Soft bake

The resist formulation was spin-coated onto a silicone wafer on which a bottom antireflective coating had been previously applied and then soft-baked for 60 seconds at 90°C on a hot plate to obtain a film thickness of 1000 nm. The resist film was then exposed to i-line radiation of 365 nm through a narrowband interference filter using a high-pressure mercury lamp and a mask aligner. Experimental samples were then baked for 60 seconds at 90°C on a hot plate and developed. The dose to clear, E0, which is the dose just sufficient to completely remove the resist film after 60 seconds immersion development in 2.38% aqueous tetramethyl ammonium hydroxide, was then determined from the measured contrast curve. Testing results are provided in Table 1. 

Many modifications of this basic chemistry have been explored to tailor these resists to deep-UV radiation. For example, changes have been made in the sensitizer so that it bleaches in this wavelength region. Early work in this area was performed on diazo-Meldrum s acid (54) (see structure). This compound functioned as a deep-UV-bleachable dissolution inhibitor however, it was somewhat volatile and, consequently, could be depleted via evaporation during soft bake. More-recent studies have therefore focused on less-volatile sensitizers incorporating heteroatom substitution (55) and on increases in molecular weight (56). 

In a first step, the negative working photoresist SU-8 is spin-coated on to the disk and soft baked [110]. The disk is then UV-exposed to pattern the bottom layer. A silver thin metal layer is thereafter evaporated. The metal layer is spin-coated with an AZ-type photoresist, dried, exposed and developed. In this way, the metal layer can be developed independently from the patterning of the SU-8 layer underneath. The metal layer is patterned by wet-chemical etching. As a next step, a second SU-8 layer is deposited, soft-baked and exposed. Top and bottom layers are now developed. After a hard bake, a second CaF2 disk is attached to the stack and sealed by a light-curing epoxy resin. 

The photoresist SU-8 has been used directly to fabricate microstructures. In order to create 400-pm-high micropillars, a 400-pm-thick SU-8 layer was used. To reduce the risk of bubbles and to ensure uniformity in the SU-8 layer, two layers of 200-pm SU-8 were consecutively deposited, each with a post-exposure soft-bake step. The exposed SU-8 layer was developed using propylene glycol methyl ether acetate [236]. 

What are the purposes of soft bake and hard bake after photoresist is coated on a wafer [321] (4 marks)... 

A 30 wt% solution of the chromophore was mixed with the Step 2 product dissolved in cyclopentanone and then spin-coated onto 2" glass wafers coated with indium tin oxide the wafer soft baked at 100°C. A corona voltage of 5 kV was applied to the film while it was heated to 220°C for 10 minutes. While at this temperature the voltage was... 

A electrooptic polymer crosslinked film was formed by spin-coating a 25 wt% of the Step 12 product in cyclopentanone onto an ITO covered glass slide. The solution was filtered through a 0.2 pm nylon filter, spin-coated at 500 rpm for 6 seconds and 1000 rpm for 30 seconds, and then soft baked at 50°C overnight under vacuum to give a 3.2 pm thick film. The film was corona poled with a needle at 20 kVand heated to 220°C for 5 minutes for crosslinking. The film was then cooled to ambient temperature under the applied field to give an electro-optic film with an raa of 36 pm/V at 1.31 pm. 

After the photoresist layer is spin-coated onto a substrate, before it is exposed to radiation and further development, a soft-bake step often takes place. In this soft-bake step, the photoresist-coated substrate is placed inside a low-temperature oven to be baked at a temperature range approximately 90-100°C for a few minutes to about 30 min. This soft-bake procedure intends to remove the solvent from the photoresist, enhance the adhesion to the substrate, and release the internal stress of the photoresist film. In essence. 

Soft bake of the resist to remove residual solvents in the resist, using either a hot plate or an oven. 

Exposure. Before the coated wafers are exposed, they undergo a mild bake (called a soft bake) in order to remove the solvent of the resist and to suppress mechanical stress. Once the wafers are soft baked, they are transferred to some type of illumination or exposure system. In the simplest case, an exposure system is a UV lamp illuminating the resist-coated wafer through a mask without any lenses in between. The purpose of the illumination system is to deliver light to the wafer with the proper intensity, directionality, spectral characteristics, and uniformity across the wafer so as to transfer (also print) the mask image as perfectly as possible onto the resist in the form of a latent image. 

Figure 7.7 SEM of high-resolution images printed in AZ 7900 (DNQ-sulfonate/novolac positive i-line resist). Processing conditions resist thickness 0.748 iJim, BARC(BARLi) thickness 1920 A, soft bake at 90°C for 90 s, exposure tool Canon 300014, NA 0.63, sigma 0.65, PEB 110°C for 60-s, 64 s single puddle NMD-W developer at 20.5X. (Courtesy of R. Damrnel. )...

Figures 11.14(a)-(d) show the mechanisms of different physical processes taking place during the first 10 seconds of soft baking a typical resist on a vacuum hot plate. Figure 11.14(a) shows that the temperature of the wafer increases with an initial rapid rise during the first seconds of baking, before leveling off after three seconds. Figure 11.14(b) shows that the solvent loss during this time...

Typical soft bake temperatures are 90-120°C for 60 seconds on a hot plate, followed by a cooling step on a chill plate to achieve uniform wafer temperature control. The boiling points of most resist solvents are on the order of... 

Figure 11.16 Effect of soft bake temperature on dissolution rates of novolac resin and exposed DNQ/novolac resist. (Reprinted with permission from American Chemical Society. )...

The soft bake process can affect the solubility properties of some resists in the developing solvent. For instance, in DNQ/novolac resists, the solubility of the exposed resist as a function of prebake temperatures shows a maximum at around 120°C (Fig. 11.16). Four zones can be distinguished in this plot (i) a no-bake zone where residual solvent and dissolution rates are high, (ii) a low-temperature zone (up to 80°C) where the dissolution rate shows an appreciable decrease due to solvent removal, (iii) a mid-temperature zone (80-110°C) where the DNQ is thermally and preferentially converted to indene carboxylic acid, leading to an increase in dissolution rate, and (iv) a high-temperature zone (>120°C) where the film densification takes place, DNQ is thermally decomposed, the film is depleted of water, and the novolac resin is cross-linked, resulting in dramatic dissolution inhibition. ... 

Once the resist-coated wafer has been soft baked in the track system, it is cooled and sent into the wafer stage of the exposure tool. Resist processing equipment is commonly interfaced directly with an exposure tool, in which case the wafer... 

It should be pointed out that the solubility change in all chemically amplified resists (CARs) occurs only in the dark reaction during the PEB. Some of the CARs such as acetal and ketal systems have low activation energy, so deprotection can occur with low-temperature PEBs or even at room temperature. In the absence of thermally driven diffusion such as in acetal and ketal resist systems, BARCs must be used for resists with such low PEB temperatures, which are significantly lower than the soft bake temperature, in order to control reflectivity issues associated with standing waves. 

Figure 13.36 Resolution capability of AZ HiR 1075 i-line photoresist used in printing line/space features with 1 1.5 pitch. Processing conditions Film thickness 0.66 iJim on 1300A AZ BARLi II BARC, soft bake (proximity) 90°C/60 seconds (proximity). Exposure tool ASML/400 Scanner. Exposure conditions Dose 170 mJ/cm, annular illumination, NA 0.65, partial coherence a (outer/inner) 0.85/0.55. Postexposure bake (proximity) 110°C/90 seconds. Development 2.38% tetramethyl ammonium hydroxide developer/single puddle for 60 seconds at 21.0°C. (Courtesy of R. Dammel. °)...

Figure 13.55 Performance of Shipley s XP0589A resist on SiON BARC on Excitech 157-nm microexposure tool with NA 0.85 and u 0.3. Reticle phase-shifting mask. Soft bake 110°C/60 seconds. PEB 110°C/60 seconds. Resist thickness 200 nm. (Courtesy of the Shipley Corporation.)...

Figure 17.10 shows the process sequence of the hydrophilic overlayer (HOL) process. First, a chemically amplified or non-chemically amplified positive-tone photoresist comprising hydrophobic polymer and appropriate PAG is coated to a nominal thickness on an appropriate substrate such as a silicon wafer, followed by a soft bake to dry out the nonaqueous solvent. Next, the photoresist film is exposed to radiation of appropriate wavelength to generate photoacid from the PAG. Then the exposed film is again baked (called PEB) at the standard temperature to enhance the diffusion of the photoacid and thermolysis of the acid-labile protecting groups of the polymers. 

The cleaned and silanized substrates were selectively patterned by photolithography using an electrically insulating photoresist [49]. A thin SU-8 2000.5 photoresist layer was spun at 6,000 rpm, soft-baked, exposed using a MJB4 mask aligner (60-80 mJcm ), post-baked, and developed in PGMEA for 30 s [49]. The applied soft and post-bake procedure was as follows 1 min at 65 °C, 1 min at 92 °C, and 1 min at 65 °C. 

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