Conventional photolithography

The radiation sources employed in microlithography include conventional (>300 nm) and deep-UV (<300 nm) light, electron-beam, ion-beam and x-ray sources. By far the predominant lithographic technology is conventional photolithography which... 

Positive Photoresists. All positive photoresists used for conventional photolithography are two-component systems and operate on a mechanism that involves destruction of a dissolution inhibitor. These resists are formulated... 

Very low cost, medium performance printed electronics A less controversial area is printed (or organic) electronics. The objective in this rapidly developing field is to develop alternatives to silicon and conventional photolithography as the basis for electronic systems151"153. Initially, the devices produced using this technology would have relatively low performance, but very low cost. These devices would be directed toward applications (for example, RF ID tags154) where one-time use would dictate cost and performance. 

Conventional photolithography with a negative photoresist SU-8 on a silicon wafer was used for manufacture [56], PDMS pre-polymer and a curing agent were mixed and poured on this master. The PDMS replica was separated and inlet and outlet holes were punched into the replica. The PDMS micro structure was bonded to glass slides by oxygen plasma treatment. 

FIGURE 2.23 Scheme for fabrication of plastic microdevices from silicon master using an intermediate soft mold, (a) Silicon structures are fabricated using conventional photolithography and reactive ion etching, (b) PDMS is cured in situ over the silicon master, (c) Polystyrene is hot embossed onto the PDMS mold or polymerized in situ from partially polymerized styrene, (d) Polystyrene replica is separated from the mold [85]. Reprinted with permission from Springer Science and Business Media. 

Alternatives to conventional photolithography will enable both reduction in cost and complexity for large-area electronics fabrication and will also help enable a transition from rigid substrates to flexible platforms. The transition towards roll-to-roll processing of flexible electronics may be required to follow the roadmap for low-cost large-area electronics. 

The challenge for jet-print patterned TFT devices is achieving sufficient precision and reliability to fabricate electronic circuits having the desired feature size and performance. This section discusses the feasibility of using phase-change etch masks patterned by jet printing, in place of conventional photolithography. 

The plastic film with interconnection layers, denoted (2) in Fig. 16.1, can be made by a process similar to that used to manufacture flexible circuit boards. First, plastic films coated with copper foil are processed by a numerically controlled (NC) drilling machine to make via holes. Plating is then used to make interconnections between top and bottom sides though via holes. Finally, the copper layers are patterned by conventional photolithography and etching. Gold plating is occasionally employed to improve electronic interconnections. 

Compared with conventional photolithography, laser drilling is a dry process and keeps the surface of polyimide gate dielectric layer away from water, etching solution, or other solvent, which often degrade the polyimide surface. We have confirmed that the electronic performance of transistors with laser via holes is identical with that without laser via holes. 

Figure 14 Fabrication procedure for the pile-up microreactor. (1) Photolithography Conventional photolithography/wet etching methods were applied. The back side of the glass plate was covered with polyolefin tape during the HF treatment. (2) Drilling Penetrating holes were drilled at the inlet and outlet ports of the micro-channel circuit. (3) Thermal bonding The required number of glass plates with microchannels and one cover plate were laminated and bonded thermally at 650°C.

Over the last decade, printed electronics has received substantial attention as a potential application of inkjet technology. Conceptually, the goal is to use printing technology as a replacement for conventional photolithography-based semiconductor manufacturing. This is expected to result in a substantial cost reduction for the realization of simple semiconductor systems on cheap, flexible substrates such as plastic, steel foils, etc. 

Figure 6.65. Comparison of (a) conventional photolithography/electroplating with (b) soft lithography. Shown in (b) is replica molding which consists of the formation of a PDMS stamp, and subsequent replication of a master in a photo- or thermally curable prepoiymer. Reproduced with permission from Gates, B. D. Xu, Q. Stewart, M. Ryan, D. Willson, C. G. Whitesides, G. M. Chem. Rev. 2005, 105, 1171. Copyright 2005 American Chemical Society.

The concept of a "minimum feature size is also important in fabrication of microdevices its actual dimensions are determined by the choice of fabrication process. As discussed below, conventional photolithography (405 or 436 nm) is generally limited to features of approximately 1 pm. Deep ultraviolet (UV) (230 to 260 nm) lithography has a minimum feature size of 0.3 pm, and x-ray and e-beam can be used to generate features as small as 0.1pm. 

Fig. 13.19 SEM images of the gap electrodes (a) the initial electrode pairs with the spacing of 5 mm fabricated by conventional photolithography (b) the nanogap with a separation of 56 nm obtained at an ac sources frequency/= 260 Hz and the series resistances Rj=R = 0. kV (c) the nanogap with a separation of 28 nm obtained at/= 260 Hz and Rj = R = kV (d) the nanogap with a separation of 9 nm at/=820 Hz and R = R = kV. Reprinted with permission from ref. [125]. Copyright 2005, American Institute of Physics...

On the other hand, there are many cases where the chemical or electrochemical reactions taking place on the surface are the rate-limiting steps in the patterning process. This will limit the patterning rate or will require development of other approaches whereby the whole pattern is made at the same time. At present, the common approaches used in SECM, i.e., the direct and feedback modes, cannot compete with the conventional photolithography methods. Nonetheless, future approaches such as multitip configuration may dramatically enhance the SECM capabilities in terms of speed of surface modification. 

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