Characterization and Annealing of Sputtered AlN Films for Piezoelectric Resonators
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447 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
processing conditions. In the current work, we have prepared AIN films by sputter deposition on (100) Si substrates. Degraded crystalline and electrical properties in AIN films have been attributed to both water and oxygen contamination interacting with a porous as-deposited AIN morphology. [9] Films of AIN deposited under ultrahigh vacuum conditions yielded high quality microwave resonators. Such films had relatively low levels of C and 0 (< 1 atomic %). Higher levels of both 0 and C resulted in inferior films for piezoelectric applications.[10] A typical oxygen concentration in the sputtered AIN film is in the range of 2 to 8 atom%. Oxygen content in the AIN film is reduced by (a) an increase in the sputtering rate, (b) a reduction in total gas flow rate and (c) an increase in substrate temperature. The presence of oxygen in AIN films is generally believed to be largely attributable to the hydrolysis brought about by water vapor in the ambient air.[ 1I] Experimental The as-deposited AIN films were annealed for two hours at 1000 'C in various sequential combinations of inert, oxidizing and reducing gases. In order to assess the chemical stability of the films with respect to exposure to aqueous processing solutions, each of the films was initially characterized by the methodologies listed below, exposed to boiling DI water for a period of 20 minutes and subsequently re-characterized by the referenced methodologies. Nf (Refractive Index) Studies The following table displays the refractive index of the as-deposited and annealed films that were measured before and after the boiling DI water soak. Table I - "Refractive Index Responses as a Function of Annealing Treatments and Exposure to Boiling Water." SampleAnneal Treatment b
Pre-Soak Nf
Post Soak Nf
None
2.068
1.270
c
Inert Gas, 2 hours
2.004
1.898
bc
Oxidizing Gas, 2 hours
1.776
1.389
ac
Reducing Gas, 2hours +
2.010
2.008
1.853
1.870
Inert Gas, 2 hours abc
Reducing Gas, 2hours + Oxidizing Gas, 2 hours
abcd Reducing Gas 2hrs. 2.024 1.990 The unannealed sample b experienced a large change in the Nf value from 2.07 (pre-soak) to 1.27 (post soak). Alternatively, the annealed sample ac exhibited almost identical Nf values for both pre-soak and post soak conditions.
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X-Ray Diffraction (XRD) Studies Reactive sputtered AIN films exhibit crystalline structures that may consist of three different crystallographic orientations: (0002),(10 10) and (11 20). The (0002) orientation corresponds to the AIN primary crystal structure while the(10 10) and the (11 20) orientations correspond to AIN secondary or short range order crystal structures. Table No. II - "X-Ray Diffraction Studies."
Sample b, as deposit b, soak ac,annealed
I(0oo2) Counts
kI1
Counts
l(0oo2/lAI
20o)
90105
96.92
930
15655
65.85
238
111293
50.77
2192
20)
1385 122303 88.31 ac,soak The annealing process for sample ac resulted in a significant crystallographic change to the preferred (0002) orienta
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