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Minimum Detectable Signals of Integrated Magnetic Sensors in Bulk CMOS and SO1 Tech- nologies for Magnetic Read Heads

Jack Lau, Cuong T. Nguyen, Ping K. KO, Philip C. H. Chan

Department of Electrical & Electronic Engineering The Hong Kong University of Science & Technology, Clear Water Bay, Hong Kong

ABSTRACT

Two of the most important factors in dictating the popularity of integrated magnetic sensors are the ease of fabrication and the signal to noise ratio of the devices. Sensors fabricated in both bulk CMOS and SO1 technologies enjoy the benefit of ease of fabrications. In this paper, we study the signal to noise ratios of various CMOS magnetic sensors. We show that the minimum detectable signal (MDS) for split-drain magnetic sensor is proportional to S i ' I -3/2 while that for LCDM and LTM in SO1 technologies are S,-'l

INTRODUCTION

Hard disk read head is by far the biggest consumer of mag- netic sensors. While most part of the post processing circuit- ries are now available in IC, the sensor remains isolated. Many researches have been devoted to silicon magnetic sen- sors [l-41. The requirements of these sensors remain an intriguing question among researchers. Every time a new sensor is reported, the immediate and most natural question to ask would likely be:"Is it good enough?' Recently, the new Lateral Thyristive Magnetometer (LTM) on silicon on insulator (SOI) reported a sensitivity of 210%/Tesla. Again, is it good enough? Ultimately, the answer rests upon the minimum detectable signal (MDS). In this paper, we discuss the MDS issue with three sensors: the easily integrateable split-drain MAGFET in bulk CMOS technology, the very new Lateral Carrier Domain Magnetometer (LCDM) and the LTM fabricated on SOI. We show that the MDS for the split- drain MAGFET is proportional to S i ' I -3/2 and is propor- tional to S i l l for both the LCDM and LTM, where Sa is the absolute sensitivity and I is the biasing current. Based on our current experimental results, to have a MDS of 1 Gauss at a typical magnetic hard disk data rate, the split-drain MAGFET would require 0.1 A of biasing current; the LCDM would need a bias current of 1 A; and the LTM would only need 0.25 mA. Furthermore, we show that the MDS is dependent on the absolute sensitivity and not the rel- ative sensitivity. In the case of the split-drain MAGFET, we show that it is possible to push the relative sensitivity, S,., alone, but the MDS remains unchanged. In particular, we

report here for the first time an asymmetrical biasing mode for the split-drain MAGFETs to improve the S,. to over a 6000%/Tesla.

CMOS MAGNETIC SENSOR: SPLIT-DRAIN MAGFET

'magnetic field

Snrirre V

FIG. 1: Split-drain MAGFET is built by splitting the drain of a M O S transistor into two. With an external magnetic jield, Lorentz force dictates an imbalance of drain current. The sensitivity to magneticjield can be derived from the drain current difference. S,, the abso- lute sensitivity, is dejined as the ratio of diference of drain current over total drain current.

Split-drain MAGFET excels in its simplicity [5]. A MS is made simply by splitting the drain of a MOS transistor into two. When a magnetic field is applied, Lorentz deflection dictates that there is a difference in the two drain current (AI), through which sensitivity to magnetic field is derived (Fig. 1). Typical S, of these devices are less than 6%/Tesla. At low frequency, the 14 noise dominates, but in applica- tions such as magnetic read heads, in which typically data rate up to 30 MHz is seen [6], thermal noise is more impor- tant. Based on noise analysis, we show that the SNR=S,BK13'2, where S, is the absolute sensitivity, B is the applied magnetic field, I is the biasing current, and K is a

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constant that depends on oxide thickness, mobility, and applied biasing. MDS is defined as the signal at which the SNR drops to unity. It is of interest to point out that with this relationship, a smaller MDS would require a larger current, which comes by with a larger W L . With a larger W/L, the I / f noise component diminishes even further. Based on test results extracted from experimental device fabricated using the Orbit 2pm process [7], we find that a MDS of 1 Gauss would require 0.1 A of biasing current and 100 mGauss would need 1.25 A.

Id-Vd Charactensticsol iOOumllOOum SplitOain

v g m 3.5 I I I I , I I I ,

ldl t ld2t vdl 'd2

2

1 r 0 , 5 1 1.5 2 2.5 3 3.5 4 4.5 5

Vd

FIG. 2: The MAGFET is like a composition of three transistorS. When V d l = v d 2 , f d l =I& and Id,=o. With v d l > > vd2, kf2 iS 08 Id2 = - I&. With M2 barely O n , Id2 =0. Thus, with Lorentz defection, some Id1 got defected to Id2 and its seen as big change at 1d2.

We can also see that S, is irrelevant to the MDS. The S, fig- ure is sometimes used because it represents how easy it is to connect amplification circuit to the sensor. The split-drain MAGFET (Fig. 2) is actually very similar to a composition of three MOS transistors, two in parallel and one connecting them near the drain gap. One idea is to almost turn off one side of the split-drain device. With that, any Lorentz deflec- tion of the current will be seen as a very significant change over the weak drain current. Therefore, we can easily moni- tor the current by just noting the change of the weak drain current. Here a device is tested. S, is defined as the change of the drain current per unit of drain current. With that, the S, at the weak drain is amazingly high. In fact, with the weak drain current on the order of nA, it is easy to achieve a S, over 1000%/Tesla, albeit at the expense of difficult biasing (Fig. 3). Yet, as long as S, does not change --- as in the case for the MAGFET, SNR ratio remains the same. The SNR does not depend on the drain current without magnetic field. It is only a function of the fluctuation of drain current over

noise!

x 10.' Asymmetnoal Drain Biasing

7.4 k

5 6

-100 -80 -60 -40 -20 0 20 40 60 80 100 Gauss

FIG. 3: With M 2 almost shut ofi Id2 is close to zero, or nA in our case. S , is dejined as the change of individual drain current under magneticjield (Ald2/1&). With Id2 at 640 nA and A l d 2 of 40 nA at I00 Gauss, S, much above 6000%/Tesla is achieved, and S, is only limited by the stability ofthe small Id2.

SO1 MAGNETIC SENOSRS: LCDM and LTM

FIG. 4: The LCDM is a npn and pnp transistor connected together: Because of the need to have both transistors in forward active, the p+n base emitter junction (node 4,s) must be forward biased. And, it must be fabricated on SOL The LCDM harnesses both carrier domain effect and internal positive feedback. It has shown potential to sense smallJield, with a S, of 6%flesla reported.

Lastly, we look at the LCDM (Fig. 4) and the LTM. The superior isolation property of silicon on insulator (SOI) has made development of the carrier domain magnetometer pos- sible. [4] has reported using SO1 substrate for magnetic sen- sors. The LCDM is based on carrier domain effect and internal feedback [8]. The device is fabricated on BESOI wafers with a silicon film thickness of 0.17 pm and a buried oxide of 1 pm using a six-mask process. Sensing the differ- ence in basekollector current, we detect a Sa of 6%/Tesla.

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1 Gauss

10 Gauss

The LTM is an improved version of the LCDM and achieves a S, of 105%/Tesla 191. The device is inherently a conjunc- tion of npn transistor coupled with a pnp transistor on SO1 substrate. An equivalent circuit for the sensor is shown in Fig. 5. It can be calculated, based on bipolar noise estima- tion, that the SNR=SBIclnK’, where K’ is dependent on the p of the transistors. The noise figure is derived based on half circuit analysis. In our experimental devices, the of the pnp and the npn are 20 and 10 respectively. With that we calcu- late the required current for different MDS. Results are sum- marized in Fig. 6.

0.1 A 1 A 0.025 pA

0.06 A 0.01 A 2.5 pA

- I FIG. 5: Noise can be derived with a half circuit analy- sis. Equivalent input noise of BJT transistor is used as a starting point. At magnetic storage data rate, shot noise and thermal noise are important. Equivalent noise cur- rent at the base/collector is found.

I I I I

Drain

(Idl+Id2)

1 current 1 Collector 1 Anode I Current (IC) Current

FIG. 6: Biasing current needed for MDS of the three sen- sors based on 30 MHz data rate requirements. The MAGFET has a S,-’I-3n dependence; the SO1 sensors has a S,~’l~l”dependence:

CONCLUSIONS

Integrated magnetic sensors offer the potential for tremen- dous cost savings and reliability improvements. There is a trade-off between power and signal to noise ratio. And, the trade-off is not linear. It depends very much on device struc- ture. Split-drain MAGFET is easy to fabricate but may not be suitable for low field measurements because of its inher- ent lower absolute sensitivity. LTM is more attractive for low field sensing.

ACKNOWLEDGEMENT

This research is partially supported by the RGC Earmarked Research Grant HKUST 547/94E.

REFERENCES

[ l ] J. Lenz, “A Review of Magnetic Sensors”, IEEE Pro- ceedings, vol. 78, no. 6, June 1990, pp. 973-989. [2] H. P. Baltes and R. S. Popovic, “Integrated Semiconduc- tor Magnetic Field Sensors,” Proc. IEEE, vol. 74, p. 1107- 1132. [3] S. Middlehoek, S. A. Audet, Silicon Sensors, Academic Press, 1989, pp. 201-247. [4] R. Castagnetti, “Magnetotransistors in SO1 Technology”, Tech. Digest IEDM, 1994, pp. 147-150. [5] A. Nathan and H. Baltes, “Two-Dimensional Numerical Modeling of Magnetic-Field Sensors in CMOS Technology,” IEEE Trans. on Electron Devices, July 1985, pp. 1212-1219 [6] T. W. Pan, A. A. Abidi, “A Wide-band CMOS Read Amplifier for Magnetic Data Storage Systems”, IEEE J. Solid-state Circuits, vol. 27, no. 6, June 1992, pp. 863-873. [7] Orbit Foresight User’s Manual, 1992. [8] 9. Lau, P. KO, P. C.H. Chan, C. T. Nguyen, “Lateral Car- rier-Domain Magnetometer Fabricated on BESOI”, Proc. IEEE SO1 Conf., 1994, pp. 103-104. [9] J. Lau, C. T. Nguyen, P. KO, P. C.H. Chan, “A Highly- Sensitive and Low Power SO1 Lateral Thyristive Magnetom- eter”, IEEE Tech. Digest IEDM, 1994, pp. 143-146.

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