Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Having substrate registration feature
Reexamination Certificate
2000-02-25
2002-08-06
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Having substrate registration feature
C438S004000
Reexamination Certificate
active
06429090
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography apparatus and methods using a charged particle beam (e.g., electron beam or ion beam) as an energy beam for performing transfer-exposure of a pattern from a mask or reticle to a sensitive substrate. Such methods and apparatus are used, for example, in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to alignment and fiducial marks used in such methods and apparatus, wherein a first such mark is situated as an index mark on a first surface (e.g., reticle surface) and a second such mark is situated as a fiducial (reference) mark on a second surface (e.g., sensitive substrate such as a wafer or the like), and an alignment of the two marks is performed to achieve optimal positioning of the two marks relative to each other.
BACKGROUND OF THE INVENTION
One type of conventional charged-particle-beam (CPB) microlithography (projection-exposure) apparatus utilizes an electron beam to irradiate a pattern defined on a reticle. Electrons of the beam passing through the irradiated region of the reticle are projected and focused onto a sensitized substrate (e.g., semiconductor wafer), thereby “transferring” the pattern from the reticle to the wafer. The reticle is mounted on a reticle stage and the wafer is mounted on a wafer stage. For accurate projection-exposure of the pattern from the reticle to a particular region on the wafer, it is necessary to align accurately the wafer with the substrate. To such end, at least one “alignment mark” (also termed an “index mark”) is provided on the reticle or reticle stage, and at least one “fiducial” (reference) mark is provided on the wafer or wafer stage. In a procedure for aligning the wafer with the reticle, the index mark is aligned with the fiducial mark.
More specifically, in a representative conventional method for performing alignments as summarized above, the electron beam is caused to illuminate the index mark on the reticle or reticle stage. Electrons of the beam passing through the index mark are directed as a scanning beam to the fiducial mark on the wafer or wafer stage. Electrons of the beam that are backscattered from the fiducial mark are detected using an appropriate detector, and the relative positional relationship of the projected image of the index mark to the fiducial mark is determined. Based on the determination, the reticle and wafer are aligned as required. Such determinations also can provide data on distortion of the beam, preliminary to making appropriate corrections to the beam.
To obtain accurate determinations as summarized above, it is necessary that the location of the index mark be known accurately relative to, for example, detected positional coordinates of the reticle stage. Similarly, it is necessary that the location of the fiducial mark be accurately known relative to the detected positional coordinates of the wafer stage. Therefore, and in view of the fact that the marks are irradiated by the beam, it is important that the marks be defined on a material having as low a coefficient of thermal expansion as possible so as to undergo minimal thermal deformation when irradiated.
FIG. 1
is a schematic sectional view of a conventional fiducial mark
10
body. A fiducial mark typically is produced by forming a layer of a heavy metal such as Ta or W on the surface of a substrate
11
(made of Si or other suitable material). The elements
12
of the fiducial mark are formed by etching the heavy-metal layer appropriately.
The fiducial mark body
10
of
FIG. 1
normally is situated on the wafer or wafer stage and used in conjunction with a corresponding index mark on the reticle or reticle stage. The pattern of the index mark normally is similar to the pattern of the fiducial mark
10
. An electron beam irradiates the index mark such that an image of the alignment mark is formed on or near the fiducial mark
10
. As the electron beam scans the image of the index mark over the fiducial mark, the relative positions of the marks are determined from an electrical signal produced by a detector of electrons that are backscattered (“backscattered electrons” or BSEs) from the fiducial mark. Based on the signal, an upstream deflector can be energized appropriately to deflect the beam to achieve maximal coincidence of the marks.
In charged-particle-beam (CPB) microlithography, alignment of an index mark with a corresponding fiducial mark can be performed using either an optical-based alignment sensor (i.e., a sensor utilizing light) or a CPB-based alignment sensor (i.e., a sensor sensitive to charged particles such as BSEs from the beam). Especially whenever an optical-based alignment sensor is used, it is necessary to determine, as a calibrated “baseline,” the distance between a reference (fiducial) location of the CPB-optical system and a reference (fiducial) location of the optical-based alignment sensor. Since the optical-based alignment sensor is situated usually outside the “column” (vacuum housing) of the CPB-optical system, the distance typically is substantial.
Measurements of distances between fiducial locations can be affected adversely by apparatus vibrations. To eliminate such vibrations, it is necessary to measure simultaneously the reference location of the CPB-optical system and the reference location of the optical-based alignment sensor. The fiducial mark used in baseline measurements should have a length that is at least equal to the baseline length. That is, when an optical-based alignment sensor is used, it is important to measure a “baseline” (distance between the optical axis of the charged particle beam and the optical axis of the optical system of the alignment sensor). To such end, fiducial marks are used that can be measured at the same time. Such marks should extend over the baseline or, if spaced apart from each other at the baseline, overlap each other. However, such marks can be adversely affected easily.
Also, positional stability of the fiducial marks to changes in temperature is very important. By way of example, if the distance between the reference location of the CPB-optical system and the reference location of the optical-based alignment sensor is 20 mm, and the apparatus temperature is controlled to within ±0.5° C., the coefficient of thermal expansion of the substrate on which the fiducial mark is formed should be 1×10
−7
/°C. or less to suppress variations in measured distance between the respective fiducial marks adequately to within 1 nm or less. The coefficient of thermal expansion of Si as currently used as a substrate for fiducial marks is about 2.4×10
−5
/°C., which is unsatisfactorily high for use in obtaining accurate measurement of the distance between fiducial locations.
It has been proposed to manufacture the substrate for a fiducial mark body using a substance having a low coefficient of thermal expansion, such as ZERODUR made by Schott of Germany. However, because ZERODUR is not electrically conductive, an undesirable electrical charge tends to accumulate on it whenever it is irradiated by an electron beam. The accumulated charge forms a corresponding electrical field around the fiducial mark, which can perturb the beam incident on the mark. If substrate charging is excessive, an electrical discharge may occur which can destroy the fiducial mark.
Providing ZERODUR with a conductive metal coating has been proposed to prevent or at least reduce charge accumulation on the substrate. However, such a coating tends to reduce the contrast of the BSE signal.
Therefore, methods are required for preventing charge accumulation on a fiducial mark without adversely reducing contrast and while maintaining the low thermal-expansion characteristic of the fiducial-mark substrate.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, fiducial mark bodies that can be used for any of various applications in charged-particle-beam (CPB) microlithography.
Fujiwara Tomoharu
Hirayanagi Noriyuki
Dang Phuc T.
Klarquist & Sparkman, LLP
Nelms David
Nikon Corporation
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