Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2000-03-03
2003-02-18
Lee, John R. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492100, C250S492220
Reexamination Certificate
active
06521900
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography methods and apparatus as used, for example, in manufacturing semiconductor integrated circuits and displays, wherein a pattern defined by a mask or reticle is transferred to a substrate such as a semiconductor wafer. More specifically, the invention pertains to alignment marks and alignment methods used for aligning the position of the substrate relative to, for example, the reticle or a reticle stage on which the reticle is mounted. The methods and apparatus can be used in optical microlithography and/or microlithography performed using a charged particle beam (e.g., electron beam).
BACKGROUND OF THE INVENTION
In microlithography systems, positional alignment between a reticle or reticle stage and a substrate (e.g., wafer) being processed is achieved by using alignment sensors to detect the respective locations of alignment marks provided on the stage and on the substrate. One type of conventional optical-based alignment sensor is a two-dimensional image sensor such as a charge-coupled device (“CCD”). Such a sensor can be used for sensing an alignment mark comprising lines formed in a periodic line/space (L/S) pattern on the substrate. Image processing can be performed to determine the location of the center of the reticle, for example.
In reduced-image type electron-beam microlithography, a “reduced” (demagnified) image of a reticle pattern is projected onto a substrate. Alignment sensors in such systems employ an electron beam to perform positional sensing of an alignment mark on the substrate. The alignment mark comprises lines arranged in a periodic array. The mark is sensed in a manner similar to positional sensing performed optically. An alignment-mark pattern, corresponding to the alignment-mark pattern defined on the substrate, is provided on the reticle. A reduced (demagnified) image of the alignment-mark pattern on the reticle is projected onto the substrate by the electron beam as the electron beam scans the corresponding mark pattern on the substrate. Impingement of the electron beam on the alignment mark on the substrate generates backscattered electrons. The backscattered electrons are detected by a backscattered-electron (BSE) detector that generates a corresponding electrical signal having a signal waveform that is processed to determine the location of the center of the alignment mark.
Further details of conventional alignment-sensing methods, as summarized above, using an electron beam are depicted in FIGS.
5
(
a
) and
5
(
b
) and described further below. In FIG.
5
(
a
), elements of an image (as formed on the substrate) of an alignment-mark pattern as defined on a reticle or reticle stage are denoted by the reference numeral
3
. The reference number
4
denotes features (“elements”) of a corresponding alignment mark defined (e.g., in a layer of heavy metal or the like) on the substrate or substrate stage. For simplicity, only two elements of the alignment-mark pattern are shown; however, it will be understood that an actual alignment mark on the reticle and the corresponding alignment mark on the substrate each comprise many elements. In FIG.
5
(
a
), the image of each element
3
(as formed on the substrate) has a width of 0.2 &mgr;m and a pitch (“period”) of 0.6 &mgr;m. These dimensions are the same as the corresponding dimensions of the elements
4
on the substrate.
As an alignment mark (defining the elements
3
) on the reticle is irradiated by an electron beam, images of the alignment-mark elements
3
are formed on the substrate. Meanwhile, the electron beam is deflected, using a deflector, as required to scan the images of the alignment-mark elements
3
across the elements
4
of the corresponding alignment mark on the substrate (scanning is performed in the horizontal direction in the figure). At moments during such scanning, overlaps occur (on the substrate) of the alignment-mark elements
4
with the images of the alignment-mark elements
3
. Backscattered or secondary electrons are emitted from regions of overlap. The number of backscattered electrons actually produced is generally proportional to the surface area of the overlap of the alignment-mark elements
4
with the alignment-mark elements
3
on the substrate. The corresponding signal waveform produced by a BSE detector is shown in FIG.
5
(
b
), representing a signal waveform produced when the mark on the reticle and the mark on the substrate each have five elements
3
,
4
, respectively.
By locating a point in the scan at which the output from the BSE detector is maximum (i.e., the center, or “center of gravity,” of the detector-output waveform), the corresponding point at which the elements
3
of the alignment-mark image are superposed on the elements
4
of the alignment mark on the substrate is determined. Meanwhile, the magnitude of beam deflection is monitored. From the magnitude of beam deflection at the point of superposition, the relative positions of the reticle and substrate can be determined.
In semiconductor-device fabrication, it is sometimes necessary to employ both electron-beam microlithography and optical microlithography, depending upon the type of device being fabricated. Some conventional electron-beam microlithography systems utilize optical alignment sensors (“optical-based sensors”; i.e., sensors employing light) in addition to alignment sensors that employ a charged particle beam (“CPB-based sensors”). Whenever semiconductor devices are being fabricated on a wafer, the position of a wafer with one type of layer may be detected more accurately using an optical-based sensor, whereas position of a wafer with another type of layer may be detected more accurately using a CPB-based sensor. Under such conditions, it is desirable to be able to perform alignments using the particular alignment sensor (optical-based or CPB-based) that will provide the more accurate measurement for the particular layer. It is also desirable to be able to detect the same alignment marks using both types of sensors.
In electron-beam microlithography apparatus that include both an optical-based sensor and a CPB-based sensor, the CPB-based sensor is situated inside the CPB “column” (vacuum chamber housing the array of lenses and deflectors of the CPB-optical system), but the optical-based sensor normally is situated outside the CPB column. With such a configuration, alignments performed using the optical-based sensor require that the distance between the respective centers of each of the two sensor systems be accurately known or measured. The most accurate way to measure the positions of the two sensors is to measure an alignment mark using the optical-based alignment sensor, measure the same mark using the CPB-based sensor, and then measure the distance traversed by the stage between the two measured positions. To perform such a measurement, however, an alignment mark is required that can be detected by both types of sensors.
With a line-and-space (L/S) alignment mark comprising elements with a period of approximately 6 &mgr;m, for example, it is possible for the same mark to be detected by both an optical-based alignment sensor and a CPB-based sensor. With mark-sensing methods that employ an electron beam, however, it is best to keep the range over which the mark is scanned by the electron beam (i.e., the beam-scan step) as small as possible. Limiting the beam-scan step improves detection accuracy and reduces possibly adverse effects of the electron beam on areas surrounding the mark. However, the array of elements of the alignment mark has a period of 6 &mgr;m, for example, a beam-scan step of approximately 6 &mgr;m normally is required. Such a large beam-scan step creates problems in that an excessively large number of data points is required to obtain the necessary detection accuracy. Also, an electron beam scanned over such a large area can exert various unwanted side effects on neighboring structure.
SUMMARY AND GENERAL ASPECTS OF THE INVENTION
In view of the shortcomings of the prior art summarized above,
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
Vanore David A.
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