Effects of process history and aging on the properties of polyimide films
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Polyimide is used extensively in a variety of integrated circuit packaging applications. It is a good dielectric material with excellent planarizing capabilities, but like most polymers, it absorbs moisture. This hygroscopic behavior can lead to reliability problems in integrated circuit packages. The effects of variations in process history on moisture uptake are examined using gravimetric measurement techniques. In particular, the effects of cure schedule and exposure to high temperature/high humidity environments (85 °C/85% RH) on steady state moisture uptake are reported. Steady state moisture uptake is shown to be a decreasing function of cure temperature. Samples cured at 250 °C absorb 25% more moisture by weight than do samples cured at 400 °C. Moreover, the steady state moisture uptake in polyimide is greater after the samples have been "aged" in a high temperature and humidity ambient. The bulk and surface chemical composition are also monitored as a function of aging using Fourier transform infrared spectroscopy (FTIR) and electron spectroscopy for chemical analysis (ESCA), respectively. The PI surface chemistry degrades after 700 h in an 85 °C/85% RH environment. The bulk chemical composition appears to be unaffected.
I. INTRODUCTION Polyimide (PI) is used extensively in the microelectronics industry in a wide variety of applications which include interlayer dielectrics in integrated circuits,1^1 inter-metal insulators in high density interconnect packaging schemes,4"6 and thermal-mechanical passivation buffer protection layers.1'3 PI has several advantages that make it attractive for microelectronics applications. It has a low dielectric permittivity (er is 3.2 to 3.4 at audio frequencies7), it is an excellent planarizer,3'8"10 and it is thermally stable up to 400 °C.1>3 Moreover, the driving forces in VLSI technology include the use of dry etching and low temperature processing. PI can be plasma etched11 and fully processed at temperatures of 400 °C or below. In spite of these advantages, there are potential problems associated with PI usage. The foremost of these is that PI absorbs moisture.7 This hygroscopic behavior can lead to reliability problems in integrated circuits such as increased insulator conductivity, loss of adhesion, and corrosion.10 In this paper, we characterize the hygroscopic behavior of PI as a function of process history using gravimetric and spectroscopic measurements. The gravimetric measurement technique yields a direct indication of the amount of water absorbed by the PI film. The samples are prepared using standard integrated circuit fabrication techniques, and the measurements are in situ and nondestructive. Therefore, the results provide insight into the behavior of PI films as they are used J. Mater. Res., Vol. 6, No. 12, Dec 1991
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in microelectronic applications. The PI structures under investigation are exposed to different ambient relative humidity (RH) values and the weight change is monitored. We investigate the
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