LDW-GLASS PHOTOMASK BLANKS

One Step Fabrication of A True Gray Level Mask

One Step 3D Photolithography Replaces Multi-Step Binary Photolithography

DOE FABRICATION: 3D SHAPING

Laser Direct Write (LDW) glass offers the advantages of a one step fabrication of a true gray level mask. The exposure of this gray level mask is done in a laser writing tool. This allows the use of the existing software previously written to support mask making and direct write on resist approaches for the fabrication of diffractive optical elements (DOEs). The so generated gray level mask can be used in an optical lithographic exposure tool (e.g., a G-line stepper, or a contact aligner) to mass fabricate resist profiles.

Using the LDW gray level mask fabrication and a following optical lithographic exposure, alignment errors are avoided, since the mask is written in a single step using different energy densities of a laser beam to generate gray levels. This new approach also allows a very economical mask fabrication. Instead of fabricating a set of 5 binary chrome masks with all the involved resist processing and wet etching, a single writing step without the need for any processing is needed. This single mask then contains all the necessary information previously contained in a set of 5 binary masks. Misalignment due to sequential printing of 5 binary masks in a set is completely avoided. After the LDW gray level mask is fabricated a series of single exposure in a step-and-repeat system can generate hundreds of DOEs on the same wafer. This wafer can then be processed with a single CAIBE step to transfer the DOE structure of a large number of different elements simultaneously into the substrate. Since the complete DOE structure is transferred into the substrate there is no need for a resist stripping step after the etching process. After dicing the wafer hundreds of monolithic multilevel DOEs have been generated by a process which cut the involved processing steps by more than a factor of 5.

LDW-glass photomask blanks are monolithic silicate glass plates with no coating of any kind. Standard LDW plates, i.e., LDW-HR plate -Type I and LDW-IR plate -Type I have been chemically treated to contain a large number density of coloring specks of silver within 1µm in the thickness dimension into the glass surface. A focused laser beam of any wavelength in the spectral range of near uv, visible (e.g., 514 nm, 632 nm and 647 nm), near infrared (e.g. 820 nm and 1060 nm) and infrared (e.g. 10.6µm) can be used to heat erase these coloring specks, causing a portion or all of the coloring specks of silver in glass to become colorless silver ions. The transmittance of LDW-glass plates increases with increasing writing-energy density of a focused laser beam. The required writing energy density is a function of the wavelength of write beam, writing velocity, i.e., the speed of laser sweep, the intensity profile of the focused laser beam and the value of %T at the desired gray level. For example, having been exposed to an energy density of 2j/cm² using a write beam at the wavelength of 514 nm and a writing velocity of 4 meters/sec, LDW-HR plate-type I becomes totally transparent.

At any given writing velocity, there exists an erasure-threshold-intensity IETh below which there is no change in optical density of LDW-glass plates even with multiple retraces. Using a write-beam intensity above the erasure-threshold-intensity IETh, the optical density of LDW-glass plates reduces with each additional retrace and the LDW-glass plates can be erased to a transparent state with multiple retraces. As the write-beam intensity increases further above the erasure-threshold-intensity IETh, retraces needed to bring about the transparent state decrease in number. LDW-glass plates are made transparent in one laser sweep i.e., no retraces at a full-erasure-intensity I_FE.

At any given writing velocity, there also exists an abrasion-threshold-intensity I_ATh at and above which the LDW-glass plates are abraded or damaged on the glass surface due to excessive temperature (>800°C) at the laser focused spot. However, the abrasion is not a pure thermal effect, since the abrasion-threshold-intensity IATh is lower using a write beam of a shorter wavelength.

At a given writing velocity, the write-latitude is defined as the difference IATh - IFE between the abrasion-threshold-intensity and the full-erasure-intensity. The write-latitude increases with decreasing writing velocity and also increases with a write-beam of a longer wavelength.

At a writing velocity of 1 to 4 meter/sec the required writing energy density for full erasure is 2 to 4 joule/cm2 using a write-beam whose wavelength in the spectral range of 488 nm to 1060 nm, provided the optical density of the LDW-glass plate is in excess of 0.5 at the wavelength of the write-beam.

The values of the writing energy density cited are based on experimental data using write-beams having a Gaussian intensity profile at the focused laser spot. One can expect the required writing energy density to reduce by a factor of more than 2 and the write-latitude increases, when a flat top intensity profile is utilized.

Multigray levels were written in LDW-glass plates using the writing velocity (e.g., the clock rate) or laser beam intensity or multiple retraces or a combination thereof as variable parameters.

A LDW-glass photomask with multi-gray levels is ideally suited for fabrication of diffractive optical elements (DOE), Micro-electro-mechanical (MEM) devices, Micro-Opto-electro-mechanical (MOEM) devices. A mask for 32 phase levels of DOE is made by exposing in a laser beam writer with predetermined energy density levels according to a depth versus optical density calibration curve such as that shown in Fig. 1 and Fig. 2. Fig. 1 shows the remaining thickness after development of Shipley S1650 photoresist versus optical density at 436 nm wavelength of LDW-HR plate. Fig. 2 shows the remaining thickness after development of OCG OeBR-514 photoresist versus optical density of LDW-HR plate at 436 nm wavelength. Both calibration curves were done with a vacuum contact aligner.