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ANNEALING THIN FILM PARYLENE COATINGS FOR MEDIA COMPATIBLE PRESSURE SENSORS

IP.com Disclosure Number: IPCOM000007775D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-23
Document File: 3 page(s) / 190K

Publishing Venue

Motorola

Related People

Gordon Bitko: AUTHOR [+4]

Abstract

Parylene C is being explored as an encapsulant coating for bulk micromachined silicon piezoresistive pressure sensor devices that will be used in harsh environments, including: salt water, acids, bases, and fuels. However, coated pressure sensors have been observed to undergo change in both sensitivity (volt- age output vs. pressure slope) and zero pressure out- put (offset) following the initial excursion above the glass transition temperature for parylene C (Ta s 80 - 100°C [l]). It is believed that during the annealing process, as a result of relaxation of resid- ual stresses formed during deposition and/or a recrystallization of the polymer chains, the parylene film undergoes changes in material properties, thus causing a shift in electrical output of the coated device. Once the coated sensors have been properly annealed, subsequent heating of devices above the Ts does not shift the electrical parameters. Further- more, the magnitude of the shift in offset can be correlated to the thickness of the parylene and the 2e+7 As-Deposited easurement maximum annealing temperature. This has been shown through two different experimental methods: time-based measurement of device electrical param- eters as a hmction of film thickness and anneal tem- perature, and wafer curvature stress measurements as a function of the same parameters. Also, finite element analysis (FEA) has shown the same results, and has been used to predict the expected parametric variations caused by known process variability.

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8 M-LA Technical Developments

ANNEALING THIN FILM PARYLENE COATINGS FOR MEDIA COMPATIBLE PRESSURE SENSORS

by Gordon Bitko, David J. Monk, H.S. Toh and John Wertz

  Parylene C is being explored as an encapsulant coating for bulk micromachined silicon piezoresistive pressure sensor devices that will be used in harsh environments, including: salt water, acids, bases, and fuels. However, coated pressure sensors have been observed to undergo change in both sensitivity (volt- age output vs. pressure slope) and zero pressure out- put (offset) following the initial excursion above the glass transition temperature for parylene C (Ta s 80 - 100°C [l]). It is believed that during the annealing process, as a result of relaxation of resid- ual stresses formed during deposition and/or a recrystallization of the polymer chains, the parylene film undergoes changes in material properties, thus causing a shift in electrical output of the coated device. Once the coated sensors have been properly annealed, subsequent heating of devices above the Ts does not shift the electrical parameters. Further- more, the magnitude of the shift in offset can be correlated to the thickness of the parylene and the

2e+7

As-Deposited easurement

maximum annealing temperature. This has been shown through two different experimental methods: time-based measurement of device electrical param- eters as a hmction of film thickness and anneal tem- perature, and wafer curvature stress measurements as a function of the same parameters. Also, finite element analysis (FEA) has shown the same results, and has been used to predict the expected parametric variations caused by known process variability.

  Modeling of the effect of parylene coatings on sensitivity has been published elsewhere [2], and it is in reasonably good agreement with experimental data. Zero pressure offset is affected to a greater degree than sensitivity by parylene annealing. Unfortunately, modeling of the effects of encapsulants on zero pressure offset is not nearly so advanced. From Figure 1, it is apparent that the zero pressure offset is related to the thermal stress of the applied thin film(s).

100 200 -100 0 100 200

Temperature ('73 Temperature ("C)

(a) (b)

Fig. 1 Measurement of (a) parylene coated wafers and (b) parylene coated pressure sensors. (a) Radius of curvature is measured using a FleXus wafer bowing system. These measurements are converted into stress values. The thermal stress is the stress value at a particular temperature (e.g., at room tempera- ture). (b) Output voltage ofan XPX 2010D pressure sensor was observed as a hmction oftemperature. A similar trend is observed assuming that offset voltage is proportional to negative stress values.

0 Motorola. 1°C. ,996 92 A u.qust 1996

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M-LA Technical Developments

To first order, the magnitude of the offset is pro- ratio of the film

CTE of the silicon

                                        the CTE of the film (a&, the port...