Patent Application
20060232838
The present invention relates to the field of total internal reflection (TIR) holography, and in particular to TIR holography as employed for photolithography.
The prior art teaches that an important application of TIR holography is for printing high-resolution microcircuit patterns, especially on glass substrates for manufacturing certain flat panel displays (e.g. U.S. Pat. No. 4,917,497, U.S. Pat. No. 4,966,428, U.S. Pat. No. 5,640,257, U.S. Pat. No. 5,695,894 and U.S. Pat. No. 6,657,756). According to the method, a hologram mask is recorded from a conventional chrome mask bearing a pattern of features by firstly placing the mask in close proximity to a holographic recording layer on a glass plate arranged on a glass prism. The mask is then illuminated with an object laser beam whilst simultaneously illuminating the holographic recording layer with a mutually coherent reference laser beam through the prism at such an angle that the reference beam is totally internally reflected from the surface of the holographic layer. The optical interference of the light transmitted by the mask with the reference beam is recorded by the photosensitive material in the holographic recording layer, which is subsequently fixed by an appropriate processing step, to form the hologram mask. The mask pattern can afterwards be regenerated, or reconstructed, from the hologram mask by re-mounting the hologram mask on a glass prism and illuminating it through the prism with a laser beam having the same wavelength as the laser beam used for recording the hologram. The pattern may be printed by placing a substrate, such as a silicon wafer or a glass plate, coated with a layer of photoresist at the same distance from the hologram as the chrome mask was during recording.
Because of the close proximity between the holographic layer and mask during recording, and between the hologram and substrate during reconstruction, the TIR holographic method provides a very high numerical aperture (˜1) in comparison with traditional photolithographic methods which enables a relatively high resolution features to be imaged using a given exposure wavelength, for example, 0.4 μm features may be printed with a wavelength of 364 nm. Further TIR holographic lithography possesses no trade-off between feature resolution and pattern size, so it can print, for example, a 0.4 μm-resolution pattern of dimensions 150 mm×150 mm. Lithographic exposure equipment based on this technique operating at a UV wavelength of 364 nm has been developed and commercialised. A draw-back of the TIR holographic technique arises because of the type of hologram and holographic recording materials that have been used: the holograms employed have generally been “volume” holograms in which the mask pattern is recorded either by a modulation of the refractive index of the holographic recording material, in the case of, for example, photopolymers, or by a modulation of its absorption, in the case of, for example, photographic emulsions. Such recording materials, however, are not ideally robust to extended periods of illumination by a high-intensity UV laser beam, and this can be a problem if the holograms have to withstand very high and virtually continuous levels of UV light, as would be the case on a lithographic equipment employed for high-volume production of flat panel displays. Other types of hologram and holographic recording material exist, namely “surface-relief” holograms and photoresist materials respectively that permit more robust holograms, but because of the recording mechanism of the TIR holographic method, they are not readily applicable to hologram masks. U.S. patent application Ser. No. 10/009,944 describes a method for recording surface-relief hologram masks in which particular polarisations are employed for the object and reference beams but the recording process is difficult to optimise.
It is an object of this invention to provide a method and apparatus for manufacturing surface-relief hologram masks for use in lithographic equipment based on total internal reflection holography in which pattern is recorded in the hologram as a variation the thickness of the recording material. It is a second object of this invention that said hologram masks are robust to long-term and intense illumination from a laser source, particularly at UV or DUV wavelength. It is a third object of the present invention that the hologram masks thus formed can be cleaned so that any contamination to the hologram mask by, for instance, handling procedures may be readily removed by a simple cleaning process thus prolonging the life of the hologram mask.
According to a first aspect of this invention, there is provided a method for manufacturing a surface-relief hologram mask for use on a lithographic system based on TIR holography which comprises the steps of:
In the step of arranging the recording plate on the master hologram mask, it is preferable that a layer of fluid is introduced between the volume hologram and the recording layer so that the light in the exposure beam that is not diffracted by the volume hologram is transmitted into the recording layer instead of being totally internally reflected from the surface of the volume hologram.
The step of inhibiting the recording of the reflection image hologram of the volume hologram in the recording layer is preferably by inhibiting the total internal reflection of said undiffracted light in the exposure beam from a second surface of the second substrate following its passage through the recording layer. This might be achieved either by arranging an absorbing element on the second face of the second substrate, or alternatively by arranging a second coupling element such as a prism on the second surface of the second substrate, with a layer of fluid between the two, in order that the undiffracted light leaves the second substrate and so that it is subsequently transmitted through another face of the coupling element. The thickness of the second substrate may also be selected to be large enough in elation to the size of the pattern recorded in the volume hologram in order that the undiffracted light that is totally internally reflected from the second surface of the second substrate does not illuminate that part of the recording layer that records the transmission image hologram. It can further be advantageous that the coherence of the light in the exposure beam is reduced in order that spurious reflections of the exposure beam are made incoherent with the light recording the transmission image hologram in the recording layer.
In order that the recording layer and master hologram mask can be easily separated following the illumination of the master hologram mask with the exposure beam it is advantageous that at least one of the volume hologram and the recording layer is initially coated or otherwise treated on its surface in order to reduce the adhesion between the two. A coating or treatment may also be applied to at least of the surfaces in order to protect the hologram or recording layer from the ingress of fluid, from abrasion or for reducing the reflectivity of an exposure beam from the illumination system during the recording of the transmission image hologram in the recording layer.
Following the formation of the surface-relief structure in the second recording layer, it is further preferable that the surface-relief structure is transferred by a process or combination of processes into the underlying material of the second substrate. This material may either be that of the bulk substrate or it may be that of an intermediate layer provided on the surface of the bulk substrate.
The surface-relief structure may alternatively be transferred from the recording plate onto a third substrate using another process or combination of processes.
Following the formation of the surface-relief structure either in the recording material, in the underlying material of the second substrate or on the third substrate, the surface-relief structure may coated with a thin layer of a material or a treatment otherwise applied to the surface-relief structure, which conforms to the surface-relief profile, in order that, for example, the surface-relief structure can be more easily cleaned, reduces the deposition and adhesion of particles onto the structure by electrostatic forces, or to act as an anti-reflection coating in order to increase the diffraction efficiency of the hologram.
In order that the transmission image hologram is recorded uniformly into the recording layer over the surface of the layer, it is preferable that the illumination system includes a scanning system that scans the exposure beam over the surface of the master hologram mask. The beam preferably has a Gaussian intensity profile and the scanning is preferably performed in a raster pattern.
It is further advantageous that the thickness of the second recording layer and the exposure energy density produced by the illumination system are selected in order to optimise the depth of the surface-relief structure in order to maximise the diffraction efficiency of the resulting surface-relief hologram.
According to a second aspect of this invention, there is provided an apparatus for manufacturing a surface-relief hologram mask for use on a lithographic system based on TIR holography which includes:
The coupling element is preferably a refractive element such as a prism with at least 2 polished faces or alternatively may be a diffractive structure such as grating or a combination of gratings.
The means for arranging the recording plate on the master hologram mask such that the undiffracted light of the exposure beam is transmitted from the volume hologram into the recording layer is preferably a layer of fluid interposed between the volume hologram and the recording layer.
It is further preferable that a mechanical, pneumatic or other means be provided to apply pressure to the recording plate in relation to the master hologram mask in order to minimise at least one of the thickness of said fluid layer between the volume hologram and recording layer and the variation in thickness of the fluid layer across the layer.
It is preferable that the apparatus further include a mechanical means to stabilise or clamp the recording plate in relation to the master hologram mask in order that the transmission image hologram is accurately recorded in the recording layer.
The means for inhibiting the recording by an exposure beam from said illumination system of the reflection image hologram in the recording layer is preferably an absorbing plate arranged on a second surface of the second substrate with a layer of fluid between the two such that the undiffracted light in the exposure beam that is transmitted through the recording layer is absorbed by said absorbing plate. The absorbing element might alternatively be a layer of an absorbing material which has been spin coated to the second surface of the second plate. In the case that the surface-relief structure obtained in the recording layer is to be subsequently transferred to a third substrate, the means for inhibiting the recording of the reflection hologram in the recording layer may alternatively be employing the material of the second substrate which is selected to be absorbing.
Preferably, the apparatus of the invention may additionally include a layer or treatment applied to at least one of the volume hologram and recording layer before they are arranged in proximity or contact that facilitates their separation and/or cleaning following the recording of the transmission image hologram in the recording layer. Such a layer or layers might additionally or alternatively be used to render at least one of the volume hologram or recording layer more robust so that, for example, the method of the invention may be applied many times to the master hologram mask thereby enabling the transmission image hologram to be recorded a plurality of times in a plurality of recording layers on a plurality of recording plates. The layer or layers may also or alternatively be used to modify the optical properties of the volume hologram or recording layer during the exposure step, for example, the layer or layers may be used as anti-reflection coatings to suppress certain reflections. Such a layer or layers with such a function may also be disposed between the volume hologram and the first substrate or between the recording layer and the second substrate.
Advantageously, the apparatus of the present invention additionally includes means for transferring the surface-relief structure formed in the recording layer into the underlying material of the second substrate, which material might be either that of the bulk substrate or that of an intermediate layer provided on the surface of the bulk substrate, which together the second substrate. Means might alternatively be provided for transferring the surface-relief structure formed in the recording layer onto the surface of a third substrate.
Preferred embodiments of the invention will now be described in greater detail with reference to the following drawings, wherein:
a illustrates the recording mechanism of TIR holography according to the prior art.
b shows in detail the optical interference patterns recorded in the holographic recording layer according to the prior art.
In order to understand and appreciate the limitations of the prior art TIR holograms recorded using the prior art, it is necessary to consider in more detail the recording mechanism of TIR holography based on the prior art. With reference firstly to
The superposition of the 3 hologram components therefore produces a complex light distribution in the recording layer that requires a “volume” holographic recording material for it to be accurately recorded. A volume holographic recording material records an optical interference pattern either as a modulation in the refractive index of the recording material or as a modulation of its absorption. Surface-relief holographic recording materials, on the other hand, are not readily applicable to TIR holography because they are unable to record a complex variation of light intensity through the thickness of the recording layer and the overlapping interference patterns prevent a surface-relief structure of significant depth from being formed. U.S. patent application Ser. No. 10/009,944 discloses a TIR holographic recording method in which special polarisations are employed for the object and reference beams in order to suppress the reflection image hologram and Lippmann holograms relative to the transmission image hologram, so that a surface-relief structure may be formed. This holographic recording process is, however, difficult to optimise for obtaining the required performance from the hologram.
To the left of this assembly is the exposure system incorporating firstly an argon ion laser 52 operating at a wavelength of 363.8 nm, the same wavelength at which the hologram mask 30 was recorded. The output beam from the laser 52, which is in TEM00 mode with a Gaussian intensity profile, passes through a beam expander system 54 consisting of 2 lenses 56, 58 for increasing the 1/e2 diameter of the beam to ˜10 mm and also a spatial filter 60 for eliminating noise in the beam. The resulting beam is then incident on a 2-axis scanning system 62 on which are mounted a pair of mirrors the first of which, 64, reflects the beam towards the second mirror (not explicitly shown in the diagram because it is obscured by the first mirror 64) which subsequently reflects the beam so that it is arrives at the hypotenuse face of the prism 36 at normal incidence. The orthogonally configured motorised stages of the 2-axis scanning system 62 are linked to a control system (not shown) that generates a raster scan of the UV beam 66 across the hypotenuse face of the prism 36 and thence over the master hologram mask 30. The stepping distance of the beam 66 between successive scan passes in the raster pattern is selected to be ˜3 mm in order that the time-integrated energy density of the exposure is made uniform across the master hologram mask 30.
The light passing into the layer of photoresist 42 is essentially therefore the light-field 70 diffracted by the transmission hologram as well as the undiffracted light of the exposure beam 78. Since these two light-fields 70, 78 are mutually coherent, they interfere and the resulting fringe pattern 79 is recorded by the photoresist layer 44. This fringe pattern 79 corresponds to that of the transmission image hologram of the master hologram 32 with, thus allowing a surface-relief hologram to be formed. From the figure it can be seen that the individual fringes in the fringe pattern 79 are not orientated orthogonally to the plane of the substrate 43 but are tilted at a small angle, the magnitude of which depends on the refractive index of the recording layer 42. It should be further noted the tilt of the fringes at a particular location in the resulting hologram with respect to its substrate 43 are in the opposite direction to that of the planes of refractive index of the transmission image hologram in the master hologram 32 with respect to its substrate 34, in other words, the structure of the fringe pattern 79 recorded in the recording layer 42 with respect to its substrate 43 is rather the mirror image of that of the transmission image hologram 76 in the master hologram 32 with respect to its substrate 34.
The time-integrated energy density, E, of the laser exposure of the master hologram 32 is related to the power, P, of the laser beam, to the stepping distance, s, and scanning speed, v, of the raster pattern by
E=P/vs
These parameters are optimised in order that the depth of the surface-relief profile obtained in the recording layer 42 following development yields high diffraction efficiency in the resulting hologram. The depth required depends on the refractive index of the recording material after development: the two are inversely related, that is, the higher the refractive index of the material, the shallower the profile needed. The optimum depth may be determined empirically by experiment or alternatively using standard theoretical treatments known to those skilled in the art such as rigorous coupled wave theory developed by M. G. Moharam and T. K. Gaylord (“Diffraction analysis of dielectric surface-relief gratings”, J. Opt. Soc. Am., 72(10) pp. 1385-1392; 1982). Typically, the exposure parameters are selected to achieve a depth of profile in the resist of 0.15 μm.
The depth of the resulting surface-relief profile, in fact, varies across the recording layer 42 depending on the distribution of the features in the mask pattern from which the master hologram mask 30 was recorded, and thus the exposure energy density should also be selected to assure a linear recording of the light-field diffracted by the master hologram 32. The diffraction efficiency of the original hologram and the photoresist process including its thickness should also be optimised for ensuring a good linearity of recording using such procedures and analytical techniques as would be familiar to one experienced in the art. Negative photoresists may alternatively be used as the recording material
In order for method to be applied to patterns of high-resolution features (such as 0.5 μm), it is important that both the thickness of the fluid layer 44 between the master hologram 32 and the photoresist layer 42 and its variation across the layer 42 are minimised. This may be achieved in a number of ways: firstly by minimising the quantity of fluid employed, secondly by ensuring that the surfaces of the hologram substrate 34 and the recording plate substrate 43 are very flat, and thirdly by minimising the size and number of defects in, on or between the hologram 32 and recording layer 42. For the latter, it is advantageous that both the master hologram mask 30 and the surface-relief hologram mask are recorded in a high-quality clean-room environment having, for example, Class 10 conditions and that all other necessary measures and procedures are implemented for minimising particles.
After the exposure operation, the recording plate 41 is removed from the system and the fluid cleaned from its surfaces by, preferably, a spin cleaning process. The photoresist layer 42 is then be developed using processing conditions optimised for achieving the required linearity of recording and the required depth of profile for high diffraction efficiency, as previously discussed. Because the fringes 79 in the interference pattern recorded in recording layer 42 are tilted from the normal with respect to the substrate 43, as described above, the cross-sectional profiles of the resulting surface-relief hologram are also tilted with respect to the substrate 43. This is illustrated in
In order that the recording layer 81 and the master hologram 32 can be readily separated after the exposure, it is advantageous that at least one of the surfaces of the master hologram 32 and recording layer 81 are pre-coated or otherwise treated with a material to reduce adhesion between the two.
In order to strengthen and better stabilise the surface-relief structure of the photoresist in the resulting copy hologram, it may subjected to a high-temperature heat treatment either on a hot plate or in an oven. To provide a more robust hologram for use on a high-throughput lithographic equipment for, for example, the manufacture of flat panel displays, the resist profile may be transferred into the underlying substrate by further processing, such as by reactive ion etching. In this case the depth of the surface-relief profile produced in the photoresist should be such that the selectivity of the resist process (i.e. the relative etching speeds of the photoresist and substrate materials) yields the required range of depths of profile in the substrate. The gas mixture and other process conditions for the RIE etching should be selected so that the resulting surface-profile is smooth: if the etching is too aggressive a rough, granulated surface is obtained which generates undesirable scatter and noise in the image reconstructed from the copy hologram. The necessary process conditions can be readily determined by standard techniques by those skilled in the art. Furthermore, by adjusting the angle of incidence of the etching ions, the angle of tilt of the resulting surface-relief profile may be modified in order to enhance the diffractive behaviour and performance of the resulting hologram mask. More complex, multi-step processes may also be employed for transferring the profile into the underlying substrate, including using intermediate deposition, planarisation or etch-back processes. For obtaining high-quality, smooth profiles of sufficient depth in the substrate using such etching processes, it is advantageous that the substrate material is fused silica rather than a glass, or alternatively that the substrate comprises a glass plate with a layer of, for example, silicon dioxide deposited on it surface with the photoresist spin-coated onto the silicon dioxide and that the surface-relief profile then be transferred into the layer of silicon dioxide by the etching process. Further, other techniques and combinations of technologies such as, for example, shim fabrication and casting methods, using such materials as sol-gels, may be employed to transfer the surface-relief structure formed in the photoresist onto another substrate. With such a transfer the direction of tilt of the surface-relief profile relative to its substrate may also be reversed, again allowing enhancement of the image-forming properties of the resulting hologram mask.
