Stress and Microstructure in RF-Diode and DC-Magnetron Sputtered Beryllium Thin Films
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STRESS AND MICROSTRUCTURE IN RF-DIODE AND DC-MAGNETRON SPUTTERED BERYLLIUM THIN FILMS RISTO H. MUTIKAINEN Technical Research Centre of Finland, Semiconductor Laboratory, Otakaari 7 B, SF-02150 Espoo, Finland ABSTRACT The effect of deposition parameters on the properties of sputtered Be films has been studied. The parameters have been optimized to obtain stress free films. Nitrogen pulsing has been used to improve the film microstructure by suppressing the columnar grain growth. INTRODUCTION Beryllium is an interesting material to deposit by sputtering it has very low atomic mass compared with typical because sputtering gases. On the other hand, beryllium films are known to and fragile. These properties are strongly affected by be brittle the film microstrucure. In general, reducing the grain size in a polycrystalline material increases the yield strength and also raises the ductility. To get insight to the dependence of the film properties on the sputtering process, we have compared various deposition schemes for beryllium including nitrogen pulsing. The affecting the film stress and the columnar parameters microstructure have been particularly addressed. EXPERIMENTAL PROCEDURE The films were sputter deposited in a vacuum system equipped with rf-diode and dc-magnetron (Leybold PK-75) 3" dia cathodes with target to substrate distances of 56 and 48 mm, respectively. A cylindrical shield of 136 mm dia with 3 mm opening in the middle was used to suppress the film deposition to the chamber. The 3" oxidized silicon wafers were clamped to water cooled copper blocks. A turbomolecular pump was used to evacuate the chamber to a base pressure better than 100 gPa. Argon was used as sputtering gas. The sputtering targets were of 99.92% and 99.8% purity, respectively, with oxygen as the major impurity. The deposited film thicknesses ranged from 700 nm to 1.7 pin. Nitrogen pulsing was used to suppress columnar grain growth in the dc magnetron sputtered films. The pumping speed was kept constant in all cases to maintain the nitrogen pulse dynamics. The nitrogen was pulsed with a shut off valve and the gas flow was controlled with a leak valve. The "hard" pulse refers to downstream shut off valve, the "soft" pulse downstream leak valve. The The sputtering gas residence time was measured to be 0.23 s. adjusted such that the nitrogen flow during the pulse was sputtering pressure increased with 0.20 Pa. The pulse duration was set to 1 second. The hard pulse introduced a very short nitrogen burst up to 0.95 Pa followed by fast pump down. The soft pressure pulse with no overshoot is shown in figure 1 with the accompaning sputtering voltage drop in constant power mode at 300 W. Pressure was measured with an MKS manometer of 16 ms rise time. To measure the amount of nitrogen incorporated in the film, RBS analyses were carried out on samples deposited on graphite substrates. X-ray diffraction (Cu Ka, intensity vs. 20) was used to determine the crystalline structure of the films. Beryllium has hexagonal crystal structure with a=2.286 A and
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