Electricity: motive power systems – Positional servo systems – Multiple mode systems
Reexamination Certificate
1999-02-11
2001-01-30
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
Multiple mode systems
C318S685000, C310S317000
Reexamination Certificate
active
06181097
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a high precision three-dimensional alignment systems using scanning probe microscopy (SPM) techniques. In particular, the present invention is related to a three-dimensional alignment system for lithography and fabrication. The present invention also relates to a high precision alignment system for SPM inspection.
BACKGROUND OF THE INVENTION
Since the advent of scanning tunneling microscopy and atomic force microscopy, intensive research interest in the field of nanometer and submicrometer length scale surface modifications utilizing the above-mentioned techniques has been expressed. These fabrications usually involve controlled modifications of the surface morphologies which are critical and important for the fabrication of miniaturized micromechanical and electronic devices with nanometer scale dimensions. It becomes more significant when one considers the limitations encountered by the photolithography systems as device dimensions are constantly being scaled down to the submicron regime. The lack of efficient light source past the ArF laser line at 193 nm and the unavailability of good optical materials may limit prompt application of such photolithography systems. Although other techniques like the e-beam and X-ray lithography may serve as attractive alternatives, precise optimization of process parameters and technical-related issues may further hinder their participation.
Attempts to achieve small features using nanolithography systems have been described. For example, U.S. Pat. Nos. 5,517,280 and 5,666,190, disclose the use of a scribing tool (in this case a photolithographic wafer) with an array of cantilevers and tips to create features in a fabrication wafer. Waveguides extend longitudinally along the bottom surface of the cantilevers, and an aperture in the waveguide is placed at the apex of a tip on the end of each cantilever. The photolithographic wafer is brought into close proximity with the fabrication wafer such that a gap separates the tip from the surface of the fabrication wafer. A piezoresistor coupled to each cantilever allows the resonance frequency of the cantilever to vary according to the size of the gap. In order to obtain a uniformly suitable gap between every tip and the fabrication wafer i.e. to get a uniform tip height, a capacitive plate is provided above the cantilever. A DC electric field between the capacitive plate and the fabrication wafer causes the thicker portion of the cantilever to flex. The cantilever continues to flex until the gap is adjusted to the proper level, indicating the correct spacing for proper exposure of photoresist on the surface of wafer. Although this photolithography method allows a plurality of cantilevers to operate within the same photolithographic wafer, there are the following limitations to the method. Firstly, the fabrication of the cantilevers and waveguides is a complex process and therefore expensive. In addition, due to the complex fabrication process, the resulting cantilevers and tips within the same wafer are often not uniform, resulting in difficulties in obtaining uniform alignment of tip heights. This problem is partially solved by providing the capacitive plates to allow for individual adjustment of each cantilever. However, this means that numerous electrical connections and capacitive plates have to be provided along the surface of the photolithographic wafer. This not only makes the circuitry extremely complex, but also limits the number of cantilevers which can be realistically controlled within one photolithographic wafer. Secondly, if a cantilever has to be flexed to a relatively large degree before the desired gap distance can be reached, the tip may be positioned to face the fabrication wafer at an angle substantially different from normal. This may further affect the lithographic step when the reactive light is applied to photoresist surface.
U.S. Pat. No. 4,991,962 discloses a high precision alignment system for aligning a mask with a wafer in a high resolution nanolithography system using optical methods. The alignment is accomplished by observing alignment targets such as multiple diffraction gratings on the mask and the wafer to generate interference signals. It is clear that although alignment obtained using this method is in the order of a few nanometers, this method only allows two dimensional alignment. When a third dimension of alignment is required, such as for multiple tip scribing wafers, this method would not be useful for preventing tilting of scribing tools.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide a system to overcome the shortcomings as stated above and give precision three-dimensional alignment.
It is another object of the present invention to reduce the requirement for complex scribing tool designs in nanolithography.
It is a further object of the present invention to provide an SPM system which allows for multi-probe imaging of surfaces.
SUMMARY OF THE INVENTION
The present invention provides a high precision three-dimensional alignment system using SPM techniques. The system comprises a fine distance control unit for the effective three-dimensional micromovement in the micro- to nanometer range of a planar object, and proximity detection unit to monitor the alignment process.
For the purposes of understanding the present invention, substrate is defined as a structure with a surface whereon processes according to the present invention is performed. For standard applications, the substrate would have a planar surface whereon lithography or scanning is performed, and may contain features with various recesses. Lateral movement refers to the movement of the substrate or tool along the plane i.e. the X and Y axis of
FIGS. 1 and 2
. Axial movement refers to the movement in a direction substantially normal to the planar surface of the substrate or tool along the Z-axis as shown in
FIGS. 1 and 2
.
In the preferred embodiment, the fine distance control unit comprises a set of at least three strategically positioned fine distance control elements which are capable of controlled expansion and contraction in the nanometer range. The most preferred embodiment of the fine distance control element comprises a piezoelectric tube, which crystal size may be varied by varying an applied voltage. This system may be applied to nanolithography, in which case the planar object is a scribing tool having a planar base with multiple tips fabricated on one surface. These tips may be used to create features on substrates such as wafers. In order to obtain uniform features from every tip, the system according to the present invention is capable of providing three-dimensional alignment such that a uniform distance over the entire surface between scribing tool and the substrate may be achieved. A proximity detector is provided to monitor the distances between the tool and substrate, and to send information to a feedback unit to control the scribing process. A coarse distance control unit comprising a translational means such as a conventional stepper motor may also be provided to allow for initial approach of the scribing tool towards the substrate. One or more piezoelectric elements may be used to provide micromovements of the stage in which the substrate is mounted. As multiple tips are used, efficiency is highly improved over single tip microlithographic systems, while at the same time maintaining a relatively simple electronic control system. From the fine and effective distance control, well-formed uniform features are produced.
In another embodiment of the present invention, the alignment system is used in the multi-probe imaging of a surface. In this embodiment, multiple scanning probes on a probing tool are aligned with the surface to be scanned using the alignment system of the present invention. Signals generated by each scanning probe are then sent to a central SPM control unit for processing.
REFERENCES:
patent: 5099216 (1992-03-01), Pelrine
patent: 5325010 (1994-06-01), Besocke et
Li Sam Fong Yau
Ng Hou Tee
Christie Parker & Hale LLP
Institute of Materials Research and Engineering
Ro Bentsu
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