Optical: systems and elements – Holographic system or element – Having multiple object beam or diffuse object illumination
Reexamination Certificate
1997-03-03
2001-05-01
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Having multiple object beam or diffuse object illumination
C359S012000, C359S001000
Reexamination Certificate
active
06226110
ABSTRACT:
The present invention relates to a method and an apparatus for holographically recording periodic or quasi-periodic features of a mask in a holographic recording layer.
U.S. Pat. No. 4,857,425 discloses a method for the manufacture of integrated circuits using Total Internal Reflection (TIR) holography. The disclosed TIR holographic technique is characterized in that a first substrate bearing a holographic recording layer is disposed on the hypothenuse face of a prism and a second substrate containing e.g. an integrated circuit pattern is arranged in proximity to the first substrate. The distance between the first and the second substrate is usually about 100 microns. In a first step the pattern of the integrated circuit is holographically recorded in the holographic recording layer of the first substrate by illuminating said photosensitive layer with an object and a reference beam of mutually coherent light, the object beam passing through the mask window and being incident on the holographic layer at 90 degree, and the reference beam being projected through one of the shorter faces of the prism at such an angle that the light is totally internally reflected off the photosensitive layer/air interface. The interference between the object and the reference beams is recorded in the photosensitive layer which after proper development and fixation presents the hologram.
For the reconstruction the hologram is again contacted with the prism face and the second substrate is replaced e.g. by a silicon wafer bearing a photosensitive layer. Thereafter a so-called reconstruction beam is directed through the prism in the exactly reversed direction to the reference beam used for the hologram recording. For this purpose, in practice, the hologram often is turned by 180° so that the reference beam functions as reconstruction beam. The reconstruction beam produces a positive image of the circuit pattern in the photosensitive layer of the silicon wafer.
Although the holographic system of Philips allows recording of features in the submicron level the attainable practical resolution in some cases is still not high enough for certain applications, e.g. for manufacturing so-called Distributed FeedBack Lasers (DFB-lasers). Their fabrication requires the formation of a fine period grating in the order of approximately 0.2 &mgr;m on a substrate material. At present the manufacturers of DFB-lasers use electron-beam lithography to “write” the grating lines. This is a slow process and therefore very expensive and not commercially attractive for large production quantities.
A general problem of high resolution lithography is the restricted depth of focus of the imaged patterns. In holographic lithography it is therefore necessary that the separation between hologram and wafer in the hologram reconstruction step is the same as the separation between recording layer and mask in the hologram recording step. Means are therefore provided which allow measurement and adjustment between the mask and the recording layer during hologram recording and between hologram and wafer during hologram reproduction step (see e.g. U.S. Pat. No. 4,857,425 issued to Philipps).
An object of the present invention is to overcome the shortcomings of the prior art and, in particular, to provide a method and an apparatus for holographically recording an essentially periodic pattern of features with still better resolution in a holographic recording layer.
According to the invention the method comprises the following steps:
optically contacting a first substrate bearing a holographic recording layer to one face of a prism;
disposing a second substrate bearing a first mask pattern parallel and in proximity to the first substrate, the mask pattern containing periodic or quasi-periodic features in a first direction;
splitting and expanding or vice versa a light beam of a certain wavelength to generate an object beam and a reference beam
directing the reference beam to one of the remaining prism faces such that the reference light beam is totally internally reflected off the holographic recording layer/air interface and
directing the object beam to the first substrate such that it passes the mask pattern and interferes with the reference beam in the holographic recording layer to form a hologram of the mask pattern, wherein the object beam is directed to the second substrate at an off-axis angle and the wavelength used and/or the angle of incidence of the object beam are selected according to the period of the features to be recorded such that essentially just the zero and one of the first diffraction orders are present for forming the hologram.
It has been found that by using such off-axis object beam illumination the resolution capability by TIR holographic lithography can be significantly improved provided that the mask pattern comprises essentially periodic or quasi-periodic features. The theoretical limit to the smallest period which can be recorded can be as small as ≅&lgr;/2 in contrast to the conventional holographic technique where the smallest theoretically attainable period is &lgr;. In addition to this resolution advantage, since there are only two propagating diffraction orders of light, the depth of focus is significantly increased in comparison with a conventional TIR hologram recording arrangement in which the object beam is normal to the plane of the mask.
A quasi-periodic mask pattern is a pattern having a limited bandwith of spatial frequencies (e.g. +/−10% of the average spatial frequency) in contrast to a pattern having a precise or discrete period. Such a quasi-periodic mask pattern may comprise a sequence of grating segments each with a precise (and the same) period but offset with respect to each other by some fraction of the grating period. This is commonly the case for DFB Lasers.
Advantageously, the object beam is directed to the first substrate at an angle such that the normal to the plane of the first substrate essentially bisects the directions of the zero and first diffraction orders. In this manner the structures which will be formed upon illumination of the hologram in the photosensitive layer on a wafer or the like, will have essentially perpendicular sidewalls with respect to the plane of the wafer. This can be of importance for subsequent processing of the wafer.
According to an advantageous embodiment the plane of incidence of the object beam on the second substrate or mask contains the vector describing the direction of the periodicity, i.e. if the periodic features are lines in a grating, the object beam is essentially orthogonal to the grating lines. In this way the contrast the interference of the optical interference pattern recorded in the holographic layer can be maximised.
Different modifications of the inventive method are feasible. For instance, after recording a hologram according to the above method it is possible to rotate the second substrate containing the mask pattern with respect to the first substrate and to rotate the object beam such that the plane of incidence of the object beam again contains the direction of the periodicity and to record the mask pattern of the second substrate in the holographic recording layer a second time. By this procedure high-resolution grating structures in different directions may be recorded and subsequently printed. Instead of rotating the pattern containing second substrate, the first substrate bearing the holographic recording layer may be rotated.
If the second substrate is replaced by another substrate bearing a mask pattern containing periodic or quasi-periodic features in a second direction a hologram containing features of different periods and in different directions can be produced.
It is also possible to record the same or different mask patterns in two or more different holographic recording layers on two or more different first substrates for forming two or more holograms. The pattern recorded in these holograms can be subsequently reconstructed onto a photosensitive layer on a single substrate thereby composing a complex
Holtronic Technologies Ltd.
Spyrou Cassandra
Stanger Leo
Treas Jared
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