High-aspect-ratio structures

The influence of the pattern aspect ratio on particle assembly will be illustrated in two different situations. 

The first example concerns deep, enclosed structures such as those presented in Fig. 15.15a,b. As illustrated in the previous example, the dewetting mechanism in such structures is modified 

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Becker H., Heim U., Hot embossing as a method for the fabrication of polymer high aspect ratio structures, Sensor. Actuat. A-Phys. 2000 83 130-135. 

Abstract A crucial problem in the manufacturing of high aspect ratio structures in the microchip production is the collapse of photoresist patterns caused by imbalanced capillary forces. A new concept to reduce the pattern collapse bases on the reduction of the capillary forces by adsorption of a cationic surfactant. The application of a cationic surfactant rinse step in the photolithographic process leads to a reduction of the pattern collapse. Physicochemical investigations elucidate the mechanism of surfactant adsorption... 

Fig. 3 SEM picture of the high aspect ratio structure made of PMMA by the LIGA process. Typically, the structural height (depth) is 100-500 jizm and the feature size is 2-20 //m thus, the aspect ratio is between 5 and 100...

Table 1 gives typical results obtained for PTFE, PFA, FEP, and PMMA. The typical high aspect ratio structures generated by the SR etching can be seen in Fig. 15 for the PTFE case. Figure 16 shows the etching rate versus the beam current (i.e., photon flux density) at various sample temperatures, in which the PTFE was etched in vacuum by the l-/zm-thick Al-filtered beam. 

Insulating materials, such as glass, quartz, alumina, and porous silicon, and their microfabrication methods to create high-aspect-ratio structures compatible with wafer bonding... 

The benefits of the Bosch process are its accurate profile control in high aspect ratio structures and its high etch rate. Etch rates up to 50 pm/min have been reported in special cases, with 10-20 pm/ min to be considered as more typical production etch rates. 

Practical (D)RIE processes exhibit a number of limitations regarding etch rate, etchable area, feature size, and shape, all of which become more prominent for high aspect ratio structures [6]. Also the sidewall profile non-idealities are discussed. 

DRIE enables high aspect ratio structures [8, 9]. Holes, trenches, and pillars with 10 1 to 20 1 aspect ratios can be fabricated, and experimental structures with 40 1 and even higher have been demonstrated. These high aspect ratio structures are limited by RIE lag, however. Applications for high aspect ratio structures include various trapping devices. Depending on feature size, either micro- or nanobeads are trapped or macromolecules like DNA. For pillars, aspect ratios in excess of 50 1 have been demonstrated 300 nm feature size and 15 pm structure height. 

A simple printing scheme such as shadow masking (i.e., proximity or contact printing) can be used while stiU producing high aspect ratio structures with relatively straight sidewalls. 

FIGURE5.il Conformal deposition over high-aspect-ratio structures can be difficult to achieve by CVD due to keyhole effects and other difficulties posed by the long diffusion paths required to access the inner regions of such complex structures. 

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