Di rect beam J' / o 20, " - l To tally reftected. 20, X Scan / "-

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1 THE RIGAKU JOURNAL VOl. 8 / NO. 1 / 1991 Technical Note THIN FILM X-RAY DIFFRACTOMETRY H. ARAKI Rigaku Corporation. Tokvo. Japan 1. Introduction X-ray diffraction methods have been very popular in recent years for the characterization analysis of a thin film formed on the surface of materials. The thin film analysis by X-ray diffraction techniques is generally undertaken to study the following: (1) The thickness and uniformity of layers in multi-iayer films. Measurement of the long -periodicity scattering position and the width of each peak profile by use of the small angle scattering goniometer. (2) Constituent elements of the layers in multilayer films. Measurement of the long periodicity scattering intensity w ith the small angle scattering goniometer. (3) Structure of the formed layers. Qualitative analysis (i.e.: Identification analysis using JCPDS cards) with X-ray diffraction patterns measured by the wide angle goniometer with thin film attachment) (4) Variations in components or crystal structure due ta differences in distance of the layers from the surface. Qualitative analysis with diffraction patterns obtained by varying the X-ray incidence angle ta the sample surface with the wide angle goniometer equipped with the thin film attachment. ln the case of the conventional Bragg-Brentano (also called 8/28) focussing method using the wide angle goniometer, the obtained X-ray diffraction intensity fram the thin film is rather low, the background intensity is ~elat i vely high and the diffracted X-rays fram the substrate may appear. Thus, it has been difficult to obtain distinctive thin film diffraction patterns by this method. To improve the signal to background ratio of such weak diffraction rays from the thin film, the thin film X-ray diffractometry which employees the low angle incidence method [1] is availablè. Numbers (3) and (4) above represent th in film X-ray diffractometry with the wide angle goniometer equipped with the thin film attachment. Described in this paper are some precautions to be taken when conducting the characterization analysis of thin films by the low angle incidence method. 2. Optics of Thin Film X-Ray Diffractometry Fig. 1 shows the optical system of the low angle incidence method. 1 n this method, parallel X-rays Di rect beam J' / 20, X Scan / "- o 20, l?o " - l To tally reftected / " Incident X-rays 1.Diffracted X-rays 1, Diffracted X-rays : Normal X-ray di ffraction ri Detector - : Thin film X-ray diffraction ~ Fig. 1 Optical system of X-Tay difftactometer. Vol. 8 NO

2 incident h' 2reflected beam b~ma P 28 t~'2:::==~d.~ sample ~tric _ ~ ( 1_ Ir=IoS (. sinp p\[i-exp{-i'lccosec a i' \ sm a + SI n ) + cosec P) } ] S: Cross section area of incident X-ray beam /J : Linear absorption coefficient of sample la: Ditfracted X-ray intensity per unit cubic volume of sample when there is no absorption cc X-ray incident angle with respect ta the sample surface {3: X-ray reflected angle with respect ta the sample surface 28: Diffracted angle Fig. 2 Diffracted X-ray intensity of thin film sample. (from Reference 1) are commonly used as the incident and diffracted X-ray beams. In order to minimize the preferred orientation effect, sample interplanar roatation is provided. Fig. 2 shows the diffracted X-ray intensities from a thin film in the low angle incidence method. The axis of abscissa and the axis of oridinate, respectively, denote the diffraction angle and the K -value, with III and rx as parameters. As may be shown from this figure, when the thickness t of the thin film has a certain value, the diffracted X-ray intensity ratio with respect to the conventional 8/28 scanning method can be increased by setting the glancing incidence angle rx at a small value. The word "symmetric" in the figure indicates the measurement of the thim film by the 8/28 scanning method. As the curve shows, the diffracted X-ray intensity decreases as 28 increases. 3. Measurement Procedure ln carrying out measurements with a thin film diffractometer, it is essential to keep glancing in- Fig. 3 Depth of X-ray penetration when the incident angle is varied. cidence at a low angle (that is, to set the incident angle of X-ray in Fig. 1 or the rx angle in Fig. 2 at a small value). Aiso important when conducting the characterization analysis in the direction of depth from the sample surface (Fig. 3) is to enhance the parallelism of the incident X-ray beam. Normally, the optimum incident angle can be determined by calculating the penetration depth of the incident X- ray beam. For instance, about one degree is often used as the incident angle for thin films of several hundred nm. When information in the direction of depth is not so critical 0.1 to 0.2 degree may suffi ce the requirement for the parallelism of the incident X-ray beam. On the other hand, when precise information in the depth is required, an incident beam monochromator is used to improve the parallelism of the incident X-ray beam. In this way, the parallelism of the incident X-ray beam at a level below 0.1 can be obtained. For instance, in order to examine the status in the direction of a 5 nm depth when dealing with a Ti thin film, the parallelism of the incident X-ray beam must be below The following methods are available for studying the characterization of thin films in detail Utilization of Total Reflection The refractive index (n) of X-rays with respect to material is very close to 1, yet is slightly less than that. When X-ray incidence is lowerthan the critical angle 8c, which is determined by the material density and the X-ray wavelength, total reflection of X-rays takes place at the surface of the material. The relationship between the penetration depth of the incident X-ray beam and the incident angle of the X-ray beam itself is illustrated in Reference [2]. These examples illustrate that the penetration depth of the X-ray beam changes drastically around the critical angle. Under the condition of total reflection, the depth of X-ray penetration is no more than several nm from the material surface. 30 The Rigaku Journal

3 Utilizing the above characteristics, Matsushige [3] set up a condition in such a way that total reflection would take place only on the substrate, not on the surface of the thin film. Under such a condition, he attempted to collect diffraction information on thin films while minimizing diffraction from the substrate. The utilization of total reflection is important for the characterization analysis of thin films. One should note also that the occurrence of total reflection can be a hindrance in measurement when its use is not intended. In such a case, therefore, one must differentiate between a pattern due to total reflection from among the obtained diffraction patterns. Besides, depending upon the condition, measurement may be made without interplanar rotation of the sample. On such an occasion, attention should be paid to reflection caused by the edge of the sample, since it means the inclusion of information trom the substrate. This sort of total reflection is also undesirable. 3.2 Varying the Glancing Incident Angle The low angle incidence method is designed for examining the characterization of thin films by X-ray diffraction techniques. In addition, as shown in Fig. 4, one can also learn the following from X-ray diffraction patterns obtained by varying the incident angle. (1) Differentiation between diffracted X-rays from the thin film and those from the substrate. (2) The characterization of the thin film in the direction of depth. (3) Presence of preferred orientation in the thin film. Fig. 4 shows a multiple recording of diffraction patterns from a single crystal substrate with a 50 nm Au film. The incident angle was varied from 11 down to 0.2. From this data one can see that the diffracted X-rays due to the substrate become distinctive from those of the thin film. Since only the diffracted X-rays of Au were detected from the thin film, this thin film consists of Au only. And, since the intensity ratio of the diffracted X-rays trom the Au film differs according to the incidence angle, the lhin film has preferred orientation. 1 Multiple Recording 1 l O.1 K cps (1 l l ) Substrate ( 3 l l ) Substrate (4 2 2 ) T 3 0 Measuring condition: Sample na me : Au thin film 500À (Si substrate) File name : Thin Film-15 Comment Rotation 0.20 Target Cu voltage : 60kV current : 300mA Goniometer : Wide angle goniometer Sampling width : Scan speed : /min Divergence slit Scatter si it Receiving slit Attachment Rotation speed Monochromator Recieving slit Measurer : 0.40mm : 8.00mm : 8.00mm : Thin film attachment Used Empty RIGAKU Fig. 4. Diffracted X-ray intensity when the angle of incidence is varied. Vol. 8 No

4 Hence the method of varying the incident angle is one of the important analytical techniques for thin film X-ray diffractometry. 4. Sample For thin film samples, qualitative analysis is relatively easy since their composition is weil defined as is the substrate's. Nevertheless, the analysis of thin films by X-ray diffractometry is complex because the films are thin, differences in the manufacturers processes and various effects from the su bstrate. Some precautions to be exercised in conducting measurements have been mentioned above. Described below are precautions to take in the analysis of samples. 4.1 Substrate There are different types of substrates depending on their intended use, such as a single crystal substrate, an amorphous substrate, a polycrystal substrate and so forth. Among these substrates, the single crystal substrate is the most desirable for X-ray diffractometry. In this case, (hk/) of the substrate seldom meets the diffraction condition, thus, causing less diffracted rays fram the substrate. If the diffraction condition is not satisfied, the X-ray intensity at a given position (28) is nearly zero. Thus the resulting diffraction pattern has excellent signal-to-background (S/B) ratios. Consequently, the detection sensitivity of thin films on single crystal substrates are the highest among these three types. The amorphous substrate is also favorable for X-ray diffractometry. While this substrate is not superior to the single crystal substrate in terms of the detection sensitivity of thin films, it allows an easy characterization analysis in the case of a crystallized thin film because of no diffracted X-rays from the substrate. When an incident angle is constant in the case of single crystal substrates, it is difficult to distinguish between diffracted X-rays from the substrate and those from the thin film. One reason is that, as shown in Fig. 5, diffraction from the single crystal substrate takes place at a position several degrees off fram its regular position. When dealing with a single crystal substrate, a good way to discern whether or not certain diffracted X-rays are from the substrate is to obtairi diffraction patterns by varying the indicent angle, as shown in Fig Preferred Orientation It is safe to consider that most thin films have Fig. 5. Diffraction pattern of substrate. 32 The Rigaku Journal

5 RD II _ ! Fig. 6. Pole figure. preferred orientation, which is a fiber texture. Measurement of the preferred orientation is one of the weakest points for the thin film. One reason is that the positive pole figure measurement procedure for the preferred orientation measurement tech niques is to measure the intensity distribution or the density distribution of a certain crystal plane (hk/) with respect to coordinates fixed within the sam pie. 1 n other words, the use of the low angle incidence method, a fundamental of the thin film X-ray diffraction techniques, is not allowed for preferred orientation measurement. Therefore, when the thin film thickness is below 1/lm, the only available method is to surmise this preferred orientation from diffraction patterns, obtained by varying the incident angle. On the other hand, when there is an intense preferred orientation, it is still possible to determine the preferred orientation by the positive pole figure measurement method or the rocking curve measurement method even if the thin film thickness is below 1 /lm, Fig. 6 shows a positive pole figure of an Au thin film of 50 nm thickness on a Si single crystal substrate (in which no absorption correction was made). It is generally presumed that examination of the existence of preferred orientation may be made by interplanar rotation. In this way, from the relations between the rotation angle and the X-ray intensity, variations in diffracted X-ray intensity are considered to take place according to the rotation angle if there is a preferred orientation. But if this preferred orientation is a fiber texture, the diffracted X-ray intensity with respect to the rotation angle in the sample plane should remain unchanged as shown in Fig. 7. As in Fig. 6, the result of measurement of a positive pole figure regarding (111), with no absorption correction, indicates a fiber texture. One should note accordingly that the relations between the interplanar rotation angle and variations in the diffracted X-ray intensity alone are insufficient to determine the existence of preferred orientation. 5. Summary A fundamental method of thin film X-ray diffractometry is the low incidence angle method. When evaluating the diffraction pattern, one will note that there is a difference between the intensity ratio by the wide angle X-ray diffraction method (intensity ratios indicated on JCPDS cards) and that by the low angle incidence method. Also, the intensity ratio of (hkl) may differ according to the angle of incidence since preferred orientation exists Vol. 8 No

6 1 Multiple Recording 1 i ~ U:' A_~.-/ '--~_ A.. Fil'-ed, lateral direction J l'-_~_~ l'---~ Fixed, longitudinal direction ' l'----.j \. A Fixed, lateral direction ' ~~ ~ ) \ A-~ :JA- Fixed, longitudinal direction ~ 2 e 100 Measuring condition: Sam pie name : Au thin film 500Â (Si substrate) File name : Thin Film -20 Comment Fixed, longitudinal direction (R) 1.0 Target Cu voltage 60kV current 300mA Goniometer : Wide angle goniometer Sampling width Scan speed : fmin Divergence slit Scatter si it Receiving slit Attachment Rotation speed Monoc hromator Recieving slit Measurer : 0.40mm : 8.00mm _ : 8.00mm : Thin film attachment Used Empty RIGAKU Fig. 7. Diffraction patterns due to differences in the incidence direction of sample. in almost ail cases. Moreover, regarding the angle in the case of single crystal substrates, the diffracting position of the substrate can deviate by several degrees. 1 n either case, it is important to measure diffraction patterns by varying the angle of incidence, as illustrated in Fig. 4. Based on our experience we have mentioned some precautions which appear to be necessary for thin film X-ray diffractometry. We hope the above discussion may be useful to those working on thin film analyses. References [ 1] Y. Kobayashi and M. Yoshimatsu, Progress of X-ray Analysis VII. p. 49 (AGNE Technical Center). [2] M. F. Doemer, S. Brennan, J. Appl. Phys, 63 (1) m 126 (1988). [ 3 ] T. Horiuchi, H. Fukao, and K. Matsushige, Preliminary pa pers for Fall '87 Lecture Meeting by the Japan Society of Applied Physics, 34 The Rigaku Journal