Characterization of a magnetron sputtering discharge with simultaneous RF- and DC-excitation of the plasma for the deposition of transparent and conductive ZnO:Al-films

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Surface and Coatings Technology 98 ( 1998) 1251-1256

IFCHNOJDGY

Characterization of a magnetron sputtering discharge with simultaneous RF- and DC-excitation of the plasma for the deposition of transparent and conductive ZnO:Al-films K. EUrner *, R. Cebulla, R. Wendt Halm-Meitner-Institut. Dept. So/are Energetik. Glienicker Str. 100, D-J4J09 Berlin. Germany

Abstract A general drawback of the RF-magnetron sputtering technique is the small deposition rate, compared to DC-sputtering. This is caused by the low DC-voltage (40-160 V for RF-powers of 10-100 W) that develops at the target, since the sputtering rate and therefore the deposition rate in the energy range below I keY depend linearly on the acceleration voltage in the cathode fall. In order to increase the target voltage, a simultaneous excitation of the plasma by RF (13.56 MHz) and DC has been used. The DC-excitation is about 1.5 times as effective concerning the deposition rate compared to an RF-excitation. The mass selected ion energy distributions measured on a floating substrate display the characteristic differences between the two discharge modes. While the RF-excitation shows broad energy distributions, caused by the oscillating movement of the plasma sheath, and high plasma potentials (45 V), the DC-ion energy distributions exhibit sharp peaks at much lower plasma potentials (3 V). By a combination of DC- and RF-excitation the plasma potential and therefore the ion energy at a floating substrate can be shifted continuously between low (DC) and high (RF) values. ZnO- and ZnO:AI-layers from ceramic targets were prepared by this new combination for magnetron sputtering. The influence of the different ion energies at a floating substrate on the film structure and the electrical parameters is shown. © 1998 Elsevier Science S.A. Keywords: Magnetron sputtering; Simultaneous DC- and RF-excitation; Ion energy distributions; Transparent conductive oxide films; ZnO:AI

1. Introduction RF-magnetron sputtering of undoped (ZnO) and aluminium doped zinc oxide (ZnO:AI) is commonly used for the deposition of the window and contact layer for heterojunction thin film solar cells (TFSC), based on absorber materials like CulnSe 2 or CulnS 2 [1,2]. The reasons for the good properties of RF-sputtered ZnO-films on TFSC's are not yet clear. Disadvantages of the RF-sputtering technique are the small deposition rate and the high costs of ceramic targets. The low rate is caused by the small DC-voltage (40-160 V for RF-powers of 10-100 W) that is gener• ated at the target, because the sputtering rate in the energy range below I keV depends linearly on the accel• eration voltage in the cathode fall [3]. Since the discharge voltages for DC-excitation are significantly higher, a simultaneous excitation by RF (13.56 MHz) and DC has been used to increase the target voltage. By variation

* Corresponding author. 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PI150257-8972(97)00253-3

of the RFjDC power ratio a continuous tuning of the deposition mode between RF and DC is possible. This gives the opportunities, first, to investigate the plasma parameters that govern the ZnO-properties, and, second, to optimize the window deposition process for TFSC's. The process is characterized according to the ion energies at the substrate and it is shown that the structural and electrical properties of ZnO- and ZnO:AI-layers pre• pared from ceramic targets by this new combination are changed significantly. 2. Experimental For the experiments a turbopumped deposition cham• ber with two 3 in.-magnetrons and a load lock was used. The experimental details are listed in Table I and described in detail in Ref. [4]. The characterization of the magnetron discharge was made with a plasma monitor (BALZERS), a combina• tion of an electrostatic energy analyzer and a quadrupole

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K. E/lmer et al. / Surface and Coatings Teclmology 98 (1998) 1251-1256

Table I Deposition parameters Target-substrate distance Target diameter/thickness Magnetic flux parallel to the target surface Target purity Base pressure Sputtering pressure Substrate potential Substrate temperature Gas purity

65mm 76mm/6mm 0,16 T (directly above the erosion track) 4N (ZnO, ZnO:AI) 3 x 10- 7 mbar 8 x 10- 3 mbar Floating Not intentionally heated Ar( 6N )/02( 5N)

mass spectrometer. The energy resolution of the electro• static analyzer was about I eV. The detection limit of the plasma monitor was about 3 counts s - t and the dynamic range was better than 106 . For the measurement of the 1- V curves of the discharges the values displayed on the DC- and RF-power supplies have been used. The film thickness was measured by a quartz crystal monitor in situ. The layers on normal float glass have been analyzed by XRD with Cu Kex-radiation (texture, grain size, stress) and electrical measurements at room temper• ature (specific resistance, Hall coefficient).

3. Results and discussion 3. J. Discharge characteristics To characterize electrically the plasmas, the I-V curves of the discharges have been measured (Fig. I). While the pure DC-plasma is characterized by a high target voltage of 400 to 450 V, the pure RF-discharge is maintained at much lower voltages of 60 to 160 V. This

significant difference is caused by the different processes at the target and in the plasma body that govern the DC- and the RF-magnetron discharge [4,5]. The DC-excitation of a magnetron is based on the delivery of secondary electrons from the cathode. Therefore, a large discharge voltage is necessary to sustain the plasma because the secondary electron emission increases mono• tonically with the ion energy. On the other hand the RF-plasma is mainly driven by ionization through electrons in the plasma body which perform an oscillat• ing motion in the plasma. This kind of discharge excita• tion is more effective compared to the ionization by (nonoscillating) secondary electrons. Therefore, lower discharge voltages are generated at the same discharge power. The addition of RF-power to a DC-magnetron discharge leads to an abrupt decrease of the target voltages at RF-portions larger than 10 W (see Fig. I). This behaviour points towards a transition of the magne• tron discharge from the DC-mode to the RF-mode. The different excitation modes are also visible from the optical emission of the plasma in front of the target. While for a DC-plasma the discharge is confined near the target surface (1 to 2 em) the RF-excitation displays a much more extended plasma region, which is further• more brighter compared to the DC-case. The combina• tion of DC- and RF-excitation allows a continuous tuning of the discharge voltage. If the 1-V curves are fitted by a dependence I"" vn the DC-excitation is char• acterized by n=6.7, while the RF-discharge exhibits an n-value of only 0.6. According to Thornton [6] high n• values are typical for a good electron confinement in front of the target.

3.2. Plasma monitoring Additional insight into the discharge and deposition process was gained from a characterization by plasma monitoring. For the ZnO:Al-target, the ion energy distri• butions (lED) of the ions 0 +, Ar +, Zn + and ZnO+ have been measured with the extractor electrode at floating potential (Fig. 2a and Fig. 2b). The lED's of the different ions are similar in their main part. The ions Zn + and 0 + display shoulders at higher energies, which are due to the sputtering process (Zn +) or to the dissociation of the 02-molecule (0+). The lED's for the DC-excitation are much narrower compared to the RF-discharge. Furthermore, the ion energies in the DC-case are much lower than for RF-excitation. Varying the RF/DC-ratio, a continuous transition in the lED's can be achieved (Fig. 2c). From the lED's, the plasma potential was derived as the potential of the foot point of the high energy side of the energy distributions. Both discharge modes differ considerably according to the plasma and the floating potentials (Fig. 3). The potential difference Vp - v;. determines the energy of ions that arrive at a floating substrate if the ions traverse the

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1Surface and Coatings Teclmolog)/98 (1998) 1251-1256

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plasma sheath without collisions, which is the case in our pressure region. The maximum ion energy is by more than a factor of 2.5 higher for an RF- compared to a DC-discharge. The influence of the higher ion energies from the RF-discharge on the layer properties is discussed later. Integration of the ion energy distribu• tions for 0 +, ot, Zn + and ZnO + yielded the ratios 0+ lot and Zn + IZnO +, which are displayed in Fig. 4. The much higher concentration ratio Zn + IZnO + com• pared to 0 + lot is caused by the lower dissociation energy of ZnO (2.9 eV) in comparison to O2 (5.1 eV) [7], leading to a nearly complete dissociation of the ZnO-molecules during their passage of the plasma. It is clearly visible that the portion of atomic oxygen and

K. Ellmer et al. I Surlace and Coatings Teclmology 98 ( 1998) 1251-1256

1254

atomic zinc decreases with increasing DC-portion. This is caused by the more effective dissociative ionization in an RF-discharge. due to higher electron energies and the higher electron density. According to the relation Vp - Vr=kT./(2e) . In (mi/2.3m.) [8]. the electron temper• atures can be derived from the potential difference Vp - Vr· For the DC-discharge one gets T. = 3.9 eV. The RF discharge shows a significantly higher mean electron energy of 9.6 eV. It is known from the literature (Tominaga et al.) that negative ions (for instance 0-) can be accelerated to high energies in the cathode fall in the direction to the substrate and influence substan• tially the film growth of ZnO-films [9.10]. With our sputtering configuration (pressure. geometry) we were not able to detect negative ions with high energy with the plasma monitor at the substrate. Furthermore. in contrast to the work of Tominaga et al. our lateral distributions of the specific resistance showed no maxima opposite to the erosion tracks. which is. according to Tominaga et al.. due to the bombardment with energetic oxygen ions. Therefore. we believe. that these particles do not playa significant role for the film growth in our configuration. 3.3. Deposition rates The deposition rate was measured as a function of the DC-portion and is shown in Fig. 5. For both targets (ZnO. ZnO:AI) the rate is proportional to the power. RF-sputtering leads to smaller deposition rates by about a factor of 1.5. compared to DC-excitation. For sputter• ing with higher discharge powers (either DC or RF) one has to take care that the grown ZnO-films have the correct stoichiometry (Zn/O ~ 1.0). because the possi• bility exists that the films may become oxygen deficient.

which increases the absorption of the films by unoxidized Zn. Maintaining a small oxygen partial pressure (POl ~ 10- 5 mbar) during the deposition leads to transparent films. 3.4. Structuralfilm properties Thin films of ZnO and ZnO:AI were prepared using different DC-portions for the plasma excitation. The ZnO- and ZnO:AI-films had thicknesses of 120 and 350 nm. respectively. which is typical for the films used in our TFSC's. XRD-spectra of the films were taken to examine structural properties such as texture. grain size and stress. The films exclusively showed the (002)• diffraction peak (20=34.4°). due to a strong (001)• texture. i.e. the c-axes of the crystallites are oriented perpendicular to the surface. Fig. 6 shows the lattice constant cl ay• r relative to the value of bulk ZnO powder (Cbulk = 5.2066 A) and the grain sizes D OO2 as functions of the used DC-portions. The lattice constants were calculated from the peak positions and the grain sizes from the integral widths of the (002 )-peaks according to the Scherrer formula. Compared to Cbulk. our films show larger lattice constants due to compressive stress in the films. By bending measurements of cantilevers coated with ZnO-films, the compressive stress and the tendencies of the stress values have been confirmed. With increasing DC-portion. the ZnO:AI-films show increasing stress. We attribute this increase of the stress to the decrease of the energy of the ions (see Fig. 3) that impinge onto the growing film. With increasing DC-portions the ion energies decrease (see Fig. 3). Higher ion energies lead, due to a higher surface mobil• ity. to a better structural quality of the growing crystallites. Although the thickness of the undoped films is 3 times

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K. Ellmer et al. / Surface and Coatings Technology 98 (/998) /25/-/256

lower, they exhibit higher stress. Normally, for instance for metal films [II], the stress increases with increasing thickness. Therefore, it is assumed that the aluminium dopant has a positive influence on the crystallization reducing the film stress. The ZnO- and ZnO:AI-films show grain sizes of 22 and 37 nm, respectively, indepen• dent of the used DC-portion. The larger grains are due to the larger thicknesses of the doped films. 3.5. Electricalfilm properties

Fig.7(a) and Fig.7(b) display the results of the electrical measurements as functions of the used DC-portions. The doped films show specific resistivities p as low as 6.2 x 10- 4 n cm for DC-portions lower than 50%. For higher DC-portions p increases linearly up to 1.4 x 10- 3 n em. The increase of p is caused both by the decrease of the electron concentration n in the films as well as their mobility JJ.. The charge carrier concen• trations in the range of 5 x 1020 cm - 3 are by a factor of

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4. Conclusions

For the first time a magnetron sputtering discharge has been excited simultaneously by DC and RF (13.56 MHz) in order to increase the deposition rate. From the 1- V curves a transition from the DC- to the RF-mode of the magnetron operation is visible. In the RF-mode the electron confinement in front of the sput• tering target is lost. In the DC-mode the deposition rate is about 1.5 times higher compared to RF-sputtering. Plasma monitor measurements displayed the signifi• cantly different plasma potentials and ion energies at a floating substrate. While the DC-discharge is charac• terized by low plasma potentials and low ion energies, the RF-mode shows up to 2.5 times higher ion energies. The structural and electrical properties of doped and undoped ZnO-films, deposited by different RFIDC• ratios can be explained by taking into account the different ion energies during the deposition. Higher ion energies (RF-excitation) improve the structural quality and reduce the film stress, which decreases the specific resistance, both by a higher charge carrier mobility as well as higher carrier concentrations. These results can explain why RF-sputtering is almost exclusively used for the window preparation of thin film solar cells. Furthermore, the simultaneous RF-and DC-excitation can be used as an analytical tool to vary the ion energies continuously in a magnetron sputter• ing process.

5

References

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