Evaporation of Adherent Al-Films on Single Crystalline LiNbO3 Substrates
Evaporation of Adherent Al-Films on Single Crystalline LiNbO3 Substrates
Department of Electrotechnics, Technical University 0-2400 Wismar, Germany LiNbO3 is used as a single crystalline substrate material for the manufacturing of surface acoustic wave devices as frequency selective components. On the polished substrate wafer, AI with a film thickness of about 400 nm is evaporated in a high vacuum. The difficulties of the inadequate adhesion of the structured Al-film were eliminated by systematical investigations. Films that were adherent to the total surface could be prepared by evaporating an intermediate film of Cr or SiOx or, otherwise, a mixed film of AI with Si. An ultrasonic test for adhesion strength was developed that results in an assessment of local and also large-area film adhesion. I. INTRODUCTION For electronics, Lithium niobate (LiNbO3) is a very interesting material. Because of its crystal class 3m, LiNbO3 is trigonal and polar. It is a ferroelectric material with large pyroelectric, piezoelectric, electro-optic, and photo-elastic coefficients. Therefore, static electric charges may occur on the substrate surface because of temperature variations. A detailed description of the crystal structure and the physical characteristics of the LiNbO3 are given by WEIS and GAYLORD 1. The growth of the LiNbO3 single crystals can be made by the Czochralski method. To avoid areas of different polarity in a crystal, the polarity during the growing of a crystal above the Curie-temperature of 1140C can be set by an electric field 2,3. A field of application of LiNbO3 is the manufacturing of surface acoustic wave devices. For that purpose a metal film is evaporated onto a polished single crystalline substrate wafer in high vacuum. An interdigital structure is then generated by means of photolithography. The interdigital structure consists of two "combs" of different shape and size. The structural widths of electrodes of the interdigital structure are less than 15 tm. The frequency range of the filters depends on the structural width; frequencies of more than 300 MHz require electrode widths of about 1/zm. The electrical contact is created by bonding of Al-wires to the structured metal film. A good adhesion strength of the evaporated metal film on the vibrating surface is absolutely necessary for the function of the filters. Concerning the yz cut there were problems of adhesion bonds with the 38-cut. II. EXPERIMENT In production, the polished LiNbO3 wafers are given a short treatment with various chemicals. Therefore, tests of various pretments of the LiNbO3 wafers were made. After a standard cleaning with acetone in an ultrasonic bath, the wafers were treated at 20C and 50C for 5 min and 20 min with the following chemicals: 1. NH3 HF, 20% solvent 2. Chromium-sulphuric acid 3. NaOH, 20% solvent 4. H3 PO4, concentrated solvent 5. H2 F2, concentrated solvent An assessment of the efficiency of the chemicals with respect to the surface of LiNbO3 wafers resulted from electron microscopic photographs. In another test series, standardized cleaned and pre-treated glass and LiNbO3 substrates were coated under the same conditions with A1 by evaporization. These tests should show differences between the adhesion of A1 films to amorphous glass and crystalline LiNbO3. In general, the evaporation was done from W-boats on the substrates in vacuum with and without a glow discharge. For the manufacturing of a mixed film, AI and the additives Cu, Si and Ni were evaporated from separate boats. The thickness of the mixed film was 20 nm to 50 nm. Above it, pure A1 was evaporated up to a thickness of 400 nm. For the manufacturing of intermediate films, SiOx as an insulator or Cr as a metal were first evaporated to a maximum thickness of 30 nm, followed by pure AI evaporated up to a total thickness of 400 nm. To investigate the adhesion of the evaporated films a test method was developed. The samples are immersed into a solvent (e.g., acetone, xylene) and ultrasonically agitated 4. The test method is sensitive to the extent of adherence of the film to the substrate. The test method can be incorporated into a production process because it is non-distructive of fully adherent films. By this test method, non-adherent or not sufficiently adherent films are removed from the substrate and visually obscure as spots without a film. By the proper choice of solvent for sound-transmission, ultrasonic frequency and treatment time, the test method can be adapted to the adhesion bond necessary for the product4. For the assessment of the investigations, a screen of 1 mm x 1 mm was used to measure the area of substrate without a film. III. RESULTS From electron microscopic investigations, the surface of the LiNbO3 wafer appears to be affected in different ways by different chemicals. There is no surface effect due to NH3 HE The largest etching pits, which are arranged according to the crystal symmetry, are produced by H2 F2. Fig. 1 shows the classification of the chemicals EVAPORATION OF ADHERENT AI-FILMS Effect of Etching FIGURE Classification of chemicals according to the etching effect on the LiNbO3 wafer. lmNH3 HF (20% solvent), 2mChromium-sulphuric acid 3--NaOH (concentrated solvent) 4---H3 PO4 (concentrated solvent) 5--H_, F2 (concentrated solvent). according to the etching effect on the LiNbO3 wafer. It was found that the treatments of the LiNbO3 wafer with chromium-sulphuric acid and H3 PO4 will improve the adhesion of the A1 film. Fig. 2 shows clearly that the adhesion of the A1 film is essentially worse on LiNbO3 than on glass. After a test time of 10 min the A1 film has been nearly 60% dissolved from the LiNbO3, while it is still on the glass surface up to more than 90%. For the film systems with an underlying film on the substrate, it was found that the adhesion of the A1 film along with an A1Si mixed film is excellent as Fig. 3 shows. AI films with an underlying mixed film of A1Cu have better adhesion than with AINi mixed films or films of pure AI. To retain the filter function of the component, the thickness of the mixed film must not succeed 30 to 40 nm 5. An excellent adhesion is also to be found with the A1 films with an intermediate 6O Substrate 2O â1 3 4 5 6 7 Time for Ultn3scic Test rain FIGURE 2 Area without layer after ultrasonic test of pure AI film on glass- and LiNbO3-substrate in percent. 8O AINi 6O AtCu z, 9, 10 min Time for Ultrasonic Test FIGURE 3 Area without layer after ultrasonic test of AI film and AI films with below mixed films of AISi, AICu and AINi on LiNbO3 in percent. layer of the materials SiOx or Cr. After a test time of 10 min there are no dissolutions of the A1 film to be seen. A glow discharge is necessary before the evaporization of A1 on the intermediate layer SiOx6. The glow discharge is not necessary at the evaporization of A1 on the intermediate Cr layer. The excellent adhesion of the AI film can be reached with very thin intermediate layers of less than 20 nm. With thicker intermediate layers, determined changes of the filter component can be reached according to the patents 7,8. IV. CONCLUSION The investigation of the adherent metallization of LiNbO3 substrates with A1 by evaporization show that by a suitable combination of substrate pretreatment, film system, and technological methods, total-area well-adhered films can be evaporated in a reproductible way. For composite film systems, AI films with underlying films of A1Si, SiOx, or Cr are suitable. To get results of the adhesive bond of films, a test methodemploying ultrasonic agitation, was developed. The test method is applicable in the manufacturing process, as well as for testing the adhesion of films that have been structured by a photolithographic method.