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Self-Assembled Monolayers

The stiction problem during drying that was presented earlier can also be avoided if a hydrophobic layer is coated onto the structure. One method of doing this is the application of a self-assembled monolayer (SAM) [25]. The SAM precursors used for this application are straight-chain hydrocarbons, such as octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3), with a chemical group at one end that adheres to silicon, silicon-dioxide, and silicon-nitride surfaces. These head groups naturally pack tightly onto the surface and crosslink, leaving the tails sticking straight up away from the surface. The coating self-limits at one molecule of thickness and is hydrophobic.

In a SAM-coating process, the structures are released and rinsed in water as usual, then soaked in a solvent miscible with water. The wafer may be moved to an intermediate solvent compatible with the first solvent and the subsequent SAM solvent. The wafer is then placed in a solution containing the SAM precursor and held for a few minutes, during which the coating occurs. Finally, it is rinsed and dried, which may be done on a hot plate or under a heat lamp. Due to the hydrophobicity of the SAM-coated surface, the contact angle changes, and the water does not pull compliant structures down to the substrate. An added benefit is that if the structure ever does touch down during operation, it will not stick, as it might otherwise do without the coating. SAM coatings have also been studied as a dry lubricant and found to prolong the life of micromachined parts sliding in contact, eventually wearing out [25]. SAMs decompose at high temperatures (~350°C).

Processes for Micromachining-Lecture 5 – стр. 12


 

22. Технология изготовления микрошарниров.


 

32. Технология термокомпрессионной и ультразвуковой сварки внешних выводов.

Thermosonic gold bonding is a well-established technique in the integrated circuit industry, simultaneously combining the application of heat, pressure, and ultrasonic energy to the bond area. Ultrasound causes the wire to vibrate, producing localized frictional heating to aid in the bonding process. Typically, the gold wire

forms a ball bond to the aluminum bond pad on the die and a stitch bond to the package lead. The “ball bond” designation follows after the spherical shape of the wire end as it bonds to the aluminum. The stitch bond, in contrast, is a wedge-like connection as the wire is pressed into contact with the package lead (typically gold

or silver plated). The temperature of the substrate is usually near 150ºC, below the threshold of the production of gold-aluminum intermetallic compounds that cause bonds to be brittle. One of these compounds (Au5Al12) is known as purple plague and is responsible for the formation of voids—the Kirkendall voids—by the diffusion of aluminum into gold. Thermosonic gold bonding can be automated using equipment commercially available from companies such as Kulicke and Soffa Industries, Inc., of Willow Grove, Pennsylvania.

Bonding aluminum wires to aluminum bond pads is also achieved with ultrasonic energy but without heating the substrate. In this case, a stitch bond works better than a ball bond, but the process tends to be slow. This makes bonding aluminum wires economically not as attractive as bonding gold wires. However, gold wires are difficult to obtain with diameters above 50 μm (2 mils), which makes aluminum wires, available in diameters up to 560 μm (22 mils), the only solution for highcurrent applications (see Table 8.3).

The thermosonic ball bond process begins with an electric discharge or spark to melt the gold and produce a ball at the exposed wire end (see Figure 8.4). The tip—or capillary—of the wire-bonding tool descends onto the aluminum bond pad, pressing the gold ball into bonding with the bond pad. Ultrasonic energy is simultaneously applied. The capillary then rises and the wire is fed out of it to form a loop as the tip is positioned over the package lead—the next bonding target. The capillary is lowered again, deforming the wire against the package lead into the shape of a wedge—the stitch bond. As the capillary rises, special clamps close onto the wire, causing it to break immediately above the stitch bond. The size of the ball dictates a minimum in-line spacing of approximately 100 μm between adjacent bond pads on the die. This spacing decreases to 75 μm for stitch bonding.


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