Patent Grant
7420730
(1) Field of the Invention
This invention relates to a heat-minimizing spatial light modulator with image enhancement features based on diffracting digital micromirror devices (DDMDs) and to techniques for fabricating and incorporating DDMDs in projection optical imaging and patterning systems.
(2) Description of the Related Art
A maskless lithography technology that is capable of high-resolution patterning is essential for manufacturing of many microelectronic and bio-molecular devices. Lithographic techniques using conventional masks involve a multi-step process which includes defining the desired features on the mask which is then used to transfer the features onto a substrate by imaging. The process of fabricating a high-density electronic circuit involves the imaging of numerous layers (often more than 20) and requires a different mask for each layer. As the minimum feature sizes of semiconductor devices continue to shrink, the cost of masks for integrated circuit fabrication continues to increase, now exceeding a million dollars for a mask set for the leading devices.
Further, when building a prototype electronic module, the mask for each layer needs to be designed and fabricated many times in order to optimize the design of the prototype device. This leads to extremely long development times for prototyping any kind of electronic circuit. Additionally, in numerous defense applications, the number of different application-specific modules required is large, whereas the quantities needed of each type of module are not large. Since it is not possible in such a production scenario to amortize the high mask costs over large volumes, the mask costs become especially unbearable.
Thus, a maskless lithography technology that provides the required high resolution and desired patterning throughput will eliminate the need for expensive masks and remove a significant barrier to the cost-effective manufacturing of numerous electronic products. Finally, the vast majority of biotechnology applications that require rapid structuring of bio-surfaces, such as nano-texturing of bone implant surfaces, proteomics, and micro-array generation, will also be greatly enhanced if the desired patterning could be accomplished with high-resolution maskless lithography.
Spatial light modulators based on digital micromirror devices (DMDs) are used in a variety of projection optical imaging and maskless patterning applications. Such DMDs have been described with a variety of pixel elements, including tiltable, displaceable and modulatable pixel elements. Many of these DMDs can be produced in volume using semiconductor-like fabrication methods and equipment.
Special techniques of enhancing the resolution capability of DMDs, by fabricating small-area pixel mirrors, have also been described. Such techniques involve limiting the size of the reflective pixel mirror to only a small portion, such as one-eighth, of the area of the pixel element. This can result in a different problem, however, since the self-protective nature of the typical fully-mirrorized pixel element is diminished greatly—namely, heating of the spatial light modulator array by the non-reflected majority (⅞ths) of the incident radiation. Note that the typical projection light source is routinely used with suitable filters to eliminate virtually all heating by any undesired frequencies of the light source output. The desired frequencies (visible light in projection display systems, and ultra-violet radiation in patterning systems) are not usually considered dangerous sources of heat-causing energy, when used properly, i. e., without excessive absorption in any component of the system in which they are used. These desired frequencies, however, when present at power levels typical to patterning systems and projection display systems, are significant sources of heat when absorbed by the majority non-reflective areas of a spatial light modulator in which the reflective area is only a small portion of the total pixel element area. The repetitive nature of the typical patterning system, and the constant nature of the typical display system, can cause such systems to suffer damage by destructive heat buildup, e.g., degradation of the DMD, its driver electronics, optical components, and their coatings.
This invention provides a heat-rejecting capability to each pixel element of a DMD-type spatial light modulator, significantly diminishing undesirable heat build-up in the spatial light modulator array module, in the optical components of the projection module, and at the projection image device, typically the substrate where the system is put to practical use as image for viewing or as pattern optical machining.
An object of this invention is to enable very-large-scale systems of spatial light modulators and targets to operate without destructive heating effects.
Another object of this invention is to provide enhancement of the resolution in the projected image beyond that which could be achieved using simple mirror pixels in the DMD-type spatial light modulator.
A feature of the invention is the use of diffracting digital micromirror devices (DDMD) that incorporate reflective phase grating features and reflective mirror features as pixel elements in the spatial light modulator.
Another feature of the invention is the use of additional resolution enhancing features in the diffracting digital micromirror devices (DDMD).
An advantage of the invention is that the illumination radiation, incident on the non-mirror portion of the spatial light modulator DDMD pixel surface, is redirected by the reflective phase grating so that none of the incident power is absorbed in that area of the pixel element, eliminating a source of deleterious heating of the spatial light modulator.
Another advantage of the invention is that the illumination radiation incident on the non-mirror portion of the spatial light modulator DDMD pixel surface is redirected by the reflective phase grating so that none of the power in the redirected beam or beams is incident on the projection lens, eliminating a source of heating that causes image distortion or damage to the optics.
Another advantage of the invention is that the illumination radiation, incident on the non-mirror portion of the spatial light modulator DDMD pixel surface, is redirected by the reflective phase grating so that none of the power in the redirected beam or beams is incident on the substrate, eliminating a source of deleterious heating that causes substrate distortion or damage and eliminating a source of deleterious exposure of the substrate.
Another advantage of the invention is that the illumination radiation that is incident on the pixel mirror and image enhancing feature portions of the spatial light modulator DDMD pixel surface is directed through the projection lens to form an image with higher resolution than could be achieved with the simple mirror pixels of the prior art.
Other objects, features, and advantages of the invention will be apparent from the following written description, claims, abstract, and the annexed drawings.
General Considerations
Spatial light modulator (SLM) arrays are used in maskless lithography and other applications. A variety of SLMs have been employed for use with different wavelengths. The most widely investigated type of SLM comprises a 2-D array of micromirrors fabricated in Si wafers with process technologies similar to those used in IC production. In these SLMs, typically the size of an individual micromirror element (pixel) is approximately 15×15 μm. When used in lithography, the achievable system resolution and throughput depend on the pixel size and number of pixels. It is highly desirable to make the size of each pixel as small as possible and the number of pixels in an SLM array as large as possible. For example, with advanced microelectromechanical systems (MEMS) technologies, it is possible to make an SLM array with a grid size of 4×8 μm, with a total number of micromirrors as large as 108-109.
By minimizing the size of the pixels, it is possible to pack more pixels into a given area. The pixel size governs the reduction ration required to achieve a given resolution. Accordingly, by reducing the pixel size, the required reduction ratio and the corresponding demands on the projection lens design can be minimized. Due to the constraints of current MEMS and CMOS fabrication capabilities, it is difficult to reduce the pixel size to arbitrarily small dimensions (1×1 μm or smaller), which would further reduce the required reduction ratio, while maintaining the required performance and minimizing the number of nonfunctioning pixels.
However, it is possible to achieve high resolution using large pixel elements (4×8 μm or larger) that are manufacturable by limiting the size of the specularly reflective region (the mirror region) to a small portion (2×2 μm or smaller) of the 4×8 μm pixel element surface. This makes the achievable feature resolution in lithography finer and reduces the demands on the projection lens.
If the specularly reflective mirror region of the pixel surface is a small portion of the entire pixel surface, the majority of the power in the incident illumination beam falls upon the other regions of the pixel element surface. It is important that this incident power is not absorbed by the pixel element and that it be redirected away from the DMD spatial light modulator without causing any deleterious heating or optical damage. In order to avoid deleterious heating of or damage to the projection lens, it is also important that the redirected power does not impinge upon the projection lens for both angular positions of the pixel that correspond to the selected state and the non-selected state. Similarly, in order to avoid deleterious heating of the substrate or spurious exposure of the substrate, it is important that the redirected power does not impinge upon the projection lens for both angular positions of the pixel that correspond to the selected state and the non-selected state.
These conditions for redirection of the incident power can be satisfied if the non-mirror regions of the pixel surface are covered with a reflection phase grating with appropriate grating depth, grating periodicity and grating groove orientation. The grating surface is coated to be highly reflecting at the incident illumination wavelength. With proper design, the vast majority of the incident power that falls upon the grating covered regions will be redirected into the +1 and −1 diffraction orders to form beams that miss the projection lens and substrate for both selected and non-selected orientations.
A binary phase grating with quarter-wave groove depth could be used and would be advantageous due to simplicity of fabrication. However, using variable-level (gray-scale) gratings with multiple steps could provide some additional degrees of freedom for improving the printed image. The grating should be designed to seamlessly interface with the pixel mirror so that the image definition and contrast are maximized.
An important additional advantage can be obtained by surrounding the mirror region on each pixel with a resolution enhancement feature such a reflective phase shift feature that is designed to sharpen the image produced by the light reflected from it and from the mirror itself. The rest of the pixel surface would be covered by a reflection phase grating as described above. Other resolution enhancement features that could be incorporated include optical proximity correction, scattering bars, and sub-resolution assist features. It is actually easier to implement resolution enhancement techniques for this type of DMD-based maskless lithography than for conventional mask-based lithography. For DMD-based maskless lithography, the pixel mirrors are well separated from each other such that they can be considered isolated and a single optimized design for the resolution enhancement feature can be implemented for all of the pixels. In comparison, for conventional mask-based lithography, many different features and techniques must be implemented depending on the configuration of the image.
The concept of this invention works in synchrony with standard MEMS fabrication practices. It allows the DDMDs to be composed of flat plates that are carved, using lithography, into mirror surfaces, electrodes, springs, etc. The mirror substrates can be flat with the mirror and the diffractive structures comprising the reflection phase grating can be deposited on top. Alternative means of disposing of unwanted incident light would require ti
Note that the reflected beam of light 6 has significant divergence. This is due to diffraction from the small aperture of pixel mirror 4. Note that diffracted beam 10 and diffracted beam 11 also have significant divergence. This is due to diffraction from the small aperture of the pixel mirror surround grating 5. Allowances for this beam divergence must be made in the selection of the periodicity and groove orientation of grating 5 so that the diffracted beams entirely miss the projection lens 7.
In the most preferred embodiment, the grating grooves are aligned perpendicular to the axis of rotation 22 of the pixel element, as shown in
The left half of the pixel surface is covered by a grating 32 that, as in the case of
In the most preferred embodiment, the grating grooves are aligned perpendicular to the axis of rotation 33 of the pixel element, as shown in
In another preferred embodiment, the grating grooves are aligned parallel to the axis of rotation 33 of the pixel element. In this case the plane defined by the incident beam of light and the 0 order reflected beam of light incident beam of light (not shown in
In this coordinate system, as the grating is rotated by angle δ (shown in the inset in
The angle Δ′ takes on particular importance as the grating is rotated, since it provides a measure of the amount by which the diffracted ray is skewed away from the x-z plane. In some cases Δ′ can be large enough that the diffracted ray will avoid the input aperture of the projection lens entirely as the grating is rotated. This requires that Δ′>∈.
For one particular configuration, where the tilting access of the cantilever is parallel to the grating pattern, the grating grooves are parallel to the y-axis. In this configuration, which corresponds to the normal use of gratings in spectroscopy, θ′=0 and Δ′=0 and the diffracted rays are all in the x-z plane. The relationship between Φ and Φ′ is given by the well known diffraction grating formula
sin Φ′−sin Φ=nλ/Λ (Equation 1)
where n=0, ±1, ±2, . . . is the order of diffraction with n=0 corresponding to specular reflection; λ is the wavelength of the optical radiation; and Λ is the grating period. (See for instance R. A. Sawyer, Experimental Spectroscopy, Dover (1963), pg 129.) Clearly this configuration is not desirable for the application of this invention, since the diffracted rays are in the same plane as the projection lens. As the grating is rotated by angle δ, diffracted rays are likely to enter the input aperture of the projection lens unless the angular spread between the diffracted orders is larger than 2δ+2∈.
For the more desirable configuration, where the tilting axis of the cantilever is perpendicular to the grating pattern, the geometry is exactly as shown in
sin2 Φ′−sin2 Φ=(nλ/Λ)2. (Equation 2)
The angles θ′ and Δ′, that measure the deviation of the diffracted ray propagation direction from the z-x plane of incidence, are given by
tan θ′=nλ/Λ sin Φ (Equation 3)
and
tan Δ′=nλ/Λ cos Φ′. (Equation 4)
It is instructive to study two limiting cases for these results. Note that for the case of n=0, θ′=0 and Δ′=0 for all values of Φ and there is no deviation from the plane of incidence. This is as would be expected for specular reflection. Also note that for the case of normal incidence where Φ=0, Equation 2 reduces to sin Φ′=nλ/Λ and Equation 3 gives θ′=0. In this case, all of the diffracted rays are in the z-y plane, Δ′=Φ′ and Equation 4 also reduces to sin Φ′=nλ/Λ. Hence, as expected, the diffraction angles are the same as predicted by the standard diffraction grating theory as expressed in Equation 1.
As a practical example, consider the case of a grating with grooves parallel to the x-axis that is tilted by δ=20 degrees. Assuming a projection lens with an acceptance half angle of ∈=10 degrees, a good choice for the “selected” angle of incidence is Φ=20 degrees. Then the “non-selected” angle of incidence will be 40 degrees. Table 1 below shows the calculated values for Φ′, θ′, and Δ′ for n=0 and n=1 for a range of angles of incidence between 20 and 40 degrees and for several different grating frequencies. Paying particular attention to the parameter Δ′, it can be seen that a grating frequency of λ/Λ=0.2 or greater is sufficient to satisfy the condition Δ′>∈ that insures the diffracted beams miss the projection lens for all values of Φ between 20 and 40 degrees.
Fabrication Method—Diffraction Grating
The diffraction grating for elimination of heat build-up can be fabricated by the following method, as illustrated in
Step 1: Deposit a photoresist layer 90 on the pixel element surface. The resist may be deposited by spinning, for example. The thickness of the resist layer should be equal to one-fourth of the wavelength of the light source used for illumination of the pixel.
Step 2: Pattern the resist using conventional lithographic processes and produce line-space patterns 91,92 of the desired dimensions (determined as described in the Detailed Description.)
Step 3: Deposit a high-reflectivity layer on top of the patterned photoresist and on the cleared pixel areas. The high-reflectivity layer may be, for example, a metallic coating (e.g., aluminum) with an overcoat of magnesium fluoride, or a multilayer dielectric coating (e.g., using layers of various oxides, such as oxides of magnesium, hafnium, ruthenium, scandium, titanium, etc.)
Fabrication Method—Resolution Enhancement Features
The resolution enhancement features are regions of phase reversal placed adjacent to the reflective patch of the pixel element. Their typical configuration may be rectangular bars whose length is slightly less than the length of a side of the reflective patch and whose width is a fraction of the length. In prior art, such features have been widely used in numerous configurations in the application of mask-based lithography. Therefore, there exist a variety of well-known processes to fabricate such features. Accordingly, for the present invention, a typical fabrication method for providing resolution enhancement features in the spatial light modulator may have the following process steps, as illustrated in
Step 1: Deposit a layer of a suitable photoresist 95 on the pixel element surface by a conventional process such as spinning. The resist thickness and its processing conditions are selected s recommended by the resist manufacturer.
Step 2: Pattern the resist using a mask designed for the appropriate configuration of the resolution enhancement feature. The resist patterning step includes exposure and development of resist, which results in the required region 96 being opened up in the vicinity of the reflective patch.
Step 3: Deposit a coating 97 of a suitable phase-reversal material in the open region. Such a coating may be, for example, a multilayer stack of a variety of oxide materials whose thicknesses are selected for maximum reflectivity of the composite structure at the wavelength of illumination. These oxides may be one or more of magnesium oxide, hafnium oxide, scandium oxide, ruthenium oxide and titanium oxide.
Step 4: Remove the photoresist remaining 98 on the pixel element by a chemical stripping process.
As previously remarked, the methods of fabrication of the grating structures and resolution enhancement features are varied and well known; above we have provided only two examples. Many other process sequences will be familiar to those even cursorily skilled in the art.
The invention has been shown and described configured as a first embodiment and second embodiment. Certain variations have been shown without departing from the spirit and scope of the invention, as defined by the following claims:
This is a divisional application of U.S. patent application Ser. No. 11/237,382, filed Sep. 28, 2005, now U.S. Pat. No. 7,170,669, Issue date Jan. 30, 2007, Jain et al., SPATIAL LIGHT MODIULATOR ARRAY WITH HEAT MINIMIZATION AND IMAGE ENHANCEMENT FEATURES.
| Number | Name | Date | Kind |
|---|---|---|---|
| 7173751 | Yun | Feb 2007 | B2 |
| 7209276 | Hong et al. | Apr 2007 | B2 |
| Number | Date | Country | |
|---|---|---|---|
| 20080018981 A1 | Jan 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11237382 | Sep 2005 | US |
| Child | 11646891 | US |
