Patent Application
20060210886
The present invention is directed to methods for making grayscale photo masks, and methods for making grayscale optical elements using the grayscale photo masks. Examples of the grayscale optical elements include refractive, reflective, and diffractive optical elements such as micro lenses, holograms and gratings.
Newly emerging optical disc formats such as Blu-Ray discs (BD) and High-Density discs (HD) use a blue laser having a 405 nm wavelength in order to focus the laser beam with great precision and tightly pack a large amount of data onto the disc. However, in contrast to the longer wavelengths used for the previous generation of optical discs such as DVD having a 680 nm wavelength, the shorter wavelengths have a lower photo-electron transformation and the reflectance of the reflective coatings provided on the traditional optical elements tend to be insufficient for the shorter wavelength.
As a result of the aforementioned characteristics of shorter wavelengths of light, grayscale optics have recently been the subject of great interest since they are able to provide higher efficiencies to compensate for the lower amounts of laser energy. The term grayscale optics is typically used to refer to micro-optics produced using a photolithographic process while the term ordinary optics is typically used to refer to optics produced using molding techniques or other mechanical methods such as a machining process.
Over the past ten years, various methods have been proposed for producing grayscale optics. For example, a direct method using a focused ion beam (FIB) has been proposed to fabricate a micro-lens (JP63-177509). However, such a method is not very cost effective since fabrication on a lens-by-lens basis is required and since emission of a FIB requires the use of very expensive equipment.
Another direct writing method which has been proposed is to emit an electron beam at an electron sensitive resist to make a micro-Fresnel lens (JP58-210765). However, this method is also expensive. In another method, a binary structure is first prepared in a photo resist and then ion beam etching is performed using a slant angle to make a blazed grating (JP54-119526). However, according to this particular method, blazed gratings having non-uniform and arbitrary structures are difficult to produce.
Yet another method proposed is a stamping method in which a mold is fabricated by a mechanical machining procedure and the machined structure is then transferred to an optical material (JP6-306321). However, in this particular method, the resolution of the microstructure has a drawback in that it is restricted by the nature and characteristics of the transferred material.
A multi-masking method and a grayscale photo mask method have been proposed as photolithographic methods. The multi-masking method is based on the usage of a binary mask and performing multiple exposures, development and etching (JP3-312655). In the grayscale photo mask method, since the photo mask is made to be a grayscale photo mask, the process is completed in one step. In general, the mask is used in a mask aligner or stepper and mass production of elements are performed by using a step-and-repeat process.
One type of such a grayscale photo mask is a “half tone” type in which a pixel in a photo mask is divided into plural sub-pixels and a rate of opening for an aperture (each sub-pixels have 0/1 binary transmittance) determines the transmission of the exposure light (U.S. Pat. No. 5,482,800). In this method, the resolution of the pattern is limited by a size of the sub-pixels which generally cannot be made very small.
Another type of the grayscale photo mask is a “surface relief type” in which the absorbing layer for exposure light has a surface structure and the intensity of the transmitted light is modulated corresponding to the thickness of the structure (U.S. Pat. No. 6,420,073 & U.S. Pat. No. 6,613,498). This type of photo mask is fragile and not durable for performing a step-and-repeat process because the surface structure is generally a microstructure which is prone to damage by even slight contact or by mishandling. Another type of grayscale photo mask is a “density type” in which the density of the absorbing layer for the exposure light is made to be varied over a photo mask. This type of photo mask is relatively stable and is durable for general usage (U.S. Pat. No. 6,638,667 & U.S. Pat. No. 6,562,523). In practice, the method disclosed in U.S. Pat. No. 6,562,523 provides the only practical density type grayscale photo mask. However, this disclosed method completely depends on the use of a specific type of glass that is generally not easy to make, thereby prohibiting this method from being cost effective. Particularly, the glass is made from a base glass component mixed with some metal oxides which are then ion-exchanged in a solution to exchange the metal ions in the metal oxides with silver ions. To write a pattern, an electron beam writer is required that can modulate the dosage of electrons.
An object of the present invention is to provide practical methods for making a density type of grayscale photo mask that has a capability of providing a high resolution with desired characteristics and that can be produced at a low cost, and to provide methods for making grayscale optical elements using such grayscale photo masks.
According to the present invention, a method is provided for producing a grayscale photo mask by: providing a photo resist on a surface of a photo mask blank; exposing grayscale light onto the photo resist to form a predetermined pattern of exposed and unexposed portions in the photo resist; developing the photo resist to remove the exposed portions in the photo resist and produce a grayscale photo resist; implanting ions into and through the grayscale photo resist to obtain a modulated ion density in the photo mask blank; and removing the grayscale photo resist.
According to another aspect of the present invention, a method is provided for producing a grayscale photo mask by: providing a photo resist on a surface of a photo mask blank; exposing grayscale light onto the photo resist to form a predetermined pattern of exposed and unexposed portions in the photo resist; developing the photo resist to remove the exposed portions in the photo resist and produce a grayscale photo resist structure; performing one of ion beam milling and etching to transfer the grayscale photo resist structure into the photo mask blank; implanting ions into the photo mask blank to obtain a modulated ion density in the photo mask blank; and removing portions of the photo mask blank containing the transferal of the grayscale photo resist structure.
According to another aspect of the present invention, a method is provided for producing an optical grayscale element by: providing a first photo resist on a surface of a photo mask blank; exposing grayscale light onto the first photo resist to form a predetermined pattern of exposed and unexposed portions in the first photo resist; developing the first photo resist to remove the exposed portions in the first photo resist and produce a grayscale photo resist; implanting ions into and through the grayscale photo resist to obtain a modulated ion density in the photo mask blank; removing the grayscale photo resist to obtain the grayscale photo mask; providing a second photo resist on a surface of a substrate; placing the grayscale photo mask in close proximity to the second photo resist; exposing light onto and through the grayscale photo mask to transfer the modulated ion density contained in the grayscale photo mask into the second photo resist as exposed and unexposed portions; developing the second photo resist to remove the exposed portions in the second photo resist; and performing one of etching and ion beam milling to the second photo resist and photo mask blank to obtain the optical grayscale element.
According to another aspect of the present invention, a method is provided for producing an optical grayscale element by: providing a first photo resist on a surface of a photo mask blank; exposing grayscale light onto the first photo resist to form a predetermined pattern of exposed and unexposed portions in the first photo resist; developing the first photo resist to remove the exposed portions in the first photo resist and produce a grayscale photo resist structure; performing one of ion beam milling and etching to transfer the grayscale photo resist structure into the photo mask blank; implanting ions into the photo mask blank to obtain a modulated ion density in the photo mask blank; removing portions of the photo mask blank containing the transferal of the grayscale photo resist structure to produce the grayscale photo mask; providing a second photo resist on a surface of a substrate; placing the grayscale photo mask in close proximity to the second photo resist; exposing light onto and through the grayscale photo mask to transfer the modulated ion density contained in the grayscale photo mask into the second photo resist as exposed and unexposed portions; developing the second photo resist to remove the exposed portions in the second photo resist; and performing one of etching and ion beam milling to the second photo resist and photo mask blank to obtain the optical grayscale element.
According to the various aspects of the invention mentioned above, exposing of the grayscale light is performed using a grayscale direct writer.
According to the various aspects of the invention mentioned above, exposing of the grayscale light is performed using a focused laser writer.
According to the various aspects of the invention mentioned above, the removing of the grayscale photo resist is performed by stripping away the grayscale photo resist.
According to the various aspects of the invention mentioned above, the ions used for implantation are selected based on performing the exposure of light onto and through the grayscale photo mask using light having a wavelength which is one of 436 nm (G-line), 405 nm (H-line), 365 nm (I-line), 248 nm (KrF excimer laser), 193 nm (ArF excimer laser), and 0.71 nm (X-ray).
According to the various aspects of the invention mentioned above, implanting of the ions is performed by implanting a dosage of ions in a range of 1015 to 1018 atoms/cm2.
According to the various aspects of the invention mentioned above, implanting of the ions is performed by using an incident angle of implantation that is in a range of 5° to 10° with respect to a normal.
According to the various aspects of the invention mentioned above, providing the photo resist is performed by providing a positive type photo resist.
According to the various aspects of the invention mentioned above, the removal of portions of the photo mask blank containing the transferal of the grayscale photo resist is performed by polishing.
According to the various aspects of the invention mentioned above, the exposing of light onto and through the grayscale photo mask is performed using one of a mask aligner and stepper.
According to the various aspects of the invention mentioned above, the etching is performed by one of a dry etching method and a wet etching method.
According to the various aspects of the invention mentioned above, an element having at least an atomic number which provides no greater than a maximum allowable RMS radial expansion for achieving a desired resolution of the grayscale photo mask is selected as the ion for implantation.
The above and other objects, features, and advantages of present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
According to the present invention, absorbing centers are embedded in a transparent mask blank material by ion implantation through a suitable grayscale structure that controls the penetration of the color centers (implanted ions). The implanted ions in the substrate become the color centers which absorb a specific wavelength of light. As a result of this process, the distribution of the absorbing centers over the mask blank surface is modulated corresponding to the grayscale structure to create a grayscale photo mask.
As shown in
Provided next is a description of the methods of the present invention for producing a grayscale element with reference to
As shown in
Provided next is a description of various factors and characteristics to be taken into account in the aforementioned methods for making a desired grayscale photo mask according to the present invention.
One of these factors is the absorption spectrum of the photo mask blank. The absorption spectrum of the photo mask blank will vary depending on the particular type of ion implanted into the photo mask blank. Additionally, the use of different wavelengths of light for exposure onto and through the grayscale photo mask will result in different levels of absorbance. According to the present invention, the specific wavelength of an exposure light produced by a mask aligner or stepper is of particular interest. The exposure light requires modulation in order to achieve a desired grayscale exposure. The currently available exposure wavelengths are 436 nm (G-line), 405 nm (H-line), 365 nm (I-line) from a Mercury lamp, 248 nm from KrF excimer laser, 193 nm from ArF excimer laser and 0.71 nm from X-ray. In order to assure a proper and desired quality of ion implantation, a particular dynamic range of absorption of the photo mask blank is required. Accordingly, the ions used for implantation are selected based on performing the exposure of light onto and through the grayscale photo mask using light having a wavelength which is one of 436 nm (G-line), 405 nm (H-line), 365 nm (I-line), 248 nm (KrF excimer laser), 193 nm (ArF excimer laser), and 0.71 nm (X-ray).
Absolute Optical Density (da)
da=−Log10T(λ)
wherein T is the absolute transmission at a wavelength λ.
Relative Optical Density (dr)
dr=−Log10(T(λ)/T0(λ))
wherein T0 is the absolute base transmission at a wavelength λ.
Table 1 provided below contains exemplary calculations for demonstrating the relationships of the absolute optical density and the relative optical density for a base transmission of 85%. As apparent from Table 1, the absolute and relative optical densities increase as the transmission decreases.
Upon comparison of the transmittance of the fused silica without ion implantation and the transmittance of Ag and C implanted with ions in
Without paying consideration to situations in which a desired resolution of the grayscale photo mask is required (this will be described later), the particular type of ion used for implantation is not a critical factor and any ion can generally be used. For example, the following ions can be used: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo, Rh, Rd, Ag, Cd, In, Sn, Sb, Te, I, Xe, Ba, La, Hf, Ta, W, Ir, Pt, Au, Ce, Pr, Nd, Sm, Eu, Tb, and Er. The ions are typically implanted using a commercial ion implanter with several hundreds kilo-volts of power and are usually implanted using a slanted incident angle. A dosage of the implanted ions is proportional to an amount of time spent for the implantation operation. The dosage of ions typically falls within a range of approximately 1015 and 1018 atoms/cm2 and is dependent on the amount of optical density required.
The discussion will now focus on the manner in which a particular grayscale structure is designed after the maximum optical density has been obtained.
It has been determined that various factors affect the penetration depth of the ions. Such determinations were made using numerical experiments in which the penetration depth of ions were calculated using a Monte-Carlo based ion implantation simulator known as SRIM2000. The results of the numerical experiments conducted for Ag, Si, Ni, C, and Al are shown in
As shown in
As further evidence of the first notable characteristic mentioned above, an experiment was conducted in which a carbon ion beam was emitted at 100 kV towards the saw-tooth structure shown in
A second notable characteristic is that the smaller the atomic weight, the higher the magnitude of penetration. For example, for the same PMMA and the same acceleration voltage of 200 kV, the penetration depth of Si (atomic weight=28) is three times higher than the penetration depth of Ag (atomic weight=108). It is noted that a ratio of these different penetration depths roughly corresponds to a ratio of the different atomic weights.
Another characteristic is that the acceleration voltage has a linear relationship with the penetration depth. Accordingly, it is possible to select a heavier atom by increasing the acceleration voltage. Moreover, if it is desired to double the thickness of the grayscale structure, this can be achieved by doubling the acceleration voltage.
The decision of which particular ion to select can be determined based upon the required optical density and the required transmission at the wavelength of exposure light to be used. For example, when I-line exposure is to be used, Ag is a suitable candidate for use as the emitted ion since it is most absorptive for the I-line exposure. If an optical density of 1.0 is needed, a transmission of 8.5% is required and a dosage slight larger than 1017 atom/cm2 is required. However, it is noted that the dosage does not have to reach 1018 atom/cm2 in this case. The maximum thickness of PMMA is about 150 nm for Ag. For example, it is possible to make a saw-tooth grayscale structure having an optical density in the range of 0 to 1.0 using Ag ion implantation with 200 kV and with a grayscale PMMA having a maximum thickness of 150 nm. If it is difficult to make the grayscale PMMA having a thickness of 150 nm then, as mentioned earlier, the acceleration voltage can be increased above 200 kV to obtain an increase in the thickness of the grayscale PMMA.
The selection of a particular material for use as the grayscale structure is another factor which affects the magnitude of penetration depth. While any positive type photo resist can be used such as, for example, PMMA base photo resist, Shipley SC 1827, Shipley S1813, AZ-111 photo resist produced by AZ Materials, or any other positive photo resist of a series of AZ photo resists, the penetration depth of ions is considerably different depending on the particular grayscale material which is used. For example, as shown in
Next, another characteristic which affects the penetration depth is the incident angle of implantation as shown in
The resolution of a grayscale photo mask is affected by a phenomenon known as “random walk” which occurs after implantation of the ions when making the grayscale mask. Particularly, random walk defines the movement of ions after being implanted through the surface of a material.
The foregoing description provides the details for preferred methods for making a grayscale photo-mask and grayscale optical element according to the present invention. Provided next is a description of an exemplary overall procedure for making a grayscale element. First, determination of the desired optical density is performed. Next, upon ascertaining the desired optical density, then a particular ion is selected that is appropriate for the optical density and the proper dosage of the ions is determined. Subsequently, the particular material for the grayscale structure and the particular method for making the grayscale structure are selected. Next, an appropriate acceleration voltage is determined which enables the grayscale structure to be constructed with a practically usable thickness. The grayscale structure is then formed using a preferred method of formation. Subsequently, the selected ion is implanted with the determined dosage and acceleration voltage. Lastly, the grayscale structure is removed by either being stripped off or being polished away to obtain the final grayscale element.
Although the preferred embodiments of the present invention have been described and disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible without departing from the scope and spirit of the invention as set forth in the accompanying claims.
