Flip chip method

An HgCdTe layer 2, comprising photodiodes 3, is formed on a first side of a CdTe substrate 11. The detector is bonded to a silicon chip 7 by a flip-chip process. A reflection preventive film is formed on a second side, opposite to the first side, of the CdTe substrate. The film is formed by a cyclotron resonance plasma CVD method by introducing nitrogen, nitrous oxide and silane as reaction gas. The thickness of the film is selected so that the reflectivity is minimized for radiation having a wavelength to be detected by the photodiodes. 

A method to manufacture an imager having a flip-chip structure is given in JP-A-57073984. The detector substrate is thinned and a slot is used to facilitate this operation. 

Further miniaturization is enabled by direct assembly of bare dies onto circuit carriers. This kind of component is electrically connected by wire bonding. Other methods for direct chip attachment are flip chip and tape automated bonding. All three methods require special equipment for processing and inspection. 

After mechanical attachment to a substrate, a leadframe, or to the inside of a package, bare die or chip devices are electrically connected by one of five methods wire bonding, flip-chip bonding, TAB, solder attachment, or attachment with electrically conductive adhesives. Fig. 1.6 shows some of these methods. 

MIL-STD-883 Method 5011 coordinated and released Thermoplastic film adhesives Silver-glass die-attach adhesives introduced First underfills for flip-chip devices... 

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

Bond strength tensile pull (ASTM D1002), die shear (MIL-STD-883, Method 2019), flip-chip strength (MIL-STD-883, Method 2031), substrate strength (MIL-STD-883, Method 2017), centrifuge (MEL-STD-883, Method 2001)... 

Visual (MIL-STD-883, Methods 5011 and 2017) Electrical stability (biased) (MSFC SPEC 592, Method 4.5.11) Moisture absorption (IPC-TM-650, TM 2.6.3.1, and ASTM D570) Bond strength tensile pull (ASTM D1002), die shear (MIL-STD-883, Method 2019), flip-chip strength (MIL-STD-883, Method 2031), substrate strength (MIL-STD-883, Method 2017), and centrifuge (MIL-STD-883, Method 2001) Residual gas analysis (MIL-STD-883, Method 1018)... 

The combination of cured adhesive interconnection, bumps, and organic substrate provides a reliable, cost-effective flip chip CSP. It was determined that Au-stud bumps were preferable to Ni/Au bumps because of the former s compliant nature. Since the planarity control of organic chip carriers is difficult, given the nature of the material, it was simpler to change the interconnection method, ie, using Au-stud bumps. However, electroless Ni/Au bumps are still an option, if the material properties of the ACF and/or chip carrier are altered to provide compliancy. 

Flip-chip technology, as shown in Fig. 11.14, is similar to TAB technology in that successive metal layers are deposited on the wafer, ending up with solder-plated bumps over the device contacts. One possible configuration utilizes an alloy of nickel and aluminum as an interface to the aluminum bonding pads. A thin film of pure nickel is plated over the Ni/Al, followed by copper and solder. The copper is plated to a thickness of about 0.0005 in., and the solder is plated to about 0.003 in. The solder is then reflowed to form a hemispherical bump. The devices are then mounted to the substrate face down by reflow solder methods. During reflow, the face of the device is prevented from contacting the substrate metallization by the copper bump. This process is sometimes referred to as the controlled collapse process. 

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