device did not change, one could reasonably assume that the next series of wafers will have measurements that are also all the same. That being the case, then in order to increase throughput and decrease the time it takes to do measurements, the invention provides for dynamically adjusting the measurements, for example, such that every third wafer instead of every wafer is measured. This invention thus detects and adjusts for not only potential errors, which could arise for example upon a recipe change, but also for accuracy.
One or more embodiments of the present invention contemplate that the invention may be used in connection with wafer-to-wafer measurements described above, as well as, or alternatively, in connection with within-wafer measurements. Consider an example of within-wafer measurements, in 5 which measurements are taken along a radius of a 200 mm diameter wafer and the radius is measured in 10 mm increments. During processing it is noted or detected by the usual detection process that there is a large variation at the 50 mm and 60 mm points. For the next sample, the system adjusts to 10 measure another point from the sampling plan between 50 mm and 60 mm to better characterize that variation, or optionally to measure an additional point, for example, between 40 mm and 50 mm that is near the location of the variation. If the die map includes points at 45 mm and 55 mm, these points can 15 then be added as measurement points. Adjusted measurements now encompass in this example, 40 mm, 45 mm, 50 mm, 55 mm, and 60 mm. The system dynamically added the two additional points (in the example) to better characterize the measurement and/or the variation. Where there are pro- 20 vided a number of candidate points in the die map allowing points to be added or substituted, the system can select among the points any of several ways, such as selecting the closest to mean, mode, other statistical analysis, etc.
A sampling plan provides specific measure points within a 25 die, a die being the section on the wafer that will typically eventually become a single chip after processing. There are specified points within the die that are candidates for measuring. The map of the die is stored, preferably in an electronic format representing the map. One appropriate place for storing the die map information is in the factory automation system ("MES" or manufacturing execution system). The stored die map information may be advantageously retrieved and translated to determine the available points for measurement on the wafer. Referring back to the previous example proposing measurement points on the radius at 45 mm and 55 mm, if these specific points are not relevant to the current die (e.g., they are not specified by the die map), an appropriate replacement would be points selected from the candidate points specified by the die map which are close to or between 45 mm and 55 mm. Those points could be selected dynamically as well. Other criteria may be used for selecting points as well.
Dynamic metrology is performed to better meet a certain 45 specification. For example, if recipe parameters are changed on the processing device, to adjust the thickness of a film that is deposited on the wafer, it may be desirable to more closely check whether the specification is still being achieved by performing measurements. 50
In order to avoid slowing down the process, one or more embodiments of the present invention advantageously determine the appropriateness of performing additional measurements when one or more events occur that are likely to indicate an internal or external change affecting the 55 manufacturing process or results. The increase in measurements and possible corresponding decrease in processing occur on an as-needed basis and/or based on predetermined criteria.
The wafer-to-wafer variation of the invention, for example, 60 can check for events which may affect a series of wafers and may adjust the sampling plan. For example, during processing, the system determines if an increase is needed in the frequency of wafers measured for process control, for example, based on 1) a change in the processing device the 65 wafers are processed on, 2) a change in the parameters or recipe that were used by the processing device to process the
wafer, 3) large detected variations or errors in measurements, and/or 4) a significant run of wafers without errors.
Particularly regarding within-wafer variation, one or more embodiments of the present invention contemplate that the system obtains a stored die map with metrology coordinate information from the MES. As indicated, the system can provide not only for assigning the measurement points optionally dynamically, but also for de-assigning.
One or more embodiments of the present invention envision changing the sampling plan using information that is gathered from the MES and automatically using that new sampling plan, depending on, for example the type of processing device on which the wafers are processed. Advantageously, the system has stored information about a wafer that indicates, among other things, the type of chip or type of device and an associated sampling plan to be used when measuring a wafer containing a specific device. Based on the type of device, the associated sampling plan or die map can be obtained, where the die map includes a set of candidate metrology points. The system then selects metrology points for the current wafer from the set of, or responsive to, the candidate points in the die map.
With respect to the sampling plan, generation of the sampling plan can vary from device to device (chip type to chip type) and some measurements may be based on die distribution on the wafer. By dividing a wafer into regions and using regions of the wafer for measurement, one or more embodiments of the present invention provide flexibility in selecting one or more points from available points in the region. Use of regions is one way to provide a pool of candidate points, from which the system may select points that are most relevant to the desired information about the film on the wafer.
In practice, the system may, for example, measure twentytwo to twenty-five points per wafer from the pool of candidate points. For some processes the system might measure fewer points, such as eight points, because it takes longer to measure those points or the wafer processing time is faster. For other processes the system might measure one point of another type of property, such dopant concentration, which is a relatively slow measurement.
In any event, it is important to balance the time consumed in a measurement against the need to produce quality products. Manufacturers consider it to be more important to be within specifications and not produce defective product, than to rapidly produce product of suspect qualities.
Each processing device on which a wafer is processed has a different processing time, and therefore the selected standard sampling rate may depend on the speed of processing of the processing device and metrology tool. On some processing devices, measurements on every wafer will not slow down processing since the speed of the processing device is slower than the measurements by the metrology tool. For example, polishing and cleaning processing devices may consume five minutes or more to process a wafer. In that case a postprocessing measurement by the metrology tool on every wafer would often not reduce throughput.
Additionally, the system may determine whether or not to make additional measurements based on the initial and the final condition of the wafers. For example, if there is a situation in which the incoming thickness profile of a cross section of a wafer does not change very much, the system may reduce the frequency of samples of incoming profiles, wafer-to-wafer. On the other hand, if the incoming profile is changing significantly, it may be desirable to measure every entering wafer.
Reference is made to FIG. 1, illustrating an example of a flow chart for one or more embodiments of a wafer-to-wafer