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A novel integrated MIS low-pass filter device
Solid-State Electronics 47 (2003) 1685–1691 www.elsevier.com/locate/sse
A novel integrated MIS low-pass filter device Stephan Holten, Herbert Kliem
*
Institute of Electrical Engineering Physics, Saarland University, P.O. Box 151150, 66041 Saarbr€ucken, Germany Received 27 August 2001; accepted 14 April 2003
Abstract A novel method to prepare a passive integrated low-pass filter is described. The filter device is based on the layer sequence of a metal–insulator–semiconductor (MIS) structure. In contrast to the standard MIS system, the metal electrode of the filter device is structured to gain a local variation of its lateral resistance. The lateral resistance forms, together with the capacitance of the insulating film, a ladder filter RC network with low-pass characteristic. Devices with filter characteristics of fourth order and maximal attenuations of )110 dB have been realized until now. The cut-off frequencies have been varied between about 25 lHz and 1 kHz. To prepare MIS filters with cut-off frequencies in the sub-millihertz region, it is necessary to deposit metal electrodes with very high sheet resistances (up to 1015 X=). These sheet resistances could be achieved by evaporation of very thin metal films with a granular, disordered morphology. Because of their granular structure, the thin films reveal a thermally activated conductivity. 2003 Elsevier Ltd. All rights reserved. Keywords: Filter; Low-pass; Passive; Integrated; Granular; Disordered; Metal thin film
1. Introduction The metal–insulator transition of very thin metal films has been investigated since the mid of the last century [1–7]. A reduction of the thickness of metal films causes a change of their morpholbelow about 50 A ogy from a (poly)-crystalline to a granular, disordered structure. Because of the structural disorder the resistivity of the thin films increases exponentially with decreasing film thickness. Consequently, a local resistance distribution within a thin metal film can be caused by a local thickness distribution. This physical effect is used to develop a novel type of a low-pass filter device [8]. A palladium electrode structure with a variation of its local resistance is deposited onto an oxidized silicon substrate. The lateral resistance of the metal electrode layer in combination with the capacitance of the insulating film forms a filter RC network with low-pass characteristic. The filter re-
*
Corresponding author. Fax: +49-681-3022272. E-mail address: [email protected] (H. Kliem).
sponse characteristic can be adjusted by the resistance distribution, i.e., the thickness profile, of the metal electrode structure. We have established a method to deposit the described metal electrode structure within a single evaporation process through a shadow mask. Filter cut-off frequencies between about 25 lHz and 1 kHz could be achieved by sheet resistances between 1015 and 106 X= for palladium electrodes. Gold electrodes of the same geometrical dimensions show no measurable conductivity in comparison to the palladium structures. At the present time filter devices with a maximal filter attenuation of )110 dB are realized. Interesting technical applications for these passive filter structures could be the use as analogous anti-aliasing prefilter in sampling systems or the analogous bandwidth confinement of impedance meters. For the industrial fabrication it is an advantage that the preparation of the filter structures can be carried out by standard silicon process technology. In addition to the characterization of the electrical transfer characteristic of the filter devices, the conduction mechanism within the granular palladium films is
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00163-1
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investigated. The lateral resistance distribution of the palladium electrode layer, the temperature dependence of the lateral resistance, and the temperature dependence of the filter response characteristic is determined. By atomic force microscopy (AFM) the morphology of the metal electrode structures was recorded.
2. Filter preparation The metal–insulator–semiconductor filter structures are prepared by evaporation of a palladium electrode structure onto an oxidized silicon substrate (n-type). A constant SiO2 film thickness of 100 nm has been used for all devices. The palladium electrode structures consist of two low-resistance contact pads I (Fig. 1(a)), which are connected by a high-resistance region II, the filter active region. The complete electrode structure is deposited within a single evaporation process through a shadow mask at room temperature. During the evaporation process, the shadow mask is fixed a few millimeters in front of the SiO2 layer. The contact pads are prepared by direct deposition through apertures in the mask. The
granular thin film within the high-resistance region II results from that part of the vaporized metal which is scattered at the edge of the mask aperture. The larger the distance to the edge of the aperture is, the lower is the metal film thickness. The nominal electrode layer thickness is specified with respect to the thickness of the contact pads I. Contact pads with thicknesses between 25 and 75 nm are deposited. Further, the distance between the contact pads I have been varied. Two palladium MIS filter series have been prepared: A series D1.0 with a nominal distance of 1 mm between the contact pads and another series D0.5 with a distance of 0.5 mm. In order to compare the conduction behavior between palladium and gold electrode structures, gold electrodes having the same geometrical dimensions as the palladium structures are prepared also. AFM measurements have shown that the morphology of the high-resistance area II differs from the morphology of the low-resistance contact pads I. For the contact pads I we find a closed metallic film (Fig. 1(b)). But in the high-resistance area II a granular, disordered film structure has been detected (Fig. 1(c)). Corresponding to the layer profile of region II (Gaussian like profile) (Fig. 1(a)) the lateral resistance increases from the left side to the right side and forms a series filter RC network together with the capacitance of the SiO2 film (Fig. 1(d)). This equivalent RC network in its discrete form is known as Cauer filter [9].
3. Filter response function 3.1. Electrical characterization
Fig. 1. (a) MIS filter structure. I Low-resistance contact pads, II high-resistance region. (b) Morphology I recorded by AFM. (c) Morphology II recorded by AFM. (d) Equivalent circuit high-resistance region II: filter Rk C network, contact pads: Cpi , Cpo .
The filter response functions of MIS filters with palladium electrodes have been measured in a frequency range from 1 mHz to 200 kHz by a lock-in amplifier Fig. 2(i)–(iv). The filter response function Fig. 2(v) has been approximated by measurements of the capacitance of the MIS filter structure in dependence on the frequency. Measuring the capacitance between the input contact pad Cpi and the substrate yields an increasing measurable effective capacitance with decreasing frequency. This would be expected from the equivalent circuit (Fig. 1(d)). By comparison of the frequency dependent effective capacitance of the MIS filter structure Fig. 2(v) with the frequency dependent effective capacitances from the filters (i)–(iv) the filter response function Fig. 2(v) has been determined. The ratio U2 =U1 of the output voltage U2 and the input voltage U1 shows an increasing steepness of the filter response functions with increasing frequency. This behavior corresponds to the serial RC network shown as equivalent circuit in Fig. 1(d). We find cut-off frequencies from 25 lHz to 1 kHz. The thickness of the SiO2
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Fig. 2. Filter response function parameters: thickness metal electrode pad I, distance between contact pads, i.e. nominal length of region II.
film determines the capacitances in the equivalent circuit (Fig. 1(d)) and influences as well the cut-off frequency. Thus the SiO2 film thickness of the measured devices is constant, the different cut-off frequencies result from the different lateral resistance distributions caused by the electrode thicknesses and the distances between the contact pads I. The higher the lateral resistance is, the lower is the cut-off frequency. Theoretical simulations with Pspice are carried out to investigate in principle the effect of the distribution of the sheet resistance RðxÞ within the filter active region II on the filter response characteristic. To calculate the filter response function exemplary the electrode region II has been modeled by separation into 10 single RC networks. The simulations show that a constant sheet resistance within the filter active region II should result in an enhanced filter edge steepness. Fig. 3(a) yields the comparison between the filter response functions of a
MIS filter with a constant lateral resistance RðxÞ Fig. 3(a)(i) and a MIS filter with an exponential increasing RðxÞ Fig. 3(a)(iii). For the MIS filter with RðxÞ ¼ const. a steeper cut-off profile is found. Additionally an effect caused by the capacitance of the output contact pad Cpo is simulated Fig. 3(a)(ii) + (iv). With increasing pad dimensions the pad capacitance Cpo increases and causes a shift of the filter response function towards lower frequencies. Further, a reduced steepness of the filter response function can be observed. In Fig. 3(b) the order of the different response functions in dependence on the frequency is shown. The curves for RðxÞ ¼ const. exhibit a strong increase to 10th order with increasing frequency (Fig. 3(a)(i) + (ii)). This corresponds to the superposition of 10 single filter RC networks with relaxation times s ¼ R C ¼ 30 s (R ¼ 1 TX, C ¼ 30 pF). The curves calculated with an exponential increase of RðxÞ show a smaller increase of the
Fig. 3. Theoretical simulations of MIS filter structures with Pspice parameters: resistance distribution in the filter active region II, dimension output contact pad, i.e. Cpo . (a) Filter response function, (b) filter order.
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filter order with increasing frequency (Fig. 3(a)(iii) + (iv)). This is caused by the superposition of 10 single filter RC networks with exponentially increasing relaxation times from smin ¼ Rmin C ¼ 30 ns to smax ¼ Rmax C ¼ 30 s (Rmin ¼ 1 kX, Rmax ¼ 1 TX, C ¼ 30 pF). The effect of the contact pad capacitance Cpo can be identified for low frequencies in a reduced increase of the filter order (Fig. 3(a)(ii) + (iv)). This simulations yield the synthesis parameter for an optimal MIS filter structure: 1. The dimensions of the contact pads have to be minimized to neglect the effect of Cpo . 2. The resistance distribution within the filter active region RðxÞ should be constant. This could be achieved by a constant film thickness within the filter active region of the electrode structure. The Gaussian-like profile of the metal electrode structure (Fig. 1(a)) is caused by the currently used shadow mask configuration. The profile results from scattering of the vaporized metal at one aperture. An almost constant film thickness can be achieved by scattering at two symmetric arranged apertures in the shadow mask. Electrical optimization and miniaturization of the metal electrode structure will be carried out in the future by optimizing the shadow mask configuration. If two evaporation steps are allowed, the evaporation of a thin film followed by the evaporation of the pads using different masks should be possible as well. To enhance the order of the filter response function with the current metal electrode, we have prepared three single filter structures connected in series within a single evaporation process. The response functions of the single filter structures superimpose linearly to the overall response function of the filtersÕ series connection (Fig. 4). The maximal steepness of the filter cut-off profile can
Fig. 5. Temperature dependence of the MIS filter response function.
now be determined to be of )80 dB/dec. For this triple filter structure we have measured a maximal attenuation of )110 dB at 17 Hz and a cut-off frequency of 60 mHz. 3.2. Temperature dependence The temperature dependence of the filter response function has been measured between 93 and 333 K (Fig. 5). For decreasing temperature the filter response function is shifted towards lower frequencies. A frequency shift by a factor of 30 can be observed between 333 and 93 K. The shape of the response function shows almost no variation with decreasing temperature. The temperature shift of the response function is constant on a logarithmic scale, consequently an Arrhenius like temperature behavior must dominate the conduction mechanism within the high-resistance area II of the electrode structure. For technical applications it may be of advantage that a tuning of the filter response can be achieved by a temperature variation.
4. Lateral resistance 4.1. Electrical characterization
Fig. 4. Filter response functions of three single MIS filters, series connection of the three MIS filters.
The filter response characteristic of the described MIS filter structures depends on the resistance distribution of the high-resistance area II. To analyze the correlation between the lateral resistance distribution and the filter response characteristic, we have measured the lateral resistance distribution by a test needle (tip radius: 2 lm), which has been rastered from the reference contact pad to the counter contact pad (Fig. 6).
S. Holten, H. Kliem / Solid-State Electronics 47 (2003) 1685–1691
Fig. 6. Measurement set up for the lateral resistance.
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Obviously the measured curves are affected by the current flow field of the test needle. There is a characteristic peak in the curves, which can be attributed to the change from a point contact field of the test needle to a line contact field when the test needle reaches the lowresistive counter contact pad. The actually existing resistance distribution can be recalculated by finiteelement methods. We can conclude from these measurements that only a very small high-resistive part of the filter active region II determines the filter characteristic. Consequently, the electrode structures can be miniaturized with respect to the lateral length of filter active region II. A highresistive layer of a length of 50 lm (see Fig. 7) should be sufficient to realize filters with the same response characteristic. Contact pads of the same length and a broadness of 50 lm of the electrode structure would result in an overall area of the filter structure of 50 150 lm2 . 4.2. Temperature dependence Fig. 8 compares the temperature dependence of the resistivity of a granular palladium film (Fig. 8(a)) and a solid palladium (Fig. 8(b)). The solid palladium shows, as expected, an increasing resistivity with increasing temperature. Whereas the granular thin film exhibits a decreasing resistivity versus temperature. This behavior is well described by a thermal activated process, whereas two temperature ranges have to be distinguished: Temperatures above 120 K [Fig. 8(a)(i)]: RðT Þ ¼ R1 exp
W ; kT
R1 ¼ 2:28 108 X;
120 K < T < 333 K
ð1Þ
W ¼ 44 meV
Fig. 7. Lateral resistance parameter: electrode thickness. (a) Palladium electrode of MIS filter D1.0, i.e. distance between contact pads 1 mm, (b) palladium electrode of MIS filter D0.5, i.e. distance between contact pads 0.5 mm.
We find an exponential increase of the resistance versus the distance in the high resistance area II (Fig. 7). The overall resistances between the contact pads vary between about 105 and 1014 X in dependence on the electrode thickness and the distance between the contact pads. MIS filters with a gold electrode structure of the same geometrical dimensions show no measurable conductivity between the contact pads I.
Fig. 8. Temperature dependence of the resistance: (a) electrode structure MIS filter; thickness 75 nm, (b) palladium solid.
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Temperatures below 120 K (Fig. 8(a)(ii)): RðT Þ ¼ R2 exp
T0 T
R2 ¼ 3:4 108 X;
m ;
83 K < T < 120 K
m ¼ 0:92;
ð2Þ
room atmosphere and for maximal temperatures up to 365 K. To achieve stability at higher temperatures as well other electrode materials like palladium alloys could be tested.
T0 ¼ 510 K 5. Summary
The activation energy tends to increase with decreasing film thickness. It is assumed that the thermally activated conductivity origins from the granular, disordered structure of the thin films [1–4]. The wave functions of the charge carriers may become localized to the single metal clusters of the thin film. In the range of the Fermi level, the conduction band of the metal splits into single energy states. Consequently, a thermally activated hopping process of charge carriers from cluster to cluster models the measured thermally activated conductivity [5–7]. At high temperatures the charge carriers gain sufficient energy to move thermally activated and therefore an unique Arrhenius behavior (1) of the conductivity in dependence on the temperature can be measured (Fig. 8(a)(i)). At low temperatures a phonon assisted tunneling process prevails and the thermal activation process changes from the unique Arrhenius type to a weaker temperature dependence. This effect is expressed by the exponent m in the argument of the exponential function of Eq. (2). Markovic et al. [7] have found an exponent m ¼ 0:8 for palladium thin films of thicknesses between at temperatures between 0.5 and 20 K. We 10 and 13 A have found m ¼ 0:92 at temperatures between 83 and 120 K. Measurements of the conductivity at lower temperatures will be carried out in the future. The thermally activated conduction behavior of the palladium electrode can be found up to temperatures of 365 K. Above this temperature the conductivity decays with time at a constant temperature. It is still an open question if this decreasing conductivity is caused by a change of the structural order of the thin film or if it is provoked by a diffusion of hydrogen out of the palladium film. In contrast to gold, palladium can absorb hydrogen up to a ratio Pd:H equal to 1:0.7 at room temperature [10]. Gold electrode structures, with the same geometrical dimensions as the palladium structures, have shown no measurable conductivity. So the decreasing conductivity of the palladium films at high temperatures and the absence of a conductivity of gold films indicate that hydrogen could be involved in the conduction mechanism. Further experimental insight into the conduction mechanism should be gained by sheet resistance measurements of the thin films in a hydrogen atmosphere. The reliability, i.e. the physical stability of the MIS filter structures is decisive for the use in technical applications. The response behavior of the current MIS filter structures is constant for months by storage in
We have presented a novel method to prepare integrated low-pass filters by evaporating a palladium electrode structure on an oxidized silicon substrate. The low-pass characteristic is achieved by a high-resistive electrode layer in combination with the capacitance of the SiO2 film. The very high sheet resistances of the palladium electrode (up to 1015 X=) enable us to prepare filters with cut-off frequencies in the sub-millihertz region. The currently realized filter cut-off frequencies are distributed over eight decades between 25 lHz and 1 kHz. At the moment, filter characteristics of fourth order and maximal filter attenuations of )110 dB are established. It has been shown that a miniaturization of the filter structures to a chip area of 50 · 150 lm2 should be possible. The high-resistance region formed by a granular, disordered film structure causes a thermally activated temperature behavior of the lateral resistance. Especially for low frequency applications the described MIS filter structure could be a reasonable alternative to standard integrated circuits. The filter fabrication is carried out and completed within a single evaporation process of a palladium electrode structure. No wiring of external components like large resistances or capacitances is necessary. The described MIS filter structure bases only on the local resistance distribution of the metal electrode and not on the oxide or substrate type. Therefore, the filter device can also be fabricated with any other insulator– substrate combination.
Acknowledgement The authors like to thank Dipl.-Ing. Marc Nalbach for assistance in the calculation of the flow fields.
References [1] Anderson PW. Absence of diffusion in certain random lattices. Phys Rev 1958;(109):1492–505. [2] Mott NF, Davis EA. Electronic processes in non-crystalline materials. Oxford: Clarendon Press; 1979. [3] Mott NF. Metal–insulator transitions. 2nd ed. London: Taylor & Francis; 1990. [4] Lee PA. Disordered electronic systems. Rev Mod Phys 1985;(57):287–337.
S. Holten, H. Kliem / Solid-State Electronics 47 (2003) 1685–1691 [5] Lee SJ et al. Metal–insulator transition in quasi-twodimensional Mo–C films. Phys Rev B 1992;(19): 12695– 700. [6] Dynes RC et al. Two-dimensional electrical conductivity in quench-condensed metal films. Phys Rev Lett 1978;(40): 479–83. [7] Markovic N et al. Anomalous hopping exponents of ultrathin metal films. Phys Rev B 2000;(62):2195–200.
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[8] Holten S, Kliem H. An integrated low-pass filter structure. Seventh International Conference on Solid Dielectrics, Eindhoven, The Netherlands, 25–29 June 2001, Conference Proceedings: 215–218. [9] Chang CY, Sze SM. ULSI devices. New York: John Wiley & Sons; 2000. [10] Holleman-Wiberg. Lehrbuch der Anorganischen Chemie. Berlin: Walter de Gruyter, 101. Auflage, 1985.
A novel integrated MIS low-pass filter device Stephan Holten, Herbert Kliem
*
Institute of Electrical Engineering Physics, Saarland University, P.O. Box 151150, 66041 Saarbr€ucken, Germany Received 27 August 2001; accepted 14 April 2003
Abstract A novel method to prepare a passive integrated low-pass filter is described. The filter device is based on the layer sequence of a metal–insulator–semiconductor (MIS) structure. In contrast to the standard MIS system, the metal electrode of the filter device is structured to gain a local variation of its lateral resistance. The lateral resistance forms, together with the capacitance of the insulating film, a ladder filter RC network with low-pass characteristic. Devices with filter characteristics of fourth order and maximal attenuations of )110 dB have been realized until now. The cut-off frequencies have been varied between about 25 lHz and 1 kHz. To prepare MIS filters with cut-off frequencies in the sub-millihertz region, it is necessary to deposit metal electrodes with very high sheet resistances (up to 1015 X=). These sheet resistances could be achieved by evaporation of very thin metal films with a granular, disordered morphology. Because of their granular structure, the thin films reveal a thermally activated conductivity. 2003 Elsevier Ltd. All rights reserved. Keywords: Filter; Low-pass; Passive; Integrated; Granular; Disordered; Metal thin film
1. Introduction The metal–insulator transition of very thin metal films has been investigated since the mid of the last century [1–7]. A reduction of the thickness of metal films causes a change of their morpholbelow about 50 A ogy from a (poly)-crystalline to a granular, disordered structure. Because of the structural disorder the resistivity of the thin films increases exponentially with decreasing film thickness. Consequently, a local resistance distribution within a thin metal film can be caused by a local thickness distribution. This physical effect is used to develop a novel type of a low-pass filter device [8]. A palladium electrode structure with a variation of its local resistance is deposited onto an oxidized silicon substrate. The lateral resistance of the metal electrode layer in combination with the capacitance of the insulating film forms a filter RC network with low-pass characteristic. The filter re-
*
Corresponding author. Fax: +49-681-3022272. E-mail address: [email protected] (H. Kliem).
sponse characteristic can be adjusted by the resistance distribution, i.e., the thickness profile, of the metal electrode structure. We have established a method to deposit the described metal electrode structure within a single evaporation process through a shadow mask. Filter cut-off frequencies between about 25 lHz and 1 kHz could be achieved by sheet resistances between 1015 and 106 X= for palladium electrodes. Gold electrodes of the same geometrical dimensions show no measurable conductivity in comparison to the palladium structures. At the present time filter devices with a maximal filter attenuation of )110 dB are realized. Interesting technical applications for these passive filter structures could be the use as analogous anti-aliasing prefilter in sampling systems or the analogous bandwidth confinement of impedance meters. For the industrial fabrication it is an advantage that the preparation of the filter structures can be carried out by standard silicon process technology. In addition to the characterization of the electrical transfer characteristic of the filter devices, the conduction mechanism within the granular palladium films is
0038-1101/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00163-1
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investigated. The lateral resistance distribution of the palladium electrode layer, the temperature dependence of the lateral resistance, and the temperature dependence of the filter response characteristic is determined. By atomic force microscopy (AFM) the morphology of the metal electrode structures was recorded.
2. Filter preparation The metal–insulator–semiconductor filter structures are prepared by evaporation of a palladium electrode structure onto an oxidized silicon substrate (n-type). A constant SiO2 film thickness of 100 nm has been used for all devices. The palladium electrode structures consist of two low-resistance contact pads I (Fig. 1(a)), which are connected by a high-resistance region II, the filter active region. The complete electrode structure is deposited within a single evaporation process through a shadow mask at room temperature. During the evaporation process, the shadow mask is fixed a few millimeters in front of the SiO2 layer. The contact pads are prepared by direct deposition through apertures in the mask. The
granular thin film within the high-resistance region II results from that part of the vaporized metal which is scattered at the edge of the mask aperture. The larger the distance to the edge of the aperture is, the lower is the metal film thickness. The nominal electrode layer thickness is specified with respect to the thickness of the contact pads I. Contact pads with thicknesses between 25 and 75 nm are deposited. Further, the distance between the contact pads I have been varied. Two palladium MIS filter series have been prepared: A series D1.0 with a nominal distance of 1 mm between the contact pads and another series D0.5 with a distance of 0.5 mm. In order to compare the conduction behavior between palladium and gold electrode structures, gold electrodes having the same geometrical dimensions as the palladium structures are prepared also. AFM measurements have shown that the morphology of the high-resistance area II differs from the morphology of the low-resistance contact pads I. For the contact pads I we find a closed metallic film (Fig. 1(b)). But in the high-resistance area II a granular, disordered film structure has been detected (Fig. 1(c)). Corresponding to the layer profile of region II (Gaussian like profile) (Fig. 1(a)) the lateral resistance increases from the left side to the right side and forms a series filter RC network together with the capacitance of the SiO2 film (Fig. 1(d)). This equivalent RC network in its discrete form is known as Cauer filter [9].
3. Filter response function 3.1. Electrical characterization
Fig. 1. (a) MIS filter structure. I Low-resistance contact pads, II high-resistance region. (b) Morphology I recorded by AFM. (c) Morphology II recorded by AFM. (d) Equivalent circuit high-resistance region II: filter Rk C network, contact pads: Cpi , Cpo .
The filter response functions of MIS filters with palladium electrodes have been measured in a frequency range from 1 mHz to 200 kHz by a lock-in amplifier Fig. 2(i)–(iv). The filter response function Fig. 2(v) has been approximated by measurements of the capacitance of the MIS filter structure in dependence on the frequency. Measuring the capacitance between the input contact pad Cpi and the substrate yields an increasing measurable effective capacitance with decreasing frequency. This would be expected from the equivalent circuit (Fig. 1(d)). By comparison of the frequency dependent effective capacitance of the MIS filter structure Fig. 2(v) with the frequency dependent effective capacitances from the filters (i)–(iv) the filter response function Fig. 2(v) has been determined. The ratio U2 =U1 of the output voltage U2 and the input voltage U1 shows an increasing steepness of the filter response functions with increasing frequency. This behavior corresponds to the serial RC network shown as equivalent circuit in Fig. 1(d). We find cut-off frequencies from 25 lHz to 1 kHz. The thickness of the SiO2
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Fig. 2. Filter response function parameters: thickness metal electrode pad I, distance between contact pads, i.e. nominal length of region II.
film determines the capacitances in the equivalent circuit (Fig. 1(d)) and influences as well the cut-off frequency. Thus the SiO2 film thickness of the measured devices is constant, the different cut-off frequencies result from the different lateral resistance distributions caused by the electrode thicknesses and the distances between the contact pads I. The higher the lateral resistance is, the lower is the cut-off frequency. Theoretical simulations with Pspice are carried out to investigate in principle the effect of the distribution of the sheet resistance RðxÞ within the filter active region II on the filter response characteristic. To calculate the filter response function exemplary the electrode region II has been modeled by separation into 10 single RC networks. The simulations show that a constant sheet resistance within the filter active region II should result in an enhanced filter edge steepness. Fig. 3(a) yields the comparison between the filter response functions of a
MIS filter with a constant lateral resistance RðxÞ Fig. 3(a)(i) and a MIS filter with an exponential increasing RðxÞ Fig. 3(a)(iii). For the MIS filter with RðxÞ ¼ const. a steeper cut-off profile is found. Additionally an effect caused by the capacitance of the output contact pad Cpo is simulated Fig. 3(a)(ii) + (iv). With increasing pad dimensions the pad capacitance Cpo increases and causes a shift of the filter response function towards lower frequencies. Further, a reduced steepness of the filter response function can be observed. In Fig. 3(b) the order of the different response functions in dependence on the frequency is shown. The curves for RðxÞ ¼ const. exhibit a strong increase to 10th order with increasing frequency (Fig. 3(a)(i) + (ii)). This corresponds to the superposition of 10 single filter RC networks with relaxation times s ¼ R C ¼ 30 s (R ¼ 1 TX, C ¼ 30 pF). The curves calculated with an exponential increase of RðxÞ show a smaller increase of the
Fig. 3. Theoretical simulations of MIS filter structures with Pspice parameters: resistance distribution in the filter active region II, dimension output contact pad, i.e. Cpo . (a) Filter response function, (b) filter order.
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filter order with increasing frequency (Fig. 3(a)(iii) + (iv)). This is caused by the superposition of 10 single filter RC networks with exponentially increasing relaxation times from smin ¼ Rmin C ¼ 30 ns to smax ¼ Rmax C ¼ 30 s (Rmin ¼ 1 kX, Rmax ¼ 1 TX, C ¼ 30 pF). The effect of the contact pad capacitance Cpo can be identified for low frequencies in a reduced increase of the filter order (Fig. 3(a)(ii) + (iv)). This simulations yield the synthesis parameter for an optimal MIS filter structure: 1. The dimensions of the contact pads have to be minimized to neglect the effect of Cpo . 2. The resistance distribution within the filter active region RðxÞ should be constant. This could be achieved by a constant film thickness within the filter active region of the electrode structure. The Gaussian-like profile of the metal electrode structure (Fig. 1(a)) is caused by the currently used shadow mask configuration. The profile results from scattering of the vaporized metal at one aperture. An almost constant film thickness can be achieved by scattering at two symmetric arranged apertures in the shadow mask. Electrical optimization and miniaturization of the metal electrode structure will be carried out in the future by optimizing the shadow mask configuration. If two evaporation steps are allowed, the evaporation of a thin film followed by the evaporation of the pads using different masks should be possible as well. To enhance the order of the filter response function with the current metal electrode, we have prepared three single filter structures connected in series within a single evaporation process. The response functions of the single filter structures superimpose linearly to the overall response function of the filtersÕ series connection (Fig. 4). The maximal steepness of the filter cut-off profile can
Fig. 5. Temperature dependence of the MIS filter response function.
now be determined to be of )80 dB/dec. For this triple filter structure we have measured a maximal attenuation of )110 dB at 17 Hz and a cut-off frequency of 60 mHz. 3.2. Temperature dependence The temperature dependence of the filter response function has been measured between 93 and 333 K (Fig. 5). For decreasing temperature the filter response function is shifted towards lower frequencies. A frequency shift by a factor of 30 can be observed between 333 and 93 K. The shape of the response function shows almost no variation with decreasing temperature. The temperature shift of the response function is constant on a logarithmic scale, consequently an Arrhenius like temperature behavior must dominate the conduction mechanism within the high-resistance area II of the electrode structure. For technical applications it may be of advantage that a tuning of the filter response can be achieved by a temperature variation.
4. Lateral resistance 4.1. Electrical characterization
Fig. 4. Filter response functions of three single MIS filters, series connection of the three MIS filters.
The filter response characteristic of the described MIS filter structures depends on the resistance distribution of the high-resistance area II. To analyze the correlation between the lateral resistance distribution and the filter response characteristic, we have measured the lateral resistance distribution by a test needle (tip radius: 2 lm), which has been rastered from the reference contact pad to the counter contact pad (Fig. 6).
S. Holten, H. Kliem / Solid-State Electronics 47 (2003) 1685–1691
Fig. 6. Measurement set up for the lateral resistance.
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Obviously the measured curves are affected by the current flow field of the test needle. There is a characteristic peak in the curves, which can be attributed to the change from a point contact field of the test needle to a line contact field when the test needle reaches the lowresistive counter contact pad. The actually existing resistance distribution can be recalculated by finiteelement methods. We can conclude from these measurements that only a very small high-resistive part of the filter active region II determines the filter characteristic. Consequently, the electrode structures can be miniaturized with respect to the lateral length of filter active region II. A highresistive layer of a length of 50 lm (see Fig. 7) should be sufficient to realize filters with the same response characteristic. Contact pads of the same length and a broadness of 50 lm of the electrode structure would result in an overall area of the filter structure of 50 150 lm2 . 4.2. Temperature dependence Fig. 8 compares the temperature dependence of the resistivity of a granular palladium film (Fig. 8(a)) and a solid palladium (Fig. 8(b)). The solid palladium shows, as expected, an increasing resistivity with increasing temperature. Whereas the granular thin film exhibits a decreasing resistivity versus temperature. This behavior is well described by a thermal activated process, whereas two temperature ranges have to be distinguished: Temperatures above 120 K [Fig. 8(a)(i)]: RðT Þ ¼ R1 exp
W ; kT
R1 ¼ 2:28 108 X;
120 K < T < 333 K
ð1Þ
W ¼ 44 meV
Fig. 7. Lateral resistance parameter: electrode thickness. (a) Palladium electrode of MIS filter D1.0, i.e. distance between contact pads 1 mm, (b) palladium electrode of MIS filter D0.5, i.e. distance between contact pads 0.5 mm.
We find an exponential increase of the resistance versus the distance in the high resistance area II (Fig. 7). The overall resistances between the contact pads vary between about 105 and 1014 X in dependence on the electrode thickness and the distance between the contact pads. MIS filters with a gold electrode structure of the same geometrical dimensions show no measurable conductivity between the contact pads I.
Fig. 8. Temperature dependence of the resistance: (a) electrode structure MIS filter; thickness 75 nm, (b) palladium solid.
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Temperatures below 120 K (Fig. 8(a)(ii)): RðT Þ ¼ R2 exp
T0 T
R2 ¼ 3:4 108 X;
m ;
83 K < T < 120 K
m ¼ 0:92;
ð2Þ
room atmosphere and for maximal temperatures up to 365 K. To achieve stability at higher temperatures as well other electrode materials like palladium alloys could be tested.
T0 ¼ 510 K 5. Summary
The activation energy tends to increase with decreasing film thickness. It is assumed that the thermally activated conductivity origins from the granular, disordered structure of the thin films [1–4]. The wave functions of the charge carriers may become localized to the single metal clusters of the thin film. In the range of the Fermi level, the conduction band of the metal splits into single energy states. Consequently, a thermally activated hopping process of charge carriers from cluster to cluster models the measured thermally activated conductivity [5–7]. At high temperatures the charge carriers gain sufficient energy to move thermally activated and therefore an unique Arrhenius behavior (1) of the conductivity in dependence on the temperature can be measured (Fig. 8(a)(i)). At low temperatures a phonon assisted tunneling process prevails and the thermal activation process changes from the unique Arrhenius type to a weaker temperature dependence. This effect is expressed by the exponent m in the argument of the exponential function of Eq. (2). Markovic et al. [7] have found an exponent m ¼ 0:8 for palladium thin films of thicknesses between at temperatures between 0.5 and 20 K. We 10 and 13 A have found m ¼ 0:92 at temperatures between 83 and 120 K. Measurements of the conductivity at lower temperatures will be carried out in the future. The thermally activated conduction behavior of the palladium electrode can be found up to temperatures of 365 K. Above this temperature the conductivity decays with time at a constant temperature. It is still an open question if this decreasing conductivity is caused by a change of the structural order of the thin film or if it is provoked by a diffusion of hydrogen out of the palladium film. In contrast to gold, palladium can absorb hydrogen up to a ratio Pd:H equal to 1:0.7 at room temperature [10]. Gold electrode structures, with the same geometrical dimensions as the palladium structures, have shown no measurable conductivity. So the decreasing conductivity of the palladium films at high temperatures and the absence of a conductivity of gold films indicate that hydrogen could be involved in the conduction mechanism. Further experimental insight into the conduction mechanism should be gained by sheet resistance measurements of the thin films in a hydrogen atmosphere. The reliability, i.e. the physical stability of the MIS filter structures is decisive for the use in technical applications. The response behavior of the current MIS filter structures is constant for months by storage in
We have presented a novel method to prepare integrated low-pass filters by evaporating a palladium electrode structure on an oxidized silicon substrate. The low-pass characteristic is achieved by a high-resistive electrode layer in combination with the capacitance of the SiO2 film. The very high sheet resistances of the palladium electrode (up to 1015 X=) enable us to prepare filters with cut-off frequencies in the sub-millihertz region. The currently realized filter cut-off frequencies are distributed over eight decades between 25 lHz and 1 kHz. At the moment, filter characteristics of fourth order and maximal filter attenuations of )110 dB are established. It has been shown that a miniaturization of the filter structures to a chip area of 50 · 150 lm2 should be possible. The high-resistance region formed by a granular, disordered film structure causes a thermally activated temperature behavior of the lateral resistance. Especially for low frequency applications the described MIS filter structure could be a reasonable alternative to standard integrated circuits. The filter fabrication is carried out and completed within a single evaporation process of a palladium electrode structure. No wiring of external components like large resistances or capacitances is necessary. The described MIS filter structure bases only on the local resistance distribution of the metal electrode and not on the oxide or substrate type. Therefore, the filter device can also be fabricated with any other insulator– substrate combination.
Acknowledgement The authors like to thank Dipl.-Ing. Marc Nalbach for assistance in the calculation of the flow fields.
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S. Holten, H. Kliem / Solid-State Electronics 47 (2003) 1685–1691 [5] Lee SJ et al. Metal–insulator transition in quasi-twodimensional Mo–C films. Phys Rev B 1992;(19): 12695– 700. [6] Dynes RC et al. Two-dimensional electrical conductivity in quench-condensed metal films. Phys Rev Lett 1978;(40): 479–83. [7] Markovic N et al. Anomalous hopping exponents of ultrathin metal films. Phys Rev B 2000;(62):2195–200.
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[8] Holten S, Kliem H. An integrated low-pass filter structure. Seventh International Conference on Solid Dielectrics, Eindhoven, The Netherlands, 25–29 June 2001, Conference Proceedings: 215–218. [9] Chang CY, Sze SM. ULSI devices. New York: John Wiley & Sons; 2000. [10] Holleman-Wiberg. Lehrbuch der Anorganischen Chemie. Berlin: Walter de Gruyter, 101. Auflage, 1985.