TIR HOLOGRAPHY
A holographic mask aligner prints a pattern onto a substrate by illuminating a hologram mask with a laser beam. A hologram mask is simply a hologram of a standard chrome-on-glass mask, which is produced using the method based on total internal reflection (“TIR”), schematically shown in the figure below (hologram recording).
The holographic recording plate, consisting of a layer of photosensitive material on a glass plate, is placed on the upper surface of a glass prism, with a layer of a transparent fluid between the two in order that the prism and glass plate form an optically continuous body.
A
chrome mask defining the circuit pattern is placed parallel and in proximity to
the holographic recording layer. The pattern is recorded by illuminating the
mask with a laser beam, called the “object beam”, whilst simultaneously
illuminating the opposite surface of the holographic layer through the prism
with a mutually coherent “reference beam” from the same laser source. Because
the reference beam arrives at the recording layer at an angle of incidence
greater than the critical angle, it is totally internally reflected from the
surface of the layer so that it does not interact with the chrome mask. The
optical interference of the light in the object beam transmitted by the mask
with the reference beam is recorded by the photosensitive material of the
recording layer to form the hologram.
After the holographic exposure, an additional process step is required to stabilise, or “fix”, the hologram and to render the holographic layer inert to subsequent exposure. The hologram mask may then be employed for lithography. For this the hologram mask is again placed on the surface of a prism with a layer of transparent fluid introduced between the two. The pattern is reconstructed from the hologram mask by illuminating it through the prism with a single laser beam of the same wavelength as that used for recording and directed in the reverse direction to that of the reference beam during recording. The interaction of the laser beam with the microstructure within the hologram diffracts the light, generating a time-reversed light-field of the object beam that recorded it, thus accurately reconstructing an image of the mask pattern for printing onto a photo resist coated wafer or substrate.
Holtronic
Technologies has established two hologram recording systems based on the above
design, one for manufacturing hologram masks of patterns up to 6" x 6" and the
second for patterns up to 10" x 12". The systems concerned as well as the
necessary processing, inspection and testing equipment are installed in
environmentally controlled Class 100 clean room conditions.
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Off-axis “oblique” TIR holography
Higher-resolution hologram masks may be manufactured using the optical configuration in which the object beam illuminates the mask at an oblique angle.
This off-axis arrangement for recording the hologram allows a first diffraction order to be transmitted by the mask for much higher resolution structures in the mask. Since the object beam is tilted in one plane, the method is primarily targeted at periodic gratings and quasi-grating structures having substantial deviations from the pure grating form.
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Advantages
TIR holographic lithography offers a number of advantages over traditional exposure methods. Firstly, the hologram mask performs the imaging operation, so no additional, complex lens system is required. This allows for a substantial simplification of the final machine with its associated benefits.
Secondly, due to the close proximity between the mask and holographic layer during recording, and between the hologram mask and wafer or substrate during printing, the numerical aperture (NA) of the imaging system is essentially 1. However, the effective value is less because of reflection losses from the respective interfaces during both recording and printing. Since the resolution (and depth of focus) of the imaging system is governed by the same laws of optics as for conventional imaging systems, this higher NA has enabled, for example, 0.22μm features to be printed using an exposure wavelength of 364nm. With respect to the depth of focus (DoF), although the higher NA of the holographic method reduces the intrinsic value, this is mitigated by the absence of imaging aberrations such as field curvature and chromatic aberration which can reduce the effective DoF of conventional imaging systems.
A third advantage, which also arises because of the close proximity between the mask and holographic layer, is that the quality of the holographic imaging at the edge of the pattern is equivalent to that at the centre, which means that the technique can be equally well applied to large patterns of high resolution features as to small patterns, in contrast to conventional lens and mirror based exposure systems for which a trade-off exists between resolution and pattern size.
A fourth advantage is derived from the relatively large area of the holographic layer over which a particular high-resolution feature is recorded. Since the separation between the mask and holographic layer is typically ~100μm, the area of the hologram over which, for example, a 0.5μm line is recorded (using a wavelength of 364nm) has width ~100μm. This means that if a 1μm-diameter particle lands on the hologram, it can only obscure a small fraction of the light-field reconstructing the image of the particular line concerned. This redundancy aspect of the way information is stored in the hologram mask provides significant tolerance to defects.
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Exposure and focus control systems
The exposure system
comprises firstly of an Argon ion laser emitting at a wavelength of 363.8nm
and beam expansion optics to produce a collimated beam of UV light with a
Gaussian intensity profile and a diameter of ~10mm.
A 2-axis stage system scans the UV beam in a raster pattern through the
hypotenuse face of the prism and over the hologram mask. The stepping distance
of the beam between successive passes of the scanning beam is selected so that
the time-integrated illumination of the hologram mask is highly uniform
(variation < ±1%).
A second 2-axis stage system scans a beam from a focus measurement module through the vertical face of the prism in the same raster pattern as the exposure beam in order to continuously measure the local separation between the hologram mask and substrate where the exposure beam is illuminating the hologram mask. Piezo-electric transducers (PZTs) in the substrate positioning system continuously adjust the height of the substrate in response to these measurements to keep the local separation constant at 120µm, thus ensuring that the image projected from the hologram is accurately focussed onto the substrate.
A high-speed focus-control loop with a data-sampling time <3ms has been developed enabling a scanning speed of 100mm/s. A newer much more powerful focus control system with a sampling frequency of 5KHz, thus a sampling-time of 0.2ms is being developed to be integrated into future systems
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Alignment system and magnification correction
The alignment system on
the HMA employs four microscopes for viewing sets of alignment marks located
at the edges or corners of the patterns recorded in the hologram mask and on
the display substrate.
Each microscope views the alignment marks by illuminating them with an LED and imaging the reflected light onto a CCD. The stored images are automatically processed by an improved Cognex software to determine the relative positions of the alignment marks in the hologram mask and on the substrate, following which the alignment errors are corrected by translational and rotational displacements of the substrate produced by horizontal-axis piezo-electric transducers (PZTs) in the substrate positioning system.
The relative positions of the alignment marks determined by the different microscopes allow not only measurement of the alignment errors between the pattern in the hologram mask and that on the substrate but also any magnification errors between the 2 patterns caused by, for example, thermal expansion. Compensation of such errors, whose magnitude may be different in orthogonal directions and which may include a trapezoidal component, is important if accurate overlay is to be achieved over the entire pattern.
The magnification components measured by the four microscopes are corrected during the exposure operation. Exposure on the HMA systems is performed by a laser beam scanning in a raster pattern over the hologram mask mounted beneath a glass prism. Before the scanning sequence begins the substrate is displaced by the horizontal-axis PZTs in the substrate positioning stage such that patterns on the substrate and in the hologram mask are aligned at the corner where the exposure beam starts its scan. Then, while the beam scans back and forth across the hologram mask, the substrate is continuously displaced laterally in the x and y directions by the PZTs, controlled by the interferometer system, so that at all times the part of the pattern being illuminated by the beam remains accurately aligned with the corresponding part of the pattern on the substrate.
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Field to field stitching
High stitching accuracy
between patterns printed by a step-and-repeat exposure sequence is obtained
using the piezo-electric transducers in the substrate positioning system and
by measuring the displacements with a 3-axis interferometer system.
The magnitudes of the displacements required in the x and y directions depend not just on the dimensions of the pattern to be printed but also on the following parameters: i) the angular offset, φ, between the co-ordinate system of the pattern in the hologram mask and the interferometer co-ordinate system, ii) the magnification errors, Mx and My, between the two co-ordinate systems, and iii) the orthogonality error, ω, between the x and y mirrors of the interferometer system. The HMA can automatically then determine these values by using the alignment microscopes to successively align a reference alignment mark on the substrate chuck with alignment marks in the hologram mask, the values then being calculated from the displacements of the reference alignment mark, as measured by the interferometer system, between each of the alignments. The displacements of the substrate, Sx and Sy, required with respect to the x and y axes of the interferometer system for accurately stitching a pattern of dimensions Dx x Dy (according to the co-ordinate system of the hologram mask) are given by:
Sx = MxDx cosφ + MyDy sin(φ + ω)
Sy = MyDy cosφ - MxDx sinφ
The 2 interferometer beams parallel to the y axis measure the rotation of the substrate so that angular errors can be corrected during the displacement.