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Analysis of focused ion beam implantation of semiconductors by thermal microscopy
Surface and Coatings Technology 142᎐144 Ž2001. 429᎐436
Analysis of focused ion beam implantation of semiconductors by thermal microscopy b , J. Pelzl a , B.K. Bein a,U D. Dietzel a , H. Rocken ¨ a
Experimental Physik III, Solid State Spectroscopy, Ruhr-Uni¨ ersitat, ¨ D-44780 Bochum, Germany b Experimental Physik III, Ion Beam Physics, Ruhr-Uni¨ ersitat, ¨ D-44780 Bochum, Germany
Abstract Implantation effects produced by focused ion beams in semiconductor materials have been analyzed with the help of thermal microscopy, based on thermal waves and laser-modulated optical reflectance Žthermoreflectance .. Implantation profiles related to variations of the ion dose and to the halos of neutrals and differently charged particles are interpreted with respect to the local thermal and electronic transport properties. For the signal excitation, different schemes have been applied: modulated laser beam irradiation; and additional electrical AC heating, giving improved signal contrast and improved lateral resolution for imaging applications on semiconductor devices. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion implantation; Modulated optical reflectance; Thermal microscopy; Thermal waves; Charge carrier waves; Silicon
1. Introduction Ion beam techniques are a versatile tool for the spatially resolved implantation of semiconductor materials and thus for the fabrication of semiconductor devices. Structured implantations can either be performed by using contact masks scanned by a rather broad beam or directly by focussed ion beams. In order to achieve good results, it is of crucial importance to monitor the quality of implantation, which may be limited by different effects: when working with focussed ion beams, side doses to the main focus can lower the spatial resolution of the implanted structure; inhomogeneities of the ion beam can cause problems with the reproduction of the implantation mask; in both cases, halos of unfocussed neutral particles can
U
Corresponding author. Tel.: q49-234-3223769; fax: q49-2343214172. E-mail address: [email protected] ŽB.K. Bein..
limit the quality of implantation and the functionality of the semiconductor device. In this work we apply the absolutely non-contact laser modulated optical reflectance method w1x to analyze the implantation profiles related to variations of implantation doses on the scale of micrometers. Simultaneously, the halos due to neutral and differently charged particles, inherent to any ion beam technique, have also been analyzed systematically. When dealing with biased semiconductor devices, the detection based on the reflectance signal additionally offers the possibility to monitor hot-spots and leakage currents which also reveal valuable information about the implantation profiles. Subsequently in Section 2, the principles of the modulated optical reflectance method are briefly described and in Section 3 the measuring system is presented. In Section 4 imaging applications of Si-implanted Si wafers are shown and by frequency-dependent measurements the implantation effects on the thermal and electronic properties are analyzed. In Section 5, simultaneous
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optical excitation and electrical AC heating is used to visualize ion beam effects of very low contrast.
2. Principles of the modulated optical reflectance method The laser-modulated optical reflectance method, which is applied here to analyze effects of ion implantation in semiconductor material, relies on the dependence of the optical reflectance RŽ n,T . on the temperature T and on the charge carrier density, n. When a semiconductor is irradiated with photons of sufficient energy, pairs of electrons and holes are produced which ᎏ by relaxation and recombination of pairs ᎏ generate heat. Both the distribution of heat and charge carriers in the semiconductor can be described as diffusion processes, governed by the heat diffusion equation ⭸T Ž© x,t . r⭸t s ␣ ⌬T Ž© x,t . q Q Ž ª x,t . r Ž ,c .
Ž1.
and the charge carrier diffusion equation ⭸n Ž© x,t . r⭸t s Dcc ⌬ n Ž© x,t . q S Ž ª x,t . q n Ž ª x,t . rcc
Ž2.
Here ␣ is the thermal diffusivity, C s c the volume specific heat, cc the lifetime of the charge carriers, and Dcc their diffusivity. QŽ x,t . and SŽ x,t . are the source terms. When the sample is illuminated by a modulated laser beam, the wave-like distributions of the temperature and charge carrier density, the so-called thermal waves ␦T Ž ª x,t . and plasma waves ␦nŽ ª x,t . w2x, are described by the diffusion Eqs. Ž1. and Ž2. and depend on the modulation frequency of excitation. As the recombination of pairs of electrons and holes is proportional to the charge carrier density and generates heat, the thermal wave and the plasma wave are coupled, leading to oscillations of the relative reflectance ␦R 1 ⭸R Ž n,T . 1 ⭸R Ž n,T . s ⭈ ␦T q ⭈ ␦n R R ⭸T R ⭸n s CT ⭈ ␦T q Cn ⭈ ␦n
Ž3.
For the wavelength of the probe beam used in this work ŽFig. 1., the coefficients CT and Cn , which describe the temperature and the charge carrier densitydependence of the optical reflectance of silicon, have got opposite signs contributing to enhanced discrimination between electronic and thermal effects. The periodic changes of the reflectance wEq. Ž3.x can then be monitored with the help of a probe beam and information can be obtained both from the amplitudes of the reflectance oscillations and their phase shift with respect to the modulated excitation. As the reflectance signal Žamplitude and phase. depends on both the
Fig. 1. Schematic of the modulated optical reflectance measuring system with an Ar ion laser beam as the pump beam and a HeNe laser beam as the probe beam.
thermal wave and the plasma wave, which on their part depend on the thermal transport properties and the electronic properties, the modulated reflectance technique provides an ideal method to analyze the effects of ion implantation in semiconductors, which affects both the thermal and the electronic properties w3,4x.
3. Experimental setup The experimental setup used for the thermoreflectance measurements is schematically shown in Fig. 1. An Ar ion laser beam of up to 1 W power is used as a pump beam and can be modulated with the help of an acousto-optical modulator up to frequencies of approximately 2 MHz. A HeNe-laser of 10 mW is used in collinear configuration as a probe beam and the intensity of the reflected light is converted by a photodiode into an electrical signal. As the periodic variations of the optical reflectance are relatively small, the response signal has to be measured by lock-in detection, allowing to filter and to discriminate small oscillations at a given frequency from a relatively high background. The two lasers can be focussed to beam diameters of approximately 5 m, so that local thermal and electronic properties can be analyzed on a micrometer scale. As the samples are mounted on an xy y translation stage, we can perform spatially resolved measurements as well as frequency-dependent measurements, where the frequency of excitation can be changed from 100 Hz to 2 MHz. The spatially resolved measurements are performed at a constant excitation frequency and allow to detect local variations of the implantation profile as local changes of the optical reflectance. The frequency-dependent measurements are performed at fixed positions and give more detailed information
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about the effects of implantation on the thermal and electronic properties. Apart from the optical excitation we can additionally apply localized heating to semiconductor devices by connecting an AC or DC power supply to a device structured with the help of focused ion beams. AC heating can be used to detect hot-spots and leakage currents w5x by the corresponding thermal wave, and optical excitation and ACrDC heating can be combined for contrast enhancement w4x, allowing to image even minor details of ion beam effects.
4. Experimental results on wafer implantation 4.1. Spatially resol¨ ed measurements on Si-implanted Si wafers The first sample to be analyzed is a silicon wafer, which has been implanted by Si ions with constant ion energies and constant ion rate. In order to produce a sample with a larger variety of implantation effects, 18 dots of approximately 10 m in diameter have been implanted by exposing the sample during certain time intervals to the ion beam. The distance between the dots is 200 m. In Fig. 2a,b and Fig. 3a,b the modulated reflectance amplitudes and phases measured with optical excitation at 20 kHz are shown. Fig. 2a,b shows the area of strongly implanted dots Žfrom dot 1 to dot 6 counted from the top, and in addition dot 7᎐9 of weaker implantation. while in Fig. 3a,b the area of the weakly implanted dots is shown Ždot 7᎐18 counted from the top.. The implantation doses decrease by a factor of 1r2000 from dot 1 to dot 18. For a quantitative interpretation one has to consider the effects of ion implantation on the thermal and electronic properties and thus on the reflectance signal. When implanting ions in semiconductors, the defects produced by implantation act as recombination centers and lower the charge carrier lifetime. Thus, a lower plasma wave has to be expected and as the plasma wave is subtracted from the thermal wave, the signals of the implanted areas usually increase both in amplitude and phase. In Fig. 3a,b, between the dots 12 and 13, the straight line of comparatively higher amplitude and phase indicates a considerable implantation dose, which has been caused by a not screened-off ion beam while the sample was moved for 200 m from dot 12 to dot 13. It can also be seen that dot 16 is much more pronounced than its neighbor dots due to a higher implantation dose. Apart from these technical details of the implantation process, there are some more principal phenomena which can be observed in Figs. 2 and 3. Apart
Fig. 2. Amplitude images Ža. and phase images Žb. detected with the help of the modulated reflectance signal in the region of the strongly implanted dots 1᎐6. Dot 1 in red can be localized both in the amplitude Ža. and phase image Žb. at the position x f 850 m, y f 750 m.
from the dots of limited size, a large area of approximately 1 mm width of changed thermal and electronic properties is observed, which accompanies the line of dots between x f 500 m and xf 1500 m. This large area of additional but weaker implantation effects is probably due to neutral particles, not focussed or deflected by the ion optics of the ion beam device. This assumption is also supported by the fact that the width of this halo is approximately that of the last aperture in front of the target. As the halo is screened off like the ions of the beam, the implantation times of neutrals and ions are the same. Owing to the larger radius of the halo particle distribution, however, the doses of the implantation times corresponding to several neigh-
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boring dots have to be added according to 18
˙ Ý ⌬ ti H Ž < ª DŽª x. sD xyª x i < y rH .
Ž4.
is1
Here H Ž x . is the Heavyside function, ª x i the center of the halo of dot i, ⌬ t i the implantation time of dot i, ˙ the rH the halo radius of approximately 500 m, and D ion rate. According to the distribution of the halo doses wEq. Ž4.x a step-like behavior of the modulated reflectance amplitude and phase is expected on a line scan in the halo region, parallel to the line of dots. Due to saturation of the implantation effects at the top of the sample corresponding to the higher implantation doses ŽFig. 2., the step-like behavior cannot be observed. In the area of comparatively lower implanta-
Fig. 4. Line scan of the signal amplitude Ž ⌬ . in the halo region, parallel to the line of dots. The slightly higher signals close to the end of the steps are probably due increased implantation effects related to a slight overlap of the different halo regions, presented by the thin continuous line. The implantation dose of the dots Ž䢇. and the resulting implantation dose of the halos Ž ᎏ . are presented as relative doses with respect to the corresponding maximum dose.
Fig. 3. Amplitude images Ža. and phase images Žb. detected with the help of the modulated reflectance signal in the region of the weakly implanted dots 7᎐18. The dots, the side regions and the different implantation levels by the neutrals in the halo region can easily be identified.
tion doses, however, e.g. between dot 7 and dot 11, the step-like decrease of the implantation effects due to the halo doses can be identified in Fig. 3. Although the measured signals are affected by noise, it can even be seen in Fig. 4, in which the measured amplitudes and the halo doses calculated according to Eq. Ž4. are compared, that there is a small increase of the amplitudes at the end of each step, which can be explained by a model taking into account small overlaps of some 10 m between the dots of approximately 1 mm distance and a halo radius slightly above 500 m, e.g. rH s 520 m. The step-like behavior of the signal amplitudes shown in Fig. 4 has been found for the signal phases as well w6x. A further interesting effect can be observed for the shape of the implanted dots in Fig. 5a,b. While the highly implanted dots are comparatively sharp Že.g. dots 1᎐3 in Fig. 2., the dots of weaker implantation dose ŽFig. 3. seem to have larger diameters of up to 200 m. This means that apart from the well-focussed center of the dots of 10 m radius non-negligible side doses have been implanted, or the defects related to the implantation diffused concentrically away from the dot centers. For the highly implanted dots, e.g. dot 1 in Fig. 5a,b, these side effects cannot be distinguished from the effects of the halo due to the lower contrast at generally higher implantation doses, whereas the implantation structure of the dots becomes more evident for the dots of weaker implantation, e.g. dot 7 and
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transient signal decrease outside the spot has been observed for dots with lower implantation doses. 4.2. Frequency-dependent measurements on Si implanted in Si wafers In order to get deeper insight into the physical effects and to obtain quantitative information about the implantation effects and profiles, frequency-dependent modulated reflectance measurements have been done on various positions. This type of measurement gives detailed information about the electronic and thermal contributions to the signal generation process, and thus allows to distinguish the effects of implantation on the electronic and the thermal transport properties. To compare frequency-dependent measurements at different positions of the sample, a normalization procedure w7x according to
Fig. 5. Line scans of amplitudes Ža. and phases Žb. perpendicular to the line of dots, crossing dots of different implantation dose.
11 in Fig. 5a,b. As we can see, the side dose effects decrease rapidly with distance from the center of the dots, falling to the level of the implantation effects in the halo region. For the very weakly implanted dots 13᎐18, the effects of the very low halo doses are no longer detectable and only the dots with the corresponding side effects are identified, e.g. dot 13 in Fig. 5a,b. Additionally, we can observe here, that the phase contrast becomes better for the very low implantation doses ŽFig. 5b., while for higher doses the amplitude contrast is more pronounced ŽFig. 5a.. Normally, the phases are more sensitive to changes of the transport properties, whereas the amplitudes contain combined information on the optical properties and the transport properties governing the diffusion processes. Performing high resolution scans of a region of 128 = 128 m around individual dots, we have seen that strongly focussed signal-spots of the modulated reflectance signal of approximately 10 m really exist, both for the amplitudes and phases w6x. Here we have to take into account, that the measured width of the signal-spot is broadened by the radii of the pump and probe beam of our measuring device, which are roughly 5᎐10 m, and that the implantation dots can be effectively smaller. For comparatively high implanted dots, the high resolution scans show a relatively constant signal amplitude outside the spot, whereas a more
Sn Ž f . s S X 1 Ž f . rS X 2 Ž f .
Ž 5a .
nŽ f . s X 1Ž f . y X 2 Ž f .
Ž 5b .
has been applied, which means that the ratios of the signal amplitudes and the phase differences measured at different sample positions are analyzed and quantitatively interpreted based on combined solutions of the thermal waves and plasma waves w3,6x. In Fig. 6a,b the theoretical approximations of the normalized amplitudes and phases, which compare dots, halo regions and virgin wafer material, have been calculated using literature values of the thermal and electronic properties of Si as a basis and by considering the effects of implantation by decreased transport properties and shorter charge carrier recombination times due to implantation-induced defects. In principle, the measurements and theoretical approximations show a similar behavior. When normalizing the measurement of a highly implanted region vs. that of a less implanted region, the normalized amplitudes and phases increase with excitation frequency. The highest increase can be observed when normalizing the measurement of the halo region vs. that of the virgin Si surface Ž ⵜ .. This is due to the larger influence of the plasma wave on the measured reflectance signal of the virgin Si surface. In comparison to the virgin surface, the plasma wave has a comparatively minor effect on the measurement in the halo region due to the defects induced by the relatively energetic charge exchange neutrals. The plasma wave does not only affect the frequency-dependence of the normalized measured data but also the absolute level of the normalized amplitudes. As there is no strong plasma component at the higher implanted areas of the sample, there is minor competition between the plasma wave and thermal
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Fig. 6. Frequency-dependent normalized amplitudes Ža. and phases Žb., measured at different positions on a Si-implanted Si wafer, where the following implantation regions are compared by normalization of the signals: ⵜ, region of halo of neutrals vs. virgin surface region; ⌬, center of dot 1 vs. measurement on side effects of dot 1; I, center of dot 1 vs. center of dot 10; `, center of dot 1 vs. center of dot 7. The experimental data are approximated by theoretical curves based on the model of coupled thermal waves and plasma waves.
wave, resulting in a much higher signal level due to the thermal wave than in the less implanted region. This is true for the normalized measurements of the center of dot 1 vs. its neighborhood Ž ⌬ .. Due to the high implantation dose of the dot center there is no plasma component in the center, but in its neighborhood there still remains a small plasma wave component, leading to the slight increase of amplitudes and phases with the modulation frequency. This effect becomes even smaller when comparing measurements of different dot centers, e.g. the center of dot 1 with the center of the less implanted dot 10 ŽI.: due to the implantation dose of the center of dot 10, which is higher in comparison to the neighborhood of dot 1, the plasma component, although still existing, is even more reduced. When normalizing the measurement at the center of dot 1 vs. that at the center of dot 7 Ž`., there is no increase of the normalized amplitudes and phases. Instead, a small decrease of the normalized amplitudes and phases with the frequency can be observed ŽFig. 6a,b.. This slight decrease can be explained by the fact that due to the high ion doses of both dots any plasma wave contributions can be excluded, that however the comparatively higher ion dose of dot 1 contributed to higher lattice damage and relatively more reduced thermal transport properties in dot 1. Based on the normalization procedure according to Eqs. Ž5a. and Ž5b., the charge carrier effects, which decrease with increasing implantation, can be identified and quantified. Although we have seen that the normalized amplitudes and phases increase with the modulation fre-
quency, when normalizing a measurement with a low plasma wave component vs. a measurement of a higher plasma component, this behavior can change in the limit of high frequencies, namely when the plasma wave component of the second measurement becomes dominant. Such a behavior, as indicated by the theoretical approximation for the normalized amplitudes Ž ⵜ . in Fig. 6a, has been measured on a second Si-implanted sample of silicon ŽFig. 7.. Again we can observe that there is only a small increase of the normalized amplitude when comparing the measurements of two highly implanted areas Ž`., but when normalizing two measurements from a dot center Ž ⌬,x. vs. a measurement of the virgin material we first see an increase of the normalized amplitudes with the frequency, then at approximately 20 kHz a relative maximum, and for further increasing frequencies a rapid decrease of the normalized amplitude ŽFig. 7.. Interesting to mention here is the fact that although the normalized amplitudes are quite noisy at low frequencies, where plasma wave and thermal wave contributions can compete with each other, they become more stable in the high frequency limit where the plasma wave is dominant.
5. Measurements of semiconductor structures implanted on SIMOX wafers Although the laser beam modulated optical reflectance technique is very sensitive to implantation effects in semiconductors, we have found imaging ap-
Fig. 7. Frequency-dependent normalized amplitudes, measured at different positions on a Si-implanted Si wafer, where the following implantation regions are compared by normalization of the signals: ⌬,x, two measurements in a dot center vs. virgin surface region; `, center of dot vs. measurement beside the dot. The experimental data are approximated by theoretical curves based on the model of coupled thermal waves and plasma waves.
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plications, where the method failed when applied in the form as described in Sections 3 and 4. The analyzed sample, a mesa structure on a SIMOX wafer Ž500 m Si substrate, a 380-nm thick intermediate layer of SiO 2 ,
Fig. 8. Ža. Laser modulated reflectance image of an IPG transistor, monitored at 20 kHz, giving information on the contact pads Žin red. showing, however, insufficient contrast between implanted and not implanted areas in the center of the IPG transistor Žin green.. The position of the implanted gate line is schematically shown by the broken lines Žin black. Žb. Reflectance image of the IPG transistor based on the signal. amplitudes generated by electrical AC heating of 2 V at a modulation frequency of f s 5 kHz across the gate line. The hot spot is found in the region of reduced implantation ŽComp. Fig. 9.. Žc. Modulated reflectance image of the IPG transistor excited by combined optical excitation and electrical AC heating and detected at the fourth harmonic Ž4 f-detection..
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and a 200-nm p-Si layer on the top. consists of ion-beam implanted insulating lines of approximately 4-m width, written with the help of the micro-beam facility at the Dynamitron Tandem Laboratory of Ruhr-University Bochum. Here we report on measurements of an inplane gate ŽIPG. transistor ŽFig. 8., with a gate line not completely isolated due to an inhomogenous implantation. In Fig. 8a the laser modulated reflectance image of the complete system registered at a modulation frequency of f s 20 kHz is presented. It shows the four contact pads Žred areas. and the mesa Žgreen area., where the broken black lines schematically indicate the position of the gate line. As shown in Fig. 8a, the ion-implanted gate line does not produce sufficient contrast to be identifiable in the laser-modulated optical reflectance image. As shown, however, in Fig. 8b with the help of the modulated reflectance technique when applying a modulated voltage for excitation across the gate line, the leakage current generates a strong modulated heat source which is detected as a hot spot at the end of the gate line. Although the detection of such a hot spot already gives valuable information on the implantation effects, we have additionally tried to image the isolating line by applying the laser modulated optical reflectance technique alone, however, at different modulation frequencies. These tests failed, probably due to too low a contrast. This is most likely due to the layer structure of the SIMOX wafer, limiting the diffusion of the charge carriers by the electrically isolating SiO 2 layer located below the p-Si layer of only 200 nm thickness. Consequently, a larger plasma wave contribution could not be generated at the modulation frequencies accessible to our equipment applied in the usual way. In order to overcome this problem we have combined optical excitation with additional electrical DC and AC heating w4x. Such measurements can either be run by applying DC heating in addition to optical excitation and reflectance detection at the frequency of the optical excitation Ž1 f-detection. or by combining AC heating with optical excitation and by using detection at higher harmonics related to the coupling of the two excitation processes. In this case, optimal results could be obtained by combining optical excitation at 5 kHz and a 5-kHz sinusoidal AC voltage of approximately 2.5 V applied at an off-set voltage of 2.5 V across the isolating gate line ᎏ corresponding to electrical heating at 10 kHz ᎏ with the detection of the fourth harmonic Ž4 f-detection. at 20 kHz. In Fig. 8c we can see the implanted gate line with a very marked profile of decreasing amplitudes when approaching the region where we earlier have found the hot spot ŽFig. 8b.. In a separate line scan along the gate line ŽFig. 9. the signal decrease along the inhomogeneously implanted gate line has been determined quantitatively, and at the end of the isolating line the hot spot has
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Fig. 9. Line scans parallel to the gate line: diode voltage Ž`. as a measure of the laterally resolved total Žnot modulated. optical reflectance giving information on the localization of the gate line, in comparison with the modulated reflectance signal Ž䢇., generated by combined optical excitation and electrical AC heating and detected by 4 f-detection. The modulated signal gives information on the inhomogeneous implantation, strongly decreasing from y f 50 m to y f 25 m and on the hot spot region at approximately 17 m.
again been identified as a second small relative maximum. This second small maximum is probably due to the AC electrical heating, detectable at the fourth harmonic related to the non-linear voltage᎐current characteristics of the IPG transistor. We conclude that the signal amplitudes excited with the combined optical and electrical AC heating and detected at the fourth harmonic are correlated to the implantation profile along the gate line. Two possible reasons can be responsible for the achieved signal enhancement: the first can be a photo-induced current across the gate line leading to additional Joule heat sources; the second may be that the applied electrical field contributes to local changes of the photo-induced charge carrier distribution.
related to neutral particles and side effects due to focussed implantation, which are inherent to any ion beam technique. By means of frequency-dependent measurements more quantitative information was obtained on the electronic and thermal properties of critical areas, identified before by spatially resolved measurements Žimaging.. By comparing signals measured at positions of different implantation doses by a so-called normalization procedure and by approximating the resulting normalized signals by the coupled solutions of the charge carrier density waves and thermal waves w6x, it was possible to identify and quantify charge carrier effects in the presence of lattice defects. Additionally, we have found that in critical cases hidden information on implantation effects can be imaged by the combination of laser beam excitation and electrical heating, enabling a strong signal enhancement. To get more quantitative information about the implantation effects in this case, further work on the mechanisms of contrast enhancement is required, since the physical reasons are not yet understood. A further improved local resolution seems to be possible, e.g. in the detection of hot spots, when combining optical heating and electrical AC heating with thermoelastic detection-based scanning microscopy w8x. Acknowledgements The analyzed samples have been prepared in the tandem accelerator facility ŽDTL. of Ruhr-University. We acknowledge the support by A. Stephan and the DTL staff members. Additionally, we acknowledge the helpful discussions with J. Meijer, Ion Beam Physics group of Exp.-Phys. III. One of the authors, D. Dietzel, has been supported in the frame of the Graduiertenkolleg 384, DFG. References
6. Conclusion and outlook In this work we have shown, that thermal microscopy based on reflectance detection of thermal and plasma waves is a very versatile and powerful tool for the analysis of implanted semiconductor materials and devices. It offers a relatively good spatial resolution by non-contact remote detection, thus allowing non-contact measurements of both thermal and electronic transport properties on miniaturized structures which cannot be contacted due to their limited size. Imaging applications as well as analysis of electronic and thermal properties can be performed. Due to the high sensitivity of the modulated thermoreflectance signal it was possible to monitor halos
w1x A. Rosencwaig, in: A. Mandelis ŽEd.., Non-Destructive Evaluation, Prentice Hall, Englewood Cliffs, NJ, 1994, p. 2. w2x D. Fournier, in: D. Bicanic ŽEd.., Photoacoustic and Photothermal Phenomena III, Springer Ser. Opt. Sci., 69, 1982 49. w3x W. Kiepert, H.J. Obramski, J. Bolte et al., Surf. Coat. Technol. 116r119 Ž1999. 410᎐418. w4x D. Dietzel, H. Roecken, C. Crell, B.K. Bein, J. Pelzl, Combined electrical and optical heating in thermal wave microscopy of semiconductor devices, to be publ. in Analytical Sciences 2001 ŽProc. ICPPP 11th, Kyoto, Japan, June 2000.. w5x A.M. Mansanares, J.P. Roger, D. Fournier, A.C. Boccara, Appl. Phys. Lett. 64 Ž1994. 4. w6x D. Dietzel, PhD thesis, Ruhr-University Bochum, Bochum, Germany Ž2001.. w7x B.K. Bein, S. Krueger, J. Pelzl, Can. J. Phys. 64 Ž1986. 1208᎐1216. w8x J. Bolte, F. Niebisch, P. Stelmaszyk, A.D. Wieck, J. Pelzl, J. Appl. Phys. 84 Ž1998. 6917᎐6922.
Analysis of focused ion beam implantation of semiconductors by thermal microscopy b , J. Pelzl a , B.K. Bein a,U D. Dietzel a , H. Rocken ¨ a
Experimental Physik III, Solid State Spectroscopy, Ruhr-Uni¨ ersitat, ¨ D-44780 Bochum, Germany b Experimental Physik III, Ion Beam Physics, Ruhr-Uni¨ ersitat, ¨ D-44780 Bochum, Germany
Abstract Implantation effects produced by focused ion beams in semiconductor materials have been analyzed with the help of thermal microscopy, based on thermal waves and laser-modulated optical reflectance Žthermoreflectance .. Implantation profiles related to variations of the ion dose and to the halos of neutrals and differently charged particles are interpreted with respect to the local thermal and electronic transport properties. For the signal excitation, different schemes have been applied: modulated laser beam irradiation; and additional electrical AC heating, giving improved signal contrast and improved lateral resolution for imaging applications on semiconductor devices. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion implantation; Modulated optical reflectance; Thermal microscopy; Thermal waves; Charge carrier waves; Silicon
1. Introduction Ion beam techniques are a versatile tool for the spatially resolved implantation of semiconductor materials and thus for the fabrication of semiconductor devices. Structured implantations can either be performed by using contact masks scanned by a rather broad beam or directly by focussed ion beams. In order to achieve good results, it is of crucial importance to monitor the quality of implantation, which may be limited by different effects: when working with focussed ion beams, side doses to the main focus can lower the spatial resolution of the implanted structure; inhomogeneities of the ion beam can cause problems with the reproduction of the implantation mask; in both cases, halos of unfocussed neutral particles can
U
Corresponding author. Tel.: q49-234-3223769; fax: q49-2343214172. E-mail address: [email protected] ŽB.K. Bein..
limit the quality of implantation and the functionality of the semiconductor device. In this work we apply the absolutely non-contact laser modulated optical reflectance method w1x to analyze the implantation profiles related to variations of implantation doses on the scale of micrometers. Simultaneously, the halos due to neutral and differently charged particles, inherent to any ion beam technique, have also been analyzed systematically. When dealing with biased semiconductor devices, the detection based on the reflectance signal additionally offers the possibility to monitor hot-spots and leakage currents which also reveal valuable information about the implantation profiles. Subsequently in Section 2, the principles of the modulated optical reflectance method are briefly described and in Section 3 the measuring system is presented. In Section 4 imaging applications of Si-implanted Si wafers are shown and by frequency-dependent measurements the implantation effects on the thermal and electronic properties are analyzed. In Section 5, simultaneous
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optical excitation and electrical AC heating is used to visualize ion beam effects of very low contrast.
2. Principles of the modulated optical reflectance method The laser-modulated optical reflectance method, which is applied here to analyze effects of ion implantation in semiconductor material, relies on the dependence of the optical reflectance RŽ n,T . on the temperature T and on the charge carrier density, n. When a semiconductor is irradiated with photons of sufficient energy, pairs of electrons and holes are produced which ᎏ by relaxation and recombination of pairs ᎏ generate heat. Both the distribution of heat and charge carriers in the semiconductor can be described as diffusion processes, governed by the heat diffusion equation ⭸T Ž© x,t . r⭸t s ␣ ⌬T Ž© x,t . q Q Ž ª x,t . r Ž ,c .
Ž1.
and the charge carrier diffusion equation ⭸n Ž© x,t . r⭸t s Dcc ⌬ n Ž© x,t . q S Ž ª x,t . q n Ž ª x,t . rcc
Ž2.
Here ␣ is the thermal diffusivity, C s c the volume specific heat, cc the lifetime of the charge carriers, and Dcc their diffusivity. QŽ x,t . and SŽ x,t . are the source terms. When the sample is illuminated by a modulated laser beam, the wave-like distributions of the temperature and charge carrier density, the so-called thermal waves ␦T Ž ª x,t . and plasma waves ␦nŽ ª x,t . w2x, are described by the diffusion Eqs. Ž1. and Ž2. and depend on the modulation frequency of excitation. As the recombination of pairs of electrons and holes is proportional to the charge carrier density and generates heat, the thermal wave and the plasma wave are coupled, leading to oscillations of the relative reflectance ␦R 1 ⭸R Ž n,T . 1 ⭸R Ž n,T . s ⭈ ␦T q ⭈ ␦n R R ⭸T R ⭸n s CT ⭈ ␦T q Cn ⭈ ␦n
Ž3.
For the wavelength of the probe beam used in this work ŽFig. 1., the coefficients CT and Cn , which describe the temperature and the charge carrier densitydependence of the optical reflectance of silicon, have got opposite signs contributing to enhanced discrimination between electronic and thermal effects. The periodic changes of the reflectance wEq. Ž3.x can then be monitored with the help of a probe beam and information can be obtained both from the amplitudes of the reflectance oscillations and their phase shift with respect to the modulated excitation. As the reflectance signal Žamplitude and phase. depends on both the
Fig. 1. Schematic of the modulated optical reflectance measuring system with an Ar ion laser beam as the pump beam and a HeNe laser beam as the probe beam.
thermal wave and the plasma wave, which on their part depend on the thermal transport properties and the electronic properties, the modulated reflectance technique provides an ideal method to analyze the effects of ion implantation in semiconductors, which affects both the thermal and the electronic properties w3,4x.
3. Experimental setup The experimental setup used for the thermoreflectance measurements is schematically shown in Fig. 1. An Ar ion laser beam of up to 1 W power is used as a pump beam and can be modulated with the help of an acousto-optical modulator up to frequencies of approximately 2 MHz. A HeNe-laser of 10 mW is used in collinear configuration as a probe beam and the intensity of the reflected light is converted by a photodiode into an electrical signal. As the periodic variations of the optical reflectance are relatively small, the response signal has to be measured by lock-in detection, allowing to filter and to discriminate small oscillations at a given frequency from a relatively high background. The two lasers can be focussed to beam diameters of approximately 5 m, so that local thermal and electronic properties can be analyzed on a micrometer scale. As the samples are mounted on an xy y translation stage, we can perform spatially resolved measurements as well as frequency-dependent measurements, where the frequency of excitation can be changed from 100 Hz to 2 MHz. The spatially resolved measurements are performed at a constant excitation frequency and allow to detect local variations of the implantation profile as local changes of the optical reflectance. The frequency-dependent measurements are performed at fixed positions and give more detailed information
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about the effects of implantation on the thermal and electronic properties. Apart from the optical excitation we can additionally apply localized heating to semiconductor devices by connecting an AC or DC power supply to a device structured with the help of focused ion beams. AC heating can be used to detect hot-spots and leakage currents w5x by the corresponding thermal wave, and optical excitation and ACrDC heating can be combined for contrast enhancement w4x, allowing to image even minor details of ion beam effects.
4. Experimental results on wafer implantation 4.1. Spatially resol¨ ed measurements on Si-implanted Si wafers The first sample to be analyzed is a silicon wafer, which has been implanted by Si ions with constant ion energies and constant ion rate. In order to produce a sample with a larger variety of implantation effects, 18 dots of approximately 10 m in diameter have been implanted by exposing the sample during certain time intervals to the ion beam. The distance between the dots is 200 m. In Fig. 2a,b and Fig. 3a,b the modulated reflectance amplitudes and phases measured with optical excitation at 20 kHz are shown. Fig. 2a,b shows the area of strongly implanted dots Žfrom dot 1 to dot 6 counted from the top, and in addition dot 7᎐9 of weaker implantation. while in Fig. 3a,b the area of the weakly implanted dots is shown Ždot 7᎐18 counted from the top.. The implantation doses decrease by a factor of 1r2000 from dot 1 to dot 18. For a quantitative interpretation one has to consider the effects of ion implantation on the thermal and electronic properties and thus on the reflectance signal. When implanting ions in semiconductors, the defects produced by implantation act as recombination centers and lower the charge carrier lifetime. Thus, a lower plasma wave has to be expected and as the plasma wave is subtracted from the thermal wave, the signals of the implanted areas usually increase both in amplitude and phase. In Fig. 3a,b, between the dots 12 and 13, the straight line of comparatively higher amplitude and phase indicates a considerable implantation dose, which has been caused by a not screened-off ion beam while the sample was moved for 200 m from dot 12 to dot 13. It can also be seen that dot 16 is much more pronounced than its neighbor dots due to a higher implantation dose. Apart from these technical details of the implantation process, there are some more principal phenomena which can be observed in Figs. 2 and 3. Apart
Fig. 2. Amplitude images Ža. and phase images Žb. detected with the help of the modulated reflectance signal in the region of the strongly implanted dots 1᎐6. Dot 1 in red can be localized both in the amplitude Ža. and phase image Žb. at the position x f 850 m, y f 750 m.
from the dots of limited size, a large area of approximately 1 mm width of changed thermal and electronic properties is observed, which accompanies the line of dots between x f 500 m and xf 1500 m. This large area of additional but weaker implantation effects is probably due to neutral particles, not focussed or deflected by the ion optics of the ion beam device. This assumption is also supported by the fact that the width of this halo is approximately that of the last aperture in front of the target. As the halo is screened off like the ions of the beam, the implantation times of neutrals and ions are the same. Owing to the larger radius of the halo particle distribution, however, the doses of the implantation times corresponding to several neigh-
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boring dots have to be added according to 18
˙ Ý ⌬ ti H Ž < ª DŽª x. sD xyª x i < y rH .
Ž4.
is1
Here H Ž x . is the Heavyside function, ª x i the center of the halo of dot i, ⌬ t i the implantation time of dot i, ˙ the rH the halo radius of approximately 500 m, and D ion rate. According to the distribution of the halo doses wEq. Ž4.x a step-like behavior of the modulated reflectance amplitude and phase is expected on a line scan in the halo region, parallel to the line of dots. Due to saturation of the implantation effects at the top of the sample corresponding to the higher implantation doses ŽFig. 2., the step-like behavior cannot be observed. In the area of comparatively lower implanta-
Fig. 4. Line scan of the signal amplitude Ž ⌬ . in the halo region, parallel to the line of dots. The slightly higher signals close to the end of the steps are probably due increased implantation effects related to a slight overlap of the different halo regions, presented by the thin continuous line. The implantation dose of the dots Ž䢇. and the resulting implantation dose of the halos Ž ᎏ . are presented as relative doses with respect to the corresponding maximum dose.
Fig. 3. Amplitude images Ža. and phase images Žb. detected with the help of the modulated reflectance signal in the region of the weakly implanted dots 7᎐18. The dots, the side regions and the different implantation levels by the neutrals in the halo region can easily be identified.
tion doses, however, e.g. between dot 7 and dot 11, the step-like decrease of the implantation effects due to the halo doses can be identified in Fig. 3. Although the measured signals are affected by noise, it can even be seen in Fig. 4, in which the measured amplitudes and the halo doses calculated according to Eq. Ž4. are compared, that there is a small increase of the amplitudes at the end of each step, which can be explained by a model taking into account small overlaps of some 10 m between the dots of approximately 1 mm distance and a halo radius slightly above 500 m, e.g. rH s 520 m. The step-like behavior of the signal amplitudes shown in Fig. 4 has been found for the signal phases as well w6x. A further interesting effect can be observed for the shape of the implanted dots in Fig. 5a,b. While the highly implanted dots are comparatively sharp Že.g. dots 1᎐3 in Fig. 2., the dots of weaker implantation dose ŽFig. 3. seem to have larger diameters of up to 200 m. This means that apart from the well-focussed center of the dots of 10 m radius non-negligible side doses have been implanted, or the defects related to the implantation diffused concentrically away from the dot centers. For the highly implanted dots, e.g. dot 1 in Fig. 5a,b, these side effects cannot be distinguished from the effects of the halo due to the lower contrast at generally higher implantation doses, whereas the implantation structure of the dots becomes more evident for the dots of weaker implantation, e.g. dot 7 and
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transient signal decrease outside the spot has been observed for dots with lower implantation doses. 4.2. Frequency-dependent measurements on Si implanted in Si wafers In order to get deeper insight into the physical effects and to obtain quantitative information about the implantation effects and profiles, frequency-dependent modulated reflectance measurements have been done on various positions. This type of measurement gives detailed information about the electronic and thermal contributions to the signal generation process, and thus allows to distinguish the effects of implantation on the electronic and the thermal transport properties. To compare frequency-dependent measurements at different positions of the sample, a normalization procedure w7x according to
Fig. 5. Line scans of amplitudes Ža. and phases Žb. perpendicular to the line of dots, crossing dots of different implantation dose.
11 in Fig. 5a,b. As we can see, the side dose effects decrease rapidly with distance from the center of the dots, falling to the level of the implantation effects in the halo region. For the very weakly implanted dots 13᎐18, the effects of the very low halo doses are no longer detectable and only the dots with the corresponding side effects are identified, e.g. dot 13 in Fig. 5a,b. Additionally, we can observe here, that the phase contrast becomes better for the very low implantation doses ŽFig. 5b., while for higher doses the amplitude contrast is more pronounced ŽFig. 5a.. Normally, the phases are more sensitive to changes of the transport properties, whereas the amplitudes contain combined information on the optical properties and the transport properties governing the diffusion processes. Performing high resolution scans of a region of 128 = 128 m around individual dots, we have seen that strongly focussed signal-spots of the modulated reflectance signal of approximately 10 m really exist, both for the amplitudes and phases w6x. Here we have to take into account, that the measured width of the signal-spot is broadened by the radii of the pump and probe beam of our measuring device, which are roughly 5᎐10 m, and that the implantation dots can be effectively smaller. For comparatively high implanted dots, the high resolution scans show a relatively constant signal amplitude outside the spot, whereas a more
Sn Ž f . s S X 1 Ž f . rS X 2 Ž f .
Ž 5a .
nŽ f . s X 1Ž f . y X 2 Ž f .
Ž 5b .
has been applied, which means that the ratios of the signal amplitudes and the phase differences measured at different sample positions are analyzed and quantitatively interpreted based on combined solutions of the thermal waves and plasma waves w3,6x. In Fig. 6a,b the theoretical approximations of the normalized amplitudes and phases, which compare dots, halo regions and virgin wafer material, have been calculated using literature values of the thermal and electronic properties of Si as a basis and by considering the effects of implantation by decreased transport properties and shorter charge carrier recombination times due to implantation-induced defects. In principle, the measurements and theoretical approximations show a similar behavior. When normalizing the measurement of a highly implanted region vs. that of a less implanted region, the normalized amplitudes and phases increase with excitation frequency. The highest increase can be observed when normalizing the measurement of the halo region vs. that of the virgin Si surface Ž ⵜ .. This is due to the larger influence of the plasma wave on the measured reflectance signal of the virgin Si surface. In comparison to the virgin surface, the plasma wave has a comparatively minor effect on the measurement in the halo region due to the defects induced by the relatively energetic charge exchange neutrals. The plasma wave does not only affect the frequency-dependence of the normalized measured data but also the absolute level of the normalized amplitudes. As there is no strong plasma component at the higher implanted areas of the sample, there is minor competition between the plasma wave and thermal
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Fig. 6. Frequency-dependent normalized amplitudes Ža. and phases Žb., measured at different positions on a Si-implanted Si wafer, where the following implantation regions are compared by normalization of the signals: ⵜ, region of halo of neutrals vs. virgin surface region; ⌬, center of dot 1 vs. measurement on side effects of dot 1; I, center of dot 1 vs. center of dot 10; `, center of dot 1 vs. center of dot 7. The experimental data are approximated by theoretical curves based on the model of coupled thermal waves and plasma waves.
wave, resulting in a much higher signal level due to the thermal wave than in the less implanted region. This is true for the normalized measurements of the center of dot 1 vs. its neighborhood Ž ⌬ .. Due to the high implantation dose of the dot center there is no plasma component in the center, but in its neighborhood there still remains a small plasma wave component, leading to the slight increase of amplitudes and phases with the modulation frequency. This effect becomes even smaller when comparing measurements of different dot centers, e.g. the center of dot 1 with the center of the less implanted dot 10 ŽI.: due to the implantation dose of the center of dot 10, which is higher in comparison to the neighborhood of dot 1, the plasma component, although still existing, is even more reduced. When normalizing the measurement at the center of dot 1 vs. that at the center of dot 7 Ž`., there is no increase of the normalized amplitudes and phases. Instead, a small decrease of the normalized amplitudes and phases with the frequency can be observed ŽFig. 6a,b.. This slight decrease can be explained by the fact that due to the high ion doses of both dots any plasma wave contributions can be excluded, that however the comparatively higher ion dose of dot 1 contributed to higher lattice damage and relatively more reduced thermal transport properties in dot 1. Based on the normalization procedure according to Eqs. Ž5a. and Ž5b., the charge carrier effects, which decrease with increasing implantation, can be identified and quantified. Although we have seen that the normalized amplitudes and phases increase with the modulation fre-
quency, when normalizing a measurement with a low plasma wave component vs. a measurement of a higher plasma component, this behavior can change in the limit of high frequencies, namely when the plasma wave component of the second measurement becomes dominant. Such a behavior, as indicated by the theoretical approximation for the normalized amplitudes Ž ⵜ . in Fig. 6a, has been measured on a second Si-implanted sample of silicon ŽFig. 7.. Again we can observe that there is only a small increase of the normalized amplitude when comparing the measurements of two highly implanted areas Ž`., but when normalizing two measurements from a dot center Ž ⌬,x. vs. a measurement of the virgin material we first see an increase of the normalized amplitudes with the frequency, then at approximately 20 kHz a relative maximum, and for further increasing frequencies a rapid decrease of the normalized amplitude ŽFig. 7.. Interesting to mention here is the fact that although the normalized amplitudes are quite noisy at low frequencies, where plasma wave and thermal wave contributions can compete with each other, they become more stable in the high frequency limit where the plasma wave is dominant.
5. Measurements of semiconductor structures implanted on SIMOX wafers Although the laser beam modulated optical reflectance technique is very sensitive to implantation effects in semiconductors, we have found imaging ap-
Fig. 7. Frequency-dependent normalized amplitudes, measured at different positions on a Si-implanted Si wafer, where the following implantation regions are compared by normalization of the signals: ⌬,x, two measurements in a dot center vs. virgin surface region; `, center of dot vs. measurement beside the dot. The experimental data are approximated by theoretical curves based on the model of coupled thermal waves and plasma waves.
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plications, where the method failed when applied in the form as described in Sections 3 and 4. The analyzed sample, a mesa structure on a SIMOX wafer Ž500 m Si substrate, a 380-nm thick intermediate layer of SiO 2 ,
Fig. 8. Ža. Laser modulated reflectance image of an IPG transistor, monitored at 20 kHz, giving information on the contact pads Žin red. showing, however, insufficient contrast between implanted and not implanted areas in the center of the IPG transistor Žin green.. The position of the implanted gate line is schematically shown by the broken lines Žin black. Žb. Reflectance image of the IPG transistor based on the signal. amplitudes generated by electrical AC heating of 2 V at a modulation frequency of f s 5 kHz across the gate line. The hot spot is found in the region of reduced implantation ŽComp. Fig. 9.. Žc. Modulated reflectance image of the IPG transistor excited by combined optical excitation and electrical AC heating and detected at the fourth harmonic Ž4 f-detection..
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and a 200-nm p-Si layer on the top. consists of ion-beam implanted insulating lines of approximately 4-m width, written with the help of the micro-beam facility at the Dynamitron Tandem Laboratory of Ruhr-University Bochum. Here we report on measurements of an inplane gate ŽIPG. transistor ŽFig. 8., with a gate line not completely isolated due to an inhomogenous implantation. In Fig. 8a the laser modulated reflectance image of the complete system registered at a modulation frequency of f s 20 kHz is presented. It shows the four contact pads Žred areas. and the mesa Žgreen area., where the broken black lines schematically indicate the position of the gate line. As shown in Fig. 8a, the ion-implanted gate line does not produce sufficient contrast to be identifiable in the laser-modulated optical reflectance image. As shown, however, in Fig. 8b with the help of the modulated reflectance technique when applying a modulated voltage for excitation across the gate line, the leakage current generates a strong modulated heat source which is detected as a hot spot at the end of the gate line. Although the detection of such a hot spot already gives valuable information on the implantation effects, we have additionally tried to image the isolating line by applying the laser modulated optical reflectance technique alone, however, at different modulation frequencies. These tests failed, probably due to too low a contrast. This is most likely due to the layer structure of the SIMOX wafer, limiting the diffusion of the charge carriers by the electrically isolating SiO 2 layer located below the p-Si layer of only 200 nm thickness. Consequently, a larger plasma wave contribution could not be generated at the modulation frequencies accessible to our equipment applied in the usual way. In order to overcome this problem we have combined optical excitation with additional electrical DC and AC heating w4x. Such measurements can either be run by applying DC heating in addition to optical excitation and reflectance detection at the frequency of the optical excitation Ž1 f-detection. or by combining AC heating with optical excitation and by using detection at higher harmonics related to the coupling of the two excitation processes. In this case, optimal results could be obtained by combining optical excitation at 5 kHz and a 5-kHz sinusoidal AC voltage of approximately 2.5 V applied at an off-set voltage of 2.5 V across the isolating gate line ᎏ corresponding to electrical heating at 10 kHz ᎏ with the detection of the fourth harmonic Ž4 f-detection. at 20 kHz. In Fig. 8c we can see the implanted gate line with a very marked profile of decreasing amplitudes when approaching the region where we earlier have found the hot spot ŽFig. 8b.. In a separate line scan along the gate line ŽFig. 9. the signal decrease along the inhomogeneously implanted gate line has been determined quantitatively, and at the end of the isolating line the hot spot has
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Fig. 9. Line scans parallel to the gate line: diode voltage Ž`. as a measure of the laterally resolved total Žnot modulated. optical reflectance giving information on the localization of the gate line, in comparison with the modulated reflectance signal Ž䢇., generated by combined optical excitation and electrical AC heating and detected by 4 f-detection. The modulated signal gives information on the inhomogeneous implantation, strongly decreasing from y f 50 m to y f 25 m and on the hot spot region at approximately 17 m.
again been identified as a second small relative maximum. This second small maximum is probably due to the AC electrical heating, detectable at the fourth harmonic related to the non-linear voltage᎐current characteristics of the IPG transistor. We conclude that the signal amplitudes excited with the combined optical and electrical AC heating and detected at the fourth harmonic are correlated to the implantation profile along the gate line. Two possible reasons can be responsible for the achieved signal enhancement: the first can be a photo-induced current across the gate line leading to additional Joule heat sources; the second may be that the applied electrical field contributes to local changes of the photo-induced charge carrier distribution.
related to neutral particles and side effects due to focussed implantation, which are inherent to any ion beam technique. By means of frequency-dependent measurements more quantitative information was obtained on the electronic and thermal properties of critical areas, identified before by spatially resolved measurements Žimaging.. By comparing signals measured at positions of different implantation doses by a so-called normalization procedure and by approximating the resulting normalized signals by the coupled solutions of the charge carrier density waves and thermal waves w6x, it was possible to identify and quantify charge carrier effects in the presence of lattice defects. Additionally, we have found that in critical cases hidden information on implantation effects can be imaged by the combination of laser beam excitation and electrical heating, enabling a strong signal enhancement. To get more quantitative information about the implantation effects in this case, further work on the mechanisms of contrast enhancement is required, since the physical reasons are not yet understood. A further improved local resolution seems to be possible, e.g. in the detection of hot spots, when combining optical heating and electrical AC heating with thermoelastic detection-based scanning microscopy w8x. Acknowledgements The analyzed samples have been prepared in the tandem accelerator facility ŽDTL. of Ruhr-University. We acknowledge the support by A. Stephan and the DTL staff members. Additionally, we acknowledge the helpful discussions with J. Meijer, Ion Beam Physics group of Exp.-Phys. III. One of the authors, D. Dietzel, has been supported in the frame of the Graduiertenkolleg 384, DFG. References
6. Conclusion and outlook In this work we have shown, that thermal microscopy based on reflectance detection of thermal and plasma waves is a very versatile and powerful tool for the analysis of implanted semiconductor materials and devices. It offers a relatively good spatial resolution by non-contact remote detection, thus allowing non-contact measurements of both thermal and electronic transport properties on miniaturized structures which cannot be contacted due to their limited size. Imaging applications as well as analysis of electronic and thermal properties can be performed. Due to the high sensitivity of the modulated thermoreflectance signal it was possible to monitor halos
w1x A. Rosencwaig, in: A. Mandelis ŽEd.., Non-Destructive Evaluation, Prentice Hall, Englewood Cliffs, NJ, 1994, p. 2. w2x D. Fournier, in: D. Bicanic ŽEd.., Photoacoustic and Photothermal Phenomena III, Springer Ser. Opt. Sci., 69, 1982 49. w3x W. Kiepert, H.J. Obramski, J. Bolte et al., Surf. Coat. Technol. 116r119 Ž1999. 410᎐418. w4x D. Dietzel, H. Roecken, C. Crell, B.K. Bein, J. Pelzl, Combined electrical and optical heating in thermal wave microscopy of semiconductor devices, to be publ. in Analytical Sciences 2001 ŽProc. ICPPP 11th, Kyoto, Japan, June 2000.. w5x A.M. Mansanares, J.P. Roger, D. Fournier, A.C. Boccara, Appl. Phys. Lett. 64 Ž1994. 4. w6x D. Dietzel, PhD thesis, Ruhr-University Bochum, Bochum, Germany Ž2001.. w7x B.K. Bein, S. Krueger, J. Pelzl, Can. J. Phys. 64 Ž1986. 1208᎐1216. w8x J. Bolte, F. Niebisch, P. Stelmaszyk, A.D. Wieck, J. Pelzl, J. Appl. Phys. 84 Ž1998. 6917᎐6922.