This subject invention relates to a detector network for an optical metrology system in which the output of each photodetector is digitized at the detector and then collected by a centralized hub.
As geometries continue to shrink, manufacturers have increasingly turned to optical techniques such as ellipsometry and reflectometry to perform non-destructive inspection and analysis of semi-conductor wafers. Techniques of this type are commonly referred to as optical metrology and are based on the notion that a subject may be examined by analyzing the reflected energy that results when a probe beam is directed at the subject. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in magnitude are analyzed. Ellipsometry and reflectometry are effective methods for measuring a wide range of attributes including information about thickness, crystallinity, composition and refractive index. The structural details of ellipsometers are more fully described in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference.
Scatterometry is a related optical metrology technique that measures the diffraction (optical scattering) that results when a probe beam is directed at a subject. Scatterometry is an effective method for measuring the critical dimensions (CD) of structural features (such as the lines and other structures included in integrated circuits). Scatterometry can be used to analyze two periodic two-dimensional structures (e.g., line gratings) as well as periodic three-dimensional structures (e.g., patterns of vias or mesas in semiconductors). Scatterometry can also be used to perform overlay registration measurements. Overlay measurements attempt to measure the degree of alignment between successive lithographic mask layers.
As shown in
The use of multiple detectors creates multiple data streams each of which must typically be converted to digital form prior to analysis. For this reason, most optical metrology systems include an analog to digital converter (A/D converter) and a selector or multiplexer that is used to select between the different detectors. Unfortunately, the use of a single A/D converter multiplexed between multiple detectors has known drawbacks. One such drawback is possibility that errors may occur in the analog transmissions between the detectors and A/D converter. These transmissions typically take place over cable links between the detectors and the A/D converter. The cables are generally susceptible to electrical noise from a range of sources and must be carefully shielded to reduce the potential for errors. Even when shielded, errors may still occur and are often difficult to detect or repair.
A second drawback becomes apparent when the output of multiple detectors must be sampled simultaneously. When a single A/D converter is used, this type of sampling is performed serially, one detector at a time. This introduces a time-skew into the data generated for different detectors. This can reduce the accuracy of calculations that are based on the outputs of multiple detectors.
The present invention provides a detector network for optical metrology systems. For a representative implementation of the detector network, an optical metrology system is configured so that each photodetector has an associated A/D converter. Each A/D converter is connected to directly receive the output of the associated detector without the use of extensive cabling or any cabling at all. Each A/D converter is also connected to a centralized hub. The connections between the hub and the A/D converters are bi-directional. The hub uses these connections both for sending control signals to the A/D converters and for receiving the digital data generated by the A/D converters.
The output from the hub is provided to a digital data acquisition board that formats the data depending on which detector is generating the received output. The digital acquisition board then supplies the data to a processor for evaluation.
During normal operation, the hub selects one or more A/D converters for data collection. The hub instructs the selected A/D converters to return the digitized output of their associated detectors. This instruction may be synchronous, allowing the output of each detector to be sampled at the same instant. Asynchronous operation is also possible. After receiving instructions from the hub, each A/D converter samples the output of its associated detector and converts that output to digital form. The digital form is then forwarded to the hub, typically in serial fashion. The hub collects the various digital outputs and sends them to the data acquisition board. Each output is then formatted (when required) and then forwarded to the processor for analysis.
In this way, the detector network eliminates the time skew associated with traditional systems where a single A/D converter is multiplexed between detectors. Transmission errors are also eliminated since all data transmissions are performed digitally allowing any level of data integrity to be supported. Digital transmission is advantageous since it is not prone to noise interference, preserves full resolution, and with averaging, allows even greater resolution to be obtained.
As shown in
An analog to digital converter 204 is associated with each detector 202. A/D converters 204 may be selected from the complete range of commercially available or purpose built units of this type. A/D converters 204 may also have any appropriate resolution. Ideally, A/D converters 204 are selected so that their respective resolutions are equal to or greater than the desired resolution for the end measurement. Typically, this means that A/D converters 204 have at least a sixteen-bit resolution. Each A/D converter 204 is situated at or in close proximity to its associated detector 202. This minimizes the length of any cabling that is used to transmit the output of the detectors 202 to their associated A/D converters. In practice, is it generally sufficient if cables of this type are less than one meter in length. In the preferred embodiment, the detector and the A/D converter are located on the same printed circuit board assembly, thereby rendering the connection length near zero.
Each A/D converter 204 is connected to a hub 206. This interconnection may be accomplished using a range of different communication technologies. For the particular implementation shown in
Hub 206 functions as a communications locus for network 200. Commands for A/D converters 204 are distributed by hub 206. Hub 206 also receives the output of A/D converters 204. To prevent data loss, hub 206 buffers the data received from A/D converters 204. Hub 206 may also buffer commands for A/D converters 204.
Hub 206 is preferably configured to support synchronized operation of A/D converters 204. This allows each of the A/D converters 204 to sampled at the same instant-eliminating time skew between measurements taken at different detectors 202. In cases where a point-to-point interconnections are used between hub 206 and A/D converters 204, synchronized operation is implemented by simultaneously sending the same command to A/D converters 204 and sequentially reading the output of A/D converters 204 following simultaneous conversion. In cases where a daisy-chain topology, it may be assumed that each A/D converter 204 receives the command at the same instant (since the actual time of propagation for any command will be extremely small). The output of the A/D converters 204 must be read sequentially in a way that preserves the integrity of each detector's data. The hub receives data and synch signals from all measurement light detectors and multiplexes them into serial data and synch signals. In the case of multiplex signals from a common detector or CCD array, the data is assembled into 16 or 32 bits depending on the mode of data collection. The hub board provides the signal and timing to each of the remote detectors and A/D converters and controls their sequential readout.
Hub 206 is connected to a digital acquisition board 208. The hub communicates with the data acquisition board through a combination of serial data, clock and control signals. Digital acquisition board 208 acts as an interface between hub 206 and a processor 210. In most cases, this means that digital acquisition board 208 is an ISA, PCI or PCMCIA card. Other card, board or bus technologies may be used as appropriate. To prevent data loss, the data acquisition board buffers the receive data using a sufficient number of FIFO buffers. Processor 210 analyzes the data received from digital acquisition board 208. Processor 210 also sends commands to digital acquisition board 208 for distribution by hub 206 to control operation of A/D converters 204.
As shown in
In step 304, hub 206 relays the command to A/D converters 204. The A/D converters 204 then perform the requested command (see step 306). In this case, this means that A/D converters 204 sample the output of detectors 202 and produce corresponding digitally encoded outputs. A/D converters 204 receive the sample command at the same instant and perform their sampling operations in unison. In this way, time skew between detectors is minimized. A/D converters 204 then send their digital outputs to hub 206 (see step 308).
Hub 206 assembles the digitally encoded outputs received from the A/D converters 204 (see step 310) and sends assembled results back digital acquisition board 208 and processor 210 (see step 312). In step 314, processor 210 uses the command results to as pan of its metrology analysis.
In this way, detector network 200 eliminates the time skew associated with traditional systems where a single A/D converter is multiplexed between detectors. Transmission errors are also eliminated since all data transmissions are performed digitally allowing any level of data integrity to be supported.
In addition to sampling, detector network 200 may be used to perform a range other functions, such as control, configuration and calibration of detectors 202 and A/D converters 204. Detector network 200 may also be used to provide remote access to detectors 202 for external systems. It should also be appreciated that detector network 200 may be used with a range of sensors, detectors and devices and is not limited to the specific example of photo-detectors. For example, detectors can include scanned photodiodes and/or CCD arrays or photomultiplier tubes.
While the subject invention has been described with reference to a preferred embodiment, various changes and modifications could be made therein, by one skilled in the art, without varying from the scope and spirit of the subject invention as defined by the appended claims
The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/336,847, filed Nov. 1, 2001, which is incorporated herein by reference.
Number | Name | Date | Kind |
---|---|---|---|
5644141 | Hooker et al. | Jul 1997 | A |
6405591 | Colarelli et al. | Jun 2002 | B1 |
6546785 | Discenzo | Apr 2003 | B1 |
Number | Date | Country | |
---|---|---|---|
60336847 | Nov 2001 | US |