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A vertical Hall device in CMOS high-voltage technology
Sensors and Actuators A 97±98 (2002) 47±53
A vertical Hall device in CMOS high-voltage technology Enrico Schuriga,*, Michel Demierrea, Christian Schottb, Radivoje S. Popovica a
Swiss Federal Institute of Technology, Institute of Microsystems, CH-1015 Lausanne, Switzerland b SENTRON AG, CH-6300 Zug, Switzerland Received 13 June 2001; received in revised form 17 August 2001; accepted 26 September 2001
Abstract In this paper we demonstrate for the ®rst time, how vertical Hall devices manufactured in CMOS technology attain sensitivities comparable to those of conventional silicon plate-shape devices without modifying the standard process or adding any post-processing steps. This was achieved by taking advantage of the low-doped deep wells provided by a high-voltage technology and by applying additionally an unconventional doping reduction method. It is demonstrated that deliberate violation of design rules can increase sensitivity without negative in¯uences on the devices. The current-related sensitivity of the presented devices varies from 18 V/AT up to 127 V/AT for different sensor geometry and doping concentrations. The linearity error is less than 0.04% for magnetic ®elds up to 2 T. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Vertical Hall; CMOS
1. Introduction In-plane sensitive magnetic sensors are preferred in important applications, such as angular position sensing [1]. The vertical Hall device (VHD) [2] is a sensor naturally adapted to these requirements. The principle of such a device, realized in a special vertical Hall (VH) technology is shown in Fig. 1 [3]. However, production costs for high sensitivity VHDs (current-related sensitivities up to SI 400 V/AT) are high, since to obtain low-doped and deep structures, a special technology is needed. Highly sensitive VHDs in CMOS technology not only bear a cost advantage, they also open the ®eld to co-integration with electronics on the same chip. This way an integrated sensor with compensated offset (by using the spinning current method), low temperature dependence and ampli®ed sensor output comes into grasp. Previous attempts to manufacture them resulted in devices with poor sensor performances (SI 20 V/AT) or they required additional non-standard processing steps in order to increase sensitivity [4±6]. A comparison between VHDs in VH and CMOS technology can be seen in Fig. 2. The VH technology uses an n-type wafer (constant doping concentration of 2 1014 cm 3). The sensor is open downwards and allows a deep current ¯ow (e.g. 30 mm). In the CMOS technology (substrate p), the current ¯ow in the sensor is limited to 7 mm. The doping * Corresponding author. Tel.: 41-21-693-6732; fax: 41-21-693-6670. E-mail address: [email protected] (E. Schurig).
pro®le of the diffusion layer is a Gaussian one with a surface concentration of about 2 1016 cm 3. The current ¯ow is, therefore, concentrated near the surface (ca. 0±3 mm). There are two major challenges to resolve for CMOS technology. The VH devices, which we will discuss in this paper, have to overcome the small n-layer depth and the high doping concentrations with a Gaussian diffusion pro®le. 2. Sensor layout 2.1. General layout Our sensor layout (Figs. 2c and 3) is based on the original geometry given by Popovic [2]. The device consists of ®ve contacts arranged in a line on top of a low-doped, active ndiffusion region. A p-diffusion layer surrounds the active area laterally. Since a deep and low-doped active area is essential for high magnetic sensitivity, we decided to use a high-voltage CMOS process, which provides n-diffusion layers up to 7 mm deep. 2.2. Initial sensor and geometry variations A ®rst sensor (VHS) was realized in a deep n-diffusion layer (DNTUB, depth 7 mm) on a p-substrate. The plate thickness is t 9 mm, the contact width w 1:5 mm and the distance between two contacts is d 10 mm. Starting from this basic design, several variations were implemented to
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 5 9 - 7
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E. Schurig et al. / Sensors and Actuators A 97±98 (2002) 47±53
Fig. 1. Principle layout of a VH sensor. The Hall voltage is measured between the two probe contacts while a magnetic field perpendicular to the plane is applied. The image shows the current flow as well as equipotential lines [3].
increase sensor sensitivity as well as to ®nd out the limits of the technology for our application. For standard electronics like MOS transistors, these limits are given by the design rules in order to guarantee the function and reliability. However, for our Hall elements we may try to go beyond them for performance improvement. The most important limit of CMOS technology for our purpose is the small depth of the active n-region. One could assume that smaller VHDs are less in¯uenced by the small depth than bigger ones,
because of the shallower current ¯ow. So our aim became to scale down the sensor dimensions. In order to increase the sensor performance one should consider the sensitivity dependence on several physical and geometrical parameters. For the current-related sensitivity SI the following relation is given SI /
1 ; ND t
(1)
Fig. 2. (a)±(d) Comparison of VH sensors in different technologies. The VH technology uses an n-type wafer and the sensor is open downward and allows a deep current flow (e.g. 30 mm) (a). The corresponding doping profile is constant in the depth with a concentration of 2 1014 cm 3. In the CMOS technology (c) (substrate p), the current flow in the sensor is limited to 7 mm. The doping profile of the diffusion layer is a Gaussian one with a surface concentration of about 2 1016 cm 3. The current flow will, therefore, be concentrated near the surface (ca. 0±3 mm).
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Fig. 3. Photograph of the realized VH sensor on-chip with magnified image of the sensor.
while ND is the doping concentration and t the thickness of the Hall plate. Thus, the sensitivity SI can be increased either by a decrease of the sensor thickness or by a reduction of the doping concentration. For the voltage-related sensitivity SV of a Hall plate, one can ®nd w (2) SV mH G; l where mH is the Hall mobility of the majority carriers, w=l the width-to-length ratio of the equivalent rectangle, and G the geometrical correction factor. In order to increase w=l we must either decrease the contact distance or increase the input contact width. We did not pursue the second possibility since we wanted to have a structure with ®ve contacts of the same size and the output contacts should be small for a high G. In our ®rst optimization step we reduced the sensor thickness to 6 mm (VHT) and 3 mm (VHVT), which is even less than allowed by the technology design rules (5.4 mm). The VHT structure has also been further modi®ed to reduce the overlap of DNTUB and PTUB (Fig. 2c) (VHT2) in order to see, if these regions have a positive or negative in¯uence on the device sensitivity. In another design (VHSH), the sensor length was reduced by placing the contacts closer together, which should increase SV according to formula (2). Instead of the 10 mm, we used d 5 mm while all the other geometrical parameters were the same as for VHS. 2.3. Reduction of the doping concentration 2.3.1. Partial doping According to formula (1), an increase in SI can be achieved by reducing the doping concentration. A very
Fig. 4. Transformation of the conventional implantation mask (a) into one that uses partial implantation (b), which means that one part of the active area is covered during implantation. The covering stripes have to be small (1 . . . 2 mm) in order to achieve a continuous implanted layer after high temperature diffusion.
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Fig. 5. Illustration of the partial implantation technique. (a)±(c) Show the formation of the continuous layer during a high temperature diffusion process: (a) after implantation; (b) after short diffusion at 1150 8C; (c) after complete diffusion at 1150 8C, the additional p-tub layer shapes the dimensions of the n-tub region.
ef®cient, patented method to do so, is to structure the implantation mask for the n-well in stripes with a distance smaller than tolerated by the design rules [7], instead of making an opening over the entire active area. Fig. 4 shows the transformation of the conventional implantation mask into one that uses partial implantation. After high temperature diffusion, the originally separate zones join to form a continuous layer with a lower overall carrier concentration of N d 5 1015 cm 3 instead of 2 1016 cm 3 near the surface. This partial implantation is illustrated by process simulation results (Fig. 5). Applying this method to the VHS design we got a more resistive sensor (VHL). 2.3.2. Shallow n-layer A second way to achieve a lower doping level is to use a shallow n-layer (SNTUB) with a depth of 5.5 mm (VHSN). Even though this layer has a reduced depth compared to the DNTUB, the four times higher sheet resistance should increase SI considerably. 3. Experimental results 3.1. Sensor resistance and sensitivity The reference sensor (VHS) yielded a current-related sensitivity of SI 18 V/AT and a voltage-related sensitivity SV of 0.015 V/VT. This result is similar to the one achieved by other authors [3]. In the next step, we monitored the effect of a thinner device. Reducing the sensor thickness to 6 mm and ®nally 3 mm yielded an increase in SI to 29 and 39 V/AT, respec-
tively. The voltage-related sensitivity is almost the same, 0.015 and 0.012 V/VT, respectively. However, for VHVT the diffusion of the PTUB layer into the active area has obviously a negative effect on the sensitivity, and therefore the thickness should not be made smaller than about 4 mm. If we compare the results for the VHT2 (SV 0:023 V/ VT and SI 58 V/AT) with those of the sensor VHS (SV 0:015 T 1), we see that the DNTUB/PTUB overlap regions (Fig. 2c) decrease the sensitivity and should be avoided. The reduction of the distance between contacts resulted for VHSH in an increase of SV from 0.015 to 0.022 T 1 according to formula (2). SI did slightly decrease (16 V/AT) because of the lower input resistance. The partial doping technique resulted in an about four times higher sensitivity SI (77 V/AT) because of the higher resistance and a slightly higher SV due to the higher Hall mobility. Using the high resistive SNTUB layer even higher sensitivities were achieved (127 V/AT and 0.019 T 1). The presented results are summarized in Table 1. Table 1 Basic sensor characteristics Sensor name
Ibias±relative Vbias±relative Input resistance, sensitivity, SI (V/AT) sensitivity, SV (V/VT) Rin (kO)
VHS VHT VHVT VHT2 VHSH VHL VHSN
18 28 39 58 16 77 127
0.015 0.015 0.012 0.023 0.022 0.016 0.019
1.2 1.9 3.6 2.5 0.7 4.8 6.6
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Fig. 6. Current-related magnetic response of the Hall (VHL) device. The signal deviation due to non-linearity is calculated to be less than 0.04% between 0.3 and 2 T.
3.2. Non-linearity and temperature behavior Non-linearity at high magnetic ®elds up to 2 T is for all sensors smaller than 0.04% (Fig. 6). There is also a certain sensitivity drift depending on the supply current (Fig. 7). It is due to the junction ®eld effect. Since the depletion layer of the p±n junction limiting the active sensor area becomes larger with higher bias voltages, the input resistance and the current-related sensitivity of the sensor are increasing. The sensors have a temperature coef®cient of about 4 10 4 K 1 (Fig. 8), which is of the same order of magnitude as conventional silicon plate-shape devices. The offset voltage of the sensors is up to several millivolts corresponding to about 30 mT.
Fig. 7. Dependence of the supply current-related sensitivity on the bias current (VHL). The sensitivity increase of about 7%/mA is due to the junction field effect.
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Fig. 8. The dependence of sensor sensitivity and input resistance on the temperature (VHL). The calculated temperature coefficients are cT;S 4:1 10 4 and cT;R 5:6 10 3 K 1 . The sensor resistance is about 15 times more sensitive to temperature changes than the Hall sensitivity.
3.3. B-field cross-sensitivity For the use as a 2-axes sensor, it is interesting to see how the sensor responds to a magnetic ®eld parallel to a line through its contacts. To measure this effect the sensor was rotated in a constant magnetic ®eld of different values around an axis perpendicular to the surface with an angular position accuracy better than 0.0058. The cross-sensitivity in¯uence is shown in Fig. 9. It indicates the sensor output for a parasitic ®eld from 0.2 to 2 T. The calculated cross-sensitivity is about 0.01 V/AT at B 2 T and only 1/8000 of the Hall sensitivity of the sensor. Since it is about quadratic with the B-®eld and since most angle sensor applications work with ®elds in the millitesla range, the effect of cross-sensitivity can be neglected.
Fig. 9. Cross-sensitivity of the sensor VHL. The field Bcross is perpendicular to the sensitive direction of the sensor. The sensor is biased with 1 mA and the sensitivity between 0.8 and 2 T is about 0.01 V/AT. The sensor output signal for Bcross is about 0.0013% of the one at Bnorm.
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Table 2 Output resistance Rout and equivalent magnetic noise density bN for thermal noise of various VH sensors, for the sensor working at 5 V at 298 K p bN (nT/ Hz) Sensor name Rout (kO)
sensors. It helps also to see the precise diffusion behavior and understand the monitored results. For a better analysis of the results further 2D simulations (even better 3D) are necessary and will be performed in the near future.
VHS VHT VHVT VHT2 VHL VHSN
3.6. Optimized sensor
1.4 2.4 4.5 3.1 5.5 8.2
32 37 64 28 56 55
3.4. Noise To obtain an idea about the in¯uence of noise on sensor detectivity, we may concentrate on thermal noise, since lowfrequency noise can be compensated applying the spinning current method. The voltage spectral density SNV of the thermal noise because of the output resistances is given as SNV 4kTR;
(3)
with R the output resistance, T the temperature and k the Boltzmann constant. It can be related to SI and I by the equivalent magnetic noise density bN p 4kTR : (4) bN SI I The results for bN are presented in Table 2 for a constant bias current at about 5 V and 298 K. The noise equivalent ®eld is p about 50 nT/ Hz, giving a limit for the minimum detectable ®eld. 3.5. Simulations Process and current ¯ow simulations (2D), using the software package ``ISE-TCAD'', show the doping concentration pro®le of the sensor and the distribution of the current density (Fig. 10). They were executed for various sensor dimensions predicting resistances in good agreement with the real values (errors less than 30%). The information regarding the current ¯ow is very useful for sensitivity optimization of the
Although we already achieved a sensor with good SI, a widely optimized sensor can be designed from the obtained data. It should have a small thickness of about t 4 mm, contact length of d 5 mm or even smaller, small overlap region DNTUB/PTUB and the SNTUB layer as active area. If necessary, SI could be further increased by the partial implantation method. Sensitivities of about SI 400 V/VT as in the VH technology seem realizable. However, up to now it was not found how the voltage-related sensitivity might become larger than 0.05 T 1. 4. Conclusions For the ®rst time a VH sensor with a good sensor performance (SI 130 V/AT) has been realized in CMOS technology without any post-processing step. Different aspects regarding the geometry and the doping level of the active area have been presented. Combining the advantages of low doping (VHSN), partial implantation (VHL) and narrow geometry (VHVT) CMOS VH sensors can be further optimized for even higher currentrelated sensitivity and low power consumption. Certain design rules can be broken for further minimization of the devices or decreasing the doping level without diminishing the sensor function or its reliability. These violations are important for the production of highly sensitive devices. The basic recipe how to implement high quality VH sensors in CMOS technology is given by this work with the potential to make a special technology obsolete and an fully integrated sensor with high performance possible. Acknowledgements The authors would like to thank the ``Swiss Committee for Technology and Innovation'' for funding this CTI project
Fig. 10. Simulation of the technology process (left half) and the current flow (right half) of the VH sensor. The contacts to bias the sensor are under the arrows Iin and Iout. The Hall voltage is measured between the contacts S1 and S2. A positive N-value stands for n-doping and the negative for p.
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as well as the industrial partners, ``Austria Microsystems (AMS)'' and ``SENTRON AG'' (Switzerland) for their cooperation and support.
for Microsystems at the Swiss Federal Institute of Technology, Lausanne (EPFL) in1998, where he work in the field of magnetic sensor design during his PhD studies.
References
Michel Demierre was born in Lausanne, Switzerland, in 1974. He received his MSc in Micro-engineering from the Swiss Federal Institute of Technology, Lausanne (EPFL), in March 1996. He is now a research assistant in the Institute of Microsystems (EPFL), where he is engaged in magnetic microsystems. His research interests include sensors and systems.
[1] F. Burger, P.A. Besse, R.S. Popovic, New fully integrated 3D silicon Hall sensor for precise angular position measurements, Sensors Actuators A 67 (1998) 72±76. [2] R.S. Popovic, The vertical Hall-effect device, IEEE Electron Dev. Lett. EDL-5 (9) (1984) 357±358. [3] A.M.J. Huiser, H.P. Baltes, Numerical modeling of vertical Hall-effect devices, IEEE Electron Dev. Lett. EDL-5 (11) (1984) 482±484. [4] M. Paranjape, L. Ristic, W. Allegretto, A vertical Hall magnetic field sensor using CMOS-compatible micromachining techniques, in: Proceedings of the IEEE Solid-State Sensor and Actuator Workshop, 1992, pp. 32±34. [5] R. Steiner, F. Kroener, T. Olbrich, R. Baresch, H. Baltes, In-plane sensitive vertical trench-Hall device, in: Proceedings of the International Electron Devices Meeting, 1998, pp. 479±482. [6] M. Kahrizi, M. Paranjape, L.M. Landsberger, Complementary metaloxide±semiconductor-compatible micromachined two-dimensional vertical Hall magnetic field sensor: a modified design, J. Vac. Sci. Technol. A 16 (2) (1998) 873±875. [7] Procedure for the production of Hall elements. European Patent No. 1045 461 A2.
Biographies Enrico Schurig was born in 1972 in Elsterwerda, Germany. He obtained the university diploma degree in Physics in 1997 at the Martin-Luther University, Halle-Wittenberg. His first field of interest was solid-state physics for optical applications. After his graduation, he became interested in taking a research occupation abroad. That is why he joined the Institute
Christian Schott was born in 1965 in Hildesheim, Germany. He graduated from the Technische Hochschule Karlsruhe in 1992, where he got a university diploma degree in electrical engineering. After his work on several projects in technical development, he started in 1995 as a PhD student at the Swiss Federal Institute of Technology, Lausanne, where he had a special interest for magnetic field measurements with Hall sensors. At the end of 2000, he joined a Swiss company (SENTRON AG) that is specialized in magnetic Hall sensors. Radivoje S. Popovic was born in Yugoslavia (Serbia) in 1945. He obtained the Dipl. Ing. degree in applied physics from the University of Beograd, Yugoslavia, in 1969, and the MSc and DrSc degrees in electronics from the University of Nis, Yugoslavia, in 1974 and 1978, respectively. From 1969 to 1981 he was with Elektronska Industrija Corp., Nis, Yugoslavia, where he worked on research and development of semiconductor devices and later became head of the company's CMOS department. From 1982 to 1993 he worked for Landis & Gyr Corp., Central R&D, Zug, Switzerland, in the field of semiconductor sensors, interface electronic and microsystems. There he was responsible for research in semiconductor device physics (1983±1985), for microtechnology R&D (1986±1990) and was appointed Vice President (Central R&D) in 1991. In 1994, he joined the Swiss Federal Institute of Technology at Lausanne (EPFL) as Professor for microtechnology systems. He teaches conceptual products and system design and microelectronics at the Department of Microengineering of the EPFL. His current research interests include sensors for magnetic, optical, and mechanical signals, the corresponding microsystems, physics of submicron devices, and noise phenomena.
A vertical Hall device in CMOS high-voltage technology Enrico Schuriga,*, Michel Demierrea, Christian Schottb, Radivoje S. Popovica a
Swiss Federal Institute of Technology, Institute of Microsystems, CH-1015 Lausanne, Switzerland b SENTRON AG, CH-6300 Zug, Switzerland Received 13 June 2001; received in revised form 17 August 2001; accepted 26 September 2001
Abstract In this paper we demonstrate for the ®rst time, how vertical Hall devices manufactured in CMOS technology attain sensitivities comparable to those of conventional silicon plate-shape devices without modifying the standard process or adding any post-processing steps. This was achieved by taking advantage of the low-doped deep wells provided by a high-voltage technology and by applying additionally an unconventional doping reduction method. It is demonstrated that deliberate violation of design rules can increase sensitivity without negative in¯uences on the devices. The current-related sensitivity of the presented devices varies from 18 V/AT up to 127 V/AT for different sensor geometry and doping concentrations. The linearity error is less than 0.04% for magnetic ®elds up to 2 T. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Vertical Hall; CMOS
1. Introduction In-plane sensitive magnetic sensors are preferred in important applications, such as angular position sensing [1]. The vertical Hall device (VHD) [2] is a sensor naturally adapted to these requirements. The principle of such a device, realized in a special vertical Hall (VH) technology is shown in Fig. 1 [3]. However, production costs for high sensitivity VHDs (current-related sensitivities up to SI 400 V/AT) are high, since to obtain low-doped and deep structures, a special technology is needed. Highly sensitive VHDs in CMOS technology not only bear a cost advantage, they also open the ®eld to co-integration with electronics on the same chip. This way an integrated sensor with compensated offset (by using the spinning current method), low temperature dependence and ampli®ed sensor output comes into grasp. Previous attempts to manufacture them resulted in devices with poor sensor performances (SI 20 V/AT) or they required additional non-standard processing steps in order to increase sensitivity [4±6]. A comparison between VHDs in VH and CMOS technology can be seen in Fig. 2. The VH technology uses an n-type wafer (constant doping concentration of 2 1014 cm 3). The sensor is open downwards and allows a deep current ¯ow (e.g. 30 mm). In the CMOS technology (substrate p), the current ¯ow in the sensor is limited to 7 mm. The doping * Corresponding author. Tel.: 41-21-693-6732; fax: 41-21-693-6670. E-mail address: [email protected] (E. Schurig).
pro®le of the diffusion layer is a Gaussian one with a surface concentration of about 2 1016 cm 3. The current ¯ow is, therefore, concentrated near the surface (ca. 0±3 mm). There are two major challenges to resolve for CMOS technology. The VH devices, which we will discuss in this paper, have to overcome the small n-layer depth and the high doping concentrations with a Gaussian diffusion pro®le. 2. Sensor layout 2.1. General layout Our sensor layout (Figs. 2c and 3) is based on the original geometry given by Popovic [2]. The device consists of ®ve contacts arranged in a line on top of a low-doped, active ndiffusion region. A p-diffusion layer surrounds the active area laterally. Since a deep and low-doped active area is essential for high magnetic sensitivity, we decided to use a high-voltage CMOS process, which provides n-diffusion layers up to 7 mm deep. 2.2. Initial sensor and geometry variations A ®rst sensor (VHS) was realized in a deep n-diffusion layer (DNTUB, depth 7 mm) on a p-substrate. The plate thickness is t 9 mm, the contact width w 1:5 mm and the distance between two contacts is d 10 mm. Starting from this basic design, several variations were implemented to
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 5 9 - 7
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E. Schurig et al. / Sensors and Actuators A 97±98 (2002) 47±53
Fig. 1. Principle layout of a VH sensor. The Hall voltage is measured between the two probe contacts while a magnetic field perpendicular to the plane is applied. The image shows the current flow as well as equipotential lines [3].
increase sensor sensitivity as well as to ®nd out the limits of the technology for our application. For standard electronics like MOS transistors, these limits are given by the design rules in order to guarantee the function and reliability. However, for our Hall elements we may try to go beyond them for performance improvement. The most important limit of CMOS technology for our purpose is the small depth of the active n-region. One could assume that smaller VHDs are less in¯uenced by the small depth than bigger ones,
because of the shallower current ¯ow. So our aim became to scale down the sensor dimensions. In order to increase the sensor performance one should consider the sensitivity dependence on several physical and geometrical parameters. For the current-related sensitivity SI the following relation is given SI /
1 ; ND t
(1)
Fig. 2. (a)±(d) Comparison of VH sensors in different technologies. The VH technology uses an n-type wafer and the sensor is open downward and allows a deep current flow (e.g. 30 mm) (a). The corresponding doping profile is constant in the depth with a concentration of 2 1014 cm 3. In the CMOS technology (c) (substrate p), the current flow in the sensor is limited to 7 mm. The doping profile of the diffusion layer is a Gaussian one with a surface concentration of about 2 1016 cm 3. The current flow will, therefore, be concentrated near the surface (ca. 0±3 mm).
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Fig. 3. Photograph of the realized VH sensor on-chip with magnified image of the sensor.
while ND is the doping concentration and t the thickness of the Hall plate. Thus, the sensitivity SI can be increased either by a decrease of the sensor thickness or by a reduction of the doping concentration. For the voltage-related sensitivity SV of a Hall plate, one can ®nd w (2) SV mH G; l where mH is the Hall mobility of the majority carriers, w=l the width-to-length ratio of the equivalent rectangle, and G the geometrical correction factor. In order to increase w=l we must either decrease the contact distance or increase the input contact width. We did not pursue the second possibility since we wanted to have a structure with ®ve contacts of the same size and the output contacts should be small for a high G. In our ®rst optimization step we reduced the sensor thickness to 6 mm (VHT) and 3 mm (VHVT), which is even less than allowed by the technology design rules (5.4 mm). The VHT structure has also been further modi®ed to reduce the overlap of DNTUB and PTUB (Fig. 2c) (VHT2) in order to see, if these regions have a positive or negative in¯uence on the device sensitivity. In another design (VHSH), the sensor length was reduced by placing the contacts closer together, which should increase SV according to formula (2). Instead of the 10 mm, we used d 5 mm while all the other geometrical parameters were the same as for VHS. 2.3. Reduction of the doping concentration 2.3.1. Partial doping According to formula (1), an increase in SI can be achieved by reducing the doping concentration. A very
Fig. 4. Transformation of the conventional implantation mask (a) into one that uses partial implantation (b), which means that one part of the active area is covered during implantation. The covering stripes have to be small (1 . . . 2 mm) in order to achieve a continuous implanted layer after high temperature diffusion.
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Fig. 5. Illustration of the partial implantation technique. (a)±(c) Show the formation of the continuous layer during a high temperature diffusion process: (a) after implantation; (b) after short diffusion at 1150 8C; (c) after complete diffusion at 1150 8C, the additional p-tub layer shapes the dimensions of the n-tub region.
ef®cient, patented method to do so, is to structure the implantation mask for the n-well in stripes with a distance smaller than tolerated by the design rules [7], instead of making an opening over the entire active area. Fig. 4 shows the transformation of the conventional implantation mask into one that uses partial implantation. After high temperature diffusion, the originally separate zones join to form a continuous layer with a lower overall carrier concentration of N d 5 1015 cm 3 instead of 2 1016 cm 3 near the surface. This partial implantation is illustrated by process simulation results (Fig. 5). Applying this method to the VHS design we got a more resistive sensor (VHL). 2.3.2. Shallow n-layer A second way to achieve a lower doping level is to use a shallow n-layer (SNTUB) with a depth of 5.5 mm (VHSN). Even though this layer has a reduced depth compared to the DNTUB, the four times higher sheet resistance should increase SI considerably. 3. Experimental results 3.1. Sensor resistance and sensitivity The reference sensor (VHS) yielded a current-related sensitivity of SI 18 V/AT and a voltage-related sensitivity SV of 0.015 V/VT. This result is similar to the one achieved by other authors [3]. In the next step, we monitored the effect of a thinner device. Reducing the sensor thickness to 6 mm and ®nally 3 mm yielded an increase in SI to 29 and 39 V/AT, respec-
tively. The voltage-related sensitivity is almost the same, 0.015 and 0.012 V/VT, respectively. However, for VHVT the diffusion of the PTUB layer into the active area has obviously a negative effect on the sensitivity, and therefore the thickness should not be made smaller than about 4 mm. If we compare the results for the VHT2 (SV 0:023 V/ VT and SI 58 V/AT) with those of the sensor VHS (SV 0:015 T 1), we see that the DNTUB/PTUB overlap regions (Fig. 2c) decrease the sensitivity and should be avoided. The reduction of the distance between contacts resulted for VHSH in an increase of SV from 0.015 to 0.022 T 1 according to formula (2). SI did slightly decrease (16 V/AT) because of the lower input resistance. The partial doping technique resulted in an about four times higher sensitivity SI (77 V/AT) because of the higher resistance and a slightly higher SV due to the higher Hall mobility. Using the high resistive SNTUB layer even higher sensitivities were achieved (127 V/AT and 0.019 T 1). The presented results are summarized in Table 1. Table 1 Basic sensor characteristics Sensor name
Ibias±relative Vbias±relative Input resistance, sensitivity, SI (V/AT) sensitivity, SV (V/VT) Rin (kO)
VHS VHT VHVT VHT2 VHSH VHL VHSN
18 28 39 58 16 77 127
0.015 0.015 0.012 0.023 0.022 0.016 0.019
1.2 1.9 3.6 2.5 0.7 4.8 6.6
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Fig. 6. Current-related magnetic response of the Hall (VHL) device. The signal deviation due to non-linearity is calculated to be less than 0.04% between 0.3 and 2 T.
3.2. Non-linearity and temperature behavior Non-linearity at high magnetic ®elds up to 2 T is for all sensors smaller than 0.04% (Fig. 6). There is also a certain sensitivity drift depending on the supply current (Fig. 7). It is due to the junction ®eld effect. Since the depletion layer of the p±n junction limiting the active sensor area becomes larger with higher bias voltages, the input resistance and the current-related sensitivity of the sensor are increasing. The sensors have a temperature coef®cient of about 4 10 4 K 1 (Fig. 8), which is of the same order of magnitude as conventional silicon plate-shape devices. The offset voltage of the sensors is up to several millivolts corresponding to about 30 mT.
Fig. 7. Dependence of the supply current-related sensitivity on the bias current (VHL). The sensitivity increase of about 7%/mA is due to the junction field effect.
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Fig. 8. The dependence of sensor sensitivity and input resistance on the temperature (VHL). The calculated temperature coefficients are cT;S 4:1 10 4 and cT;R 5:6 10 3 K 1 . The sensor resistance is about 15 times more sensitive to temperature changes than the Hall sensitivity.
3.3. B-field cross-sensitivity For the use as a 2-axes sensor, it is interesting to see how the sensor responds to a magnetic ®eld parallel to a line through its contacts. To measure this effect the sensor was rotated in a constant magnetic ®eld of different values around an axis perpendicular to the surface with an angular position accuracy better than 0.0058. The cross-sensitivity in¯uence is shown in Fig. 9. It indicates the sensor output for a parasitic ®eld from 0.2 to 2 T. The calculated cross-sensitivity is about 0.01 V/AT at B 2 T and only 1/8000 of the Hall sensitivity of the sensor. Since it is about quadratic with the B-®eld and since most angle sensor applications work with ®elds in the millitesla range, the effect of cross-sensitivity can be neglected.
Fig. 9. Cross-sensitivity of the sensor VHL. The field Bcross is perpendicular to the sensitive direction of the sensor. The sensor is biased with 1 mA and the sensitivity between 0.8 and 2 T is about 0.01 V/AT. The sensor output signal for Bcross is about 0.0013% of the one at Bnorm.
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Table 2 Output resistance Rout and equivalent magnetic noise density bN for thermal noise of various VH sensors, for the sensor working at 5 V at 298 K p bN (nT/ Hz) Sensor name Rout (kO)
sensors. It helps also to see the precise diffusion behavior and understand the monitored results. For a better analysis of the results further 2D simulations (even better 3D) are necessary and will be performed in the near future.
VHS VHT VHVT VHT2 VHL VHSN
3.6. Optimized sensor
1.4 2.4 4.5 3.1 5.5 8.2
32 37 64 28 56 55
3.4. Noise To obtain an idea about the in¯uence of noise on sensor detectivity, we may concentrate on thermal noise, since lowfrequency noise can be compensated applying the spinning current method. The voltage spectral density SNV of the thermal noise because of the output resistances is given as SNV 4kTR;
(3)
with R the output resistance, T the temperature and k the Boltzmann constant. It can be related to SI and I by the equivalent magnetic noise density bN p 4kTR : (4) bN SI I The results for bN are presented in Table 2 for a constant bias current at about 5 V and 298 K. The noise equivalent ®eld is p about 50 nT/ Hz, giving a limit for the minimum detectable ®eld. 3.5. Simulations Process and current ¯ow simulations (2D), using the software package ``ISE-TCAD'', show the doping concentration pro®le of the sensor and the distribution of the current density (Fig. 10). They were executed for various sensor dimensions predicting resistances in good agreement with the real values (errors less than 30%). The information regarding the current ¯ow is very useful for sensitivity optimization of the
Although we already achieved a sensor with good SI, a widely optimized sensor can be designed from the obtained data. It should have a small thickness of about t 4 mm, contact length of d 5 mm or even smaller, small overlap region DNTUB/PTUB and the SNTUB layer as active area. If necessary, SI could be further increased by the partial implantation method. Sensitivities of about SI 400 V/VT as in the VH technology seem realizable. However, up to now it was not found how the voltage-related sensitivity might become larger than 0.05 T 1. 4. Conclusions For the ®rst time a VH sensor with a good sensor performance (SI 130 V/AT) has been realized in CMOS technology without any post-processing step. Different aspects regarding the geometry and the doping level of the active area have been presented. Combining the advantages of low doping (VHSN), partial implantation (VHL) and narrow geometry (VHVT) CMOS VH sensors can be further optimized for even higher currentrelated sensitivity and low power consumption. Certain design rules can be broken for further minimization of the devices or decreasing the doping level without diminishing the sensor function or its reliability. These violations are important for the production of highly sensitive devices. The basic recipe how to implement high quality VH sensors in CMOS technology is given by this work with the potential to make a special technology obsolete and an fully integrated sensor with high performance possible. Acknowledgements The authors would like to thank the ``Swiss Committee for Technology and Innovation'' for funding this CTI project
Fig. 10. Simulation of the technology process (left half) and the current flow (right half) of the VH sensor. The contacts to bias the sensor are under the arrows Iin and Iout. The Hall voltage is measured between the contacts S1 and S2. A positive N-value stands for n-doping and the negative for p.
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as well as the industrial partners, ``Austria Microsystems (AMS)'' and ``SENTRON AG'' (Switzerland) for their cooperation and support.
for Microsystems at the Swiss Federal Institute of Technology, Lausanne (EPFL) in1998, where he work in the field of magnetic sensor design during his PhD studies.
References
Michel Demierre was born in Lausanne, Switzerland, in 1974. He received his MSc in Micro-engineering from the Swiss Federal Institute of Technology, Lausanne (EPFL), in March 1996. He is now a research assistant in the Institute of Microsystems (EPFL), where he is engaged in magnetic microsystems. His research interests include sensors and systems.
[1] F. Burger, P.A. Besse, R.S. Popovic, New fully integrated 3D silicon Hall sensor for precise angular position measurements, Sensors Actuators A 67 (1998) 72±76. [2] R.S. Popovic, The vertical Hall-effect device, IEEE Electron Dev. Lett. EDL-5 (9) (1984) 357±358. [3] A.M.J. Huiser, H.P. Baltes, Numerical modeling of vertical Hall-effect devices, IEEE Electron Dev. Lett. EDL-5 (11) (1984) 482±484. [4] M. Paranjape, L. Ristic, W. Allegretto, A vertical Hall magnetic field sensor using CMOS-compatible micromachining techniques, in: Proceedings of the IEEE Solid-State Sensor and Actuator Workshop, 1992, pp. 32±34. [5] R. Steiner, F. Kroener, T. Olbrich, R. Baresch, H. Baltes, In-plane sensitive vertical trench-Hall device, in: Proceedings of the International Electron Devices Meeting, 1998, pp. 479±482. [6] M. Kahrizi, M. Paranjape, L.M. Landsberger, Complementary metaloxide±semiconductor-compatible micromachined two-dimensional vertical Hall magnetic field sensor: a modified design, J. Vac. Sci. Technol. A 16 (2) (1998) 873±875. [7] Procedure for the production of Hall elements. European Patent No. 1045 461 A2.
Biographies Enrico Schurig was born in 1972 in Elsterwerda, Germany. He obtained the university diploma degree in Physics in 1997 at the Martin-Luther University, Halle-Wittenberg. His first field of interest was solid-state physics for optical applications. After his graduation, he became interested in taking a research occupation abroad. That is why he joined the Institute
Christian Schott was born in 1965 in Hildesheim, Germany. He graduated from the Technische Hochschule Karlsruhe in 1992, where he got a university diploma degree in electrical engineering. After his work on several projects in technical development, he started in 1995 as a PhD student at the Swiss Federal Institute of Technology, Lausanne, where he had a special interest for magnetic field measurements with Hall sensors. At the end of 2000, he joined a Swiss company (SENTRON AG) that is specialized in magnetic Hall sensors. Radivoje S. Popovic was born in Yugoslavia (Serbia) in 1945. He obtained the Dipl. Ing. degree in applied physics from the University of Beograd, Yugoslavia, in 1969, and the MSc and DrSc degrees in electronics from the University of Nis, Yugoslavia, in 1974 and 1978, respectively. From 1969 to 1981 he was with Elektronska Industrija Corp., Nis, Yugoslavia, where he worked on research and development of semiconductor devices and later became head of the company's CMOS department. From 1982 to 1993 he worked for Landis & Gyr Corp., Central R&D, Zug, Switzerland, in the field of semiconductor sensors, interface electronic and microsystems. There he was responsible for research in semiconductor device physics (1983±1985), for microtechnology R&D (1986±1990) and was appointed Vice President (Central R&D) in 1991. In 1994, he joined the Swiss Federal Institute of Technology at Lausanne (EPFL) as Professor for microtechnology systems. He teaches conceptual products and system design and microelectronics at the Department of Microengineering of the EPFL. His current research interests include sensors for magnetic, optical, and mechanical signals, the corresponding microsystems, physics of submicron devices, and noise phenomena.