Chips Polyimides

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

Manufacture of Printed Wiring Boards. Printed wiring boards, or printed circuit boards, are usually thin flat panels than contain one or multiple layers of thin copper patterns that interconnect the various electronic components (e.g. integrated circuit chips, connectors, resistors) that are attached to the boards. These panels are present in almost every consumer electronic product and automobile sold today. The various photopolymer products used to manufacture the printed wiring boards include film resists, electroless plating resists (23), liquid resists, electrodeposited resists (24), solder masks (25), laser exposed photoresists (26), flexible photoimageable permanent coatings (27) and polyimide interlayer insulator films (28). Another new use of photopolymer chemistry is the selective formation of conductive patterns in polymers (29). 

Optical properties of the material are less critical for microchips hyphenated with MS than for devices with on-chip optical detection where low background absorption or fluorescence is mandatory. Thus, completely opaque polymers like glassy carbon or polyimide " can be used as microfabrication substrates. Furthermore, polymer microchips are of great interest because their potentially low manufacturing costs may allow them to be disposable. Methods used for the fabrication of plastic chips include laser ablation and molding methods. 

Metz, S., R. Holzer, and P. Renaud. Polyimide-based microfluidic devices. Lab Chip 1, 29-34 (2001). 

A coating of polyimide with a thickness of no less than 3 mil over the memory chip will practically eliminate this problem. 

Figure 21. Nine-chip microprocessor module with TFML copper-polyimide interconnections on an 80 by 80 mm ceramic substrate. Top, completed package populated with tape-automated-bonded lCs bottom, internal signal interconnection layers. (Reproduced from reference 80. Copyright 1987 American...

A novel method for achieving low thermal impedance between the chip and substrate has been developed at Stanford University (163-165). Reentrant microcapillaries (3-5 (xm wide) are etched into the substrate (either a silicon wafer or a ceramic coated with a polyimide) and partially filled with a fluid such as silicone oil. This structure increases the heat-transfer area, reduces the interfacial thermal resistance, and provides a stress-free, re-... 

S. Metz, S. Jiquet, A. Bertsch and P. Renaud, Polyimide and SU-8 microfluidic devices manufactured by heat-depolymerizable sacrificial material technique, Lab Chip, 4 (2004) 114-120. 

An excimer laser was also used to machine a PC chip (6 mm thick) to create 160-pm-wide channels (60 pm deep), [811] or on a polyimide sheet [192,811]. Another UV excimer laser (248 nm) was used to ablate microstructures within PC channels (fabricated by imprinting) [193]. 

An all-polyimide microchannel chip was created by patterning two photosensitive polyimide layers, and then by laminating the two layers based on a partially imidized interfacing layer [228,229]. 

A glassy C electrode, which was immobilized with tyrosinase, was used for amperometric detection of phenol in a polyimide chip. Phenol was enzymatically converted to catechol, which was then oxidized to quinone during detection. Chlorophenol can also be detected, but this is achieved after a dechlorination step (to phenol) using a Mg/Pd metal catalyst [229]. 

Voltammetric techniques have also been employed for on-chip EC detection. For instance, anodic stripping voltammetry (ASV) was used to detect Pb in a Si-Pyrex chip [770] or a polyimide chip [229]. 

On the other hand, electrical impedance measurement of bacterial suspensions (.Listeria innocua) was carried out on a Si-glass chip. This method was used to perform a cell viability test. This is because the metabolic products produced from viable cells modify the ionic strength of a low-conductivity medium, significantly altering its electrical characteristics. Later work also involved the detection of the presence of small numbers of bacterial cells in a Si chip 100 L. innocua cells, 200 L. monocytogenes cells, and 40 E. coli cells [93,885], In another report, impedance spectroscopy has been used for analysis of erythrocytes in a glass-polyimide chip [886]. 

A non-contact heating method called infrared-mediated temperature control has been employed for PCR. Because the chip material (polyimide) does not absorb IR, only the solution absorbs IR. Therefore, the low thermal mass of the solution allows for fast thermal cycling, and 15 cycles have been achieved in 240 s Amplification of X phage DNA (500 bp) was first conducted at 94°C for 10 s, followed by 15 cycles of 94°C (2 s), 68°C (2 s), 72°C (2 s), then finally stopped at 72°C for 10 s [192],... 

Efforts continue in the Far East, particularly in Japan, by Horie et al., on photosensitive polyimides containing epoxide groups [44,45]. These studies focus on the chemical amplification of photo crosslinks in the resulting materials for use in making mask materials in silicon chip fabrication. 

The main source of the alpha particles is trace quantities of uranium and thorium in the silica filler. Because silica fillers that did not contain these radioactive elements were not available, other methods for preventing alpha particles from reaching the active DRAM cells were devised. These early methods consisted of cov-vering the active cells with either a silicone or polyimide chip coat or with Rapton tape. These methods added extra steps to the manufacturing process which were cumbersome and labor intensive and, if not done precisely, had a negative reliability impact. These processes were not widely used once "low alpha fillers" became commercially available in 1982/1983. Initially, these "low alpha fillers", which contain <1 ppb uranium, were only available from one or two natural sources. Now, however, there are additional natural and synthetic sources of silica, all of which contain <1 ppb of uranium and have an alpha particle emission rate of less than. 001 alpha particles/hr-cm. Figure 9 shows where the industry was in 1980 and where it stands today. An improvement by a factor of 30-50 has been achieved with "lower alpha" filler and compound manufacturing. 

One method to minimize leakage and improve device reliability is the use of chip overcoats. Silicone or polyimide chip overcoats... 

Epoxy resins are almost exclusively used for such chip-attach purposes polyimides are used to a lesser extent. Polyimides provide excellent protection in accelerated humidity tests, but it is difficult to obtain void-free... 

Although the surface of most IC chips has been passivated with a layer of inorganic dielectric material such as silicon dioxide or silicon nitride (polyimides have also been used as final passivating layers), the protection provided by such layers is not sufficient to ensure reliable operation throughout the lifetime of the device. The three basic methods of protection are... 

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