Temperature
8.1 UV Lithography
were subsequently re…ned through the use of vendor (Clariant Corp.) documentation for the photoresist and further process development, will be discussed. This is followed by details in the measurement technique and preliminary results from the fabricated devices.
A summary of the results concludes this chapter.
high thermal stability (up to 150 C) to maintain accurate pattern transfer during ion mill and reactive ion etching. In addition, the AZ 5200 Series resist can image reverse to a negative tone using a simple post-exposure bake (PEB), which adds extra versatility to the process. The last two digits of the numerical designation correlate to the viscosity of the photoresist, which, in the case of 5214, would give a coating thickness of 1.4 m at 4000 rpm spin for 40 seconds. A suitable amount of resist should be applied for optimal coating of the sample without inundating the edges and back surface with the resist. It is noted here that because the substrate size is a mere 1 1 cm square, there is an aggregation of photoresist, called the edge bead, near the edges and at the corners of the sample due to surface tension, after the resist is spun on the substrate. Successful patterning of structures on the order of microns hinges on the proper removal of the edge bead.
8.1.3 Softbake
Softbaking removes most of the remaining solvent from the photoresist …lm, thereby densifying it. The softbake time and temperature can in‡uence adhesion, photospeed, and dimensional control of the photoresist. Also, they a¤ect the selectivity of the developer between exposed and unexposed regions of the photoresist. Monitoring the amount of un- exposed resist loss during development provides an e¤ective control parameter for process optimization, given that the unexposed resist loss during development changes inversely to softbake time and temperature. A softbake temperature of 95 C for only 45 seconds on a hot plate is applied to the sample to optimize the sensitivity and contrast of the photore- sist. The short bake time does not appear to compromise the adhesion and stability of the resist.
8.1.4 Photomask and Exposure
The photomask use to exposed the pattern is …rst virtually designed in the software L-Edit by Tanner Research. Other software like the more popular AutoCad can also be used. The basic idea in the mask design is to separately write the pattern for each layer into its own die. The patterning for the heterostructure injection devices includes four major stages, i.e., the CMR mesa, YBCO bridges, Au contacts, and contact wires, so four die patterns are needed in the design. Although within the design program, the layers could be viewed as overlapping patterns, it is important to add ample number of position markers through the die to allow for swift and accurate alignment of each pattern relative to its precursor. For edge bead removal, an additional die with a simple opaque square pattern nominally smaller than the sample should be included on the mask design.
The …ve die patterns are written to the same photomask (4 4in.), made of either quartz or white crown glass, with the former being the most expensive option. On one side of the glass is a layer of chrome, on which the die patterns are written. The chrome is covered with an anti-re‡ective coating. The patterns are written on the photomask either by e-beam or photolithography. E-beam lithography is used to make the photomask, because the smallest feature requirement in this design is 1 m.
The photosensitivity of the AZ 5214-E photoresist in positive tone is suitable for the 313, 334, 365, and 405 nm emission peaks of the standard mercury vapor light source.
These wavelengths can be used to achieve resolution of one micron and below in the 1.4 m thick resist …lms. It is important to achieve close contact between the mask pattern and the sample covered with the photoresist …lm; in general, soft contact mode, which involves
bringing the sample and the mask to light mechanical contact, can be used for large features in the patterning. For structures on the order of a few microns, it is necessary to activate the hard contact vacuum mode. Here a vacuum gasket seals against the photomask the volume near the sample, and air is evacuated via a small pump, pressing the sample ‡ush against the photomask to ensure very tight contact. When using this mode, it is important to use the edge bead removal die to overexpose and then develop out the edges of the photoresist, because these large edge bead, if not removed, would physically impede close contact between the sample and the photomask. Depending on the intensity of the mercury lamp, typical exposure times for AZ 5214-E resist range between 12 to 36 seconds; however, for edge bead removal, exposure of several minutes can be used.
8.1.5 Development and Rinse
The developer used is an odorless aqueous AZ inorganic sodium-based developer.
It is bu¤ered to maintain a uniform pH and to provide maximum bath life and process stability. The dilution used is 1 part developer to 1 part deionized (DI) water. This dilute concentration enables higher contrast and provides greater selectivity between the exposed and unexposed resist. A longer development time of approximately 1 minute may be necessary. The developer should be maintained at a constant temperature ( 1 C) within the range of 20-25 C. The sample upon completion of development should be rinsed immediately with DI water to halt the developing process. To dry the DI water to avoid prolonged exposure of YBCO to water, the sample can be spin dried or forced dry with …ltered nitrogen.
8.1.6 Postbake and Photoresist Removal
Postbaking, also called hard baking, will generally improve image stability and adhesion as well as plasma and chemical resistance. The postbaking parameter can vary widely, but for this application, baking on a hot plate at 105 C for 20 minutes is recom- mended. After the etch process is complete, the photoresist generally can be removed by soaking in acetone …rst for 10 minutes and then in ultrasound for an additional 5 minutes.
Isopropanol and ethanol solvents typically follow to rinse o¤ the remnant residue.