Chapter 1 Introduction
1.3 Applications
Consider a Hall effect sensor：the primary quantity measured is the component of magnetic induction perpendicular to the plane of the sensor, averaged over the active area, An example of a primary application of this world be a Hall effect Gaussmeter. At the secondary level a Hall sensor can be used in a proximity switch, wherer it is acting as a position sensor by virtue of the spatial variation of magnetic induction near a permanent magnet. Sensors are used in applications covering almost
every area of human activity. It is helpful to classify these areas, because the priorities vary widely from one to another. Areas of application for sensors is listed as follows [11]：
․Aerospace
․Automotive
․Biomedical
․Consumer/domestic
․Industrial：Chemical/Construction/Electrical/Mechanical
․Scientific research
․Surveying and prospecting
Furthermore, the thin film transistor on glass substrate accompany with the lower leakage current, the lower power consumption and the higher on-off current ratio compare to the MOSFET on silicon substrate. These advantages will promote the performance widely in the fabrication of the magnetic sensor array lately. As the foreign scientific or technical literature reported, only several few research papers mentioned the fabrication of the magnetic sensor using the thin film transistor structure. According to these literatures, we find that these devices are not the real thin film transistors, the improper device’s architecture design and without suitable fabrication flow, lead to the bad performance in these sensor devices. Consequently, our research project design a series experiments to develop the complete technology of TFT magnetic sensor device, for measuring and analyzing the magnetic field distribution of direction and density in the space. We design several kinds of geometric structures TFT magnetic sensor with different dimensions to realize and analyze the effects of the magnetic sensitivity, Hall voltage, and the signal to noise ratio on the TFT magnetic sensor. Thanks to the mature technology in TFT process, high yield and stability produced, we can integrate the sensor devices with sensor circuits in unit process flow. By means of parallel array sensor devices, it will increase the magnetic sensitivity, decrease the disturbance of environment noises and enhance the sensing analyzability.
Reference
[1] L. Dong, R.F. Yue, “A high performance single chip uncooled a-Si TFT infrared sensor”, Solid-State Sensors, Actuators and Microsystems, 12th International Conference, Vol.1, pp.312 – 315, 2003.
[2] Hara, H.; Sakurai, M,” Low temperature polycrystalline silicon TFT fingerprint sensor with integrated comparator circuit”, Proceeding of the 30th European, 21-23 Sept. 2004
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magnetic sensors”New York/VCH/c1989
Chapter 2 Theory
2.1 The Hall Effect
The Hall Effect is a physical effect arising in matter carrying electric field in the presence of a magnetic field. Fig. 2-1 shows bulk materials with free electron carriers under the applied voltage. Suppose that in this bulk material, along the +x direction, an external electric field Ee is established. Then we applied a uniform magnetic field in the +z direction. From Lorentz force we know that the electrons will be affected by the magnetic field. Electrons will be pushed towards +Y direction. The interaction is described by the Lorentz force：
) (v B q
F = ×
(2.1)
Where F is the Lorentz force expressed by a charge carrier of charge moving with velocity v in a magnetic flux density B. The force F pushes carriers toward the back side of the bulk. Consequently the carrier concentration at the back side of the bulk starts to increase, while the carrier concentration at the front of the bulk starts to decrease. Then an electric field EH appears between the two edges.
This field, EH acts on the moving carriers too, and the transverse electrical force becomes strong enough to balance the magnetic force eventually. The transverse electric field EH is called the Hall electric field. In this balance condition, we can get this equation:
0 This means that the Hall field is a function of the velocity of the carriers and the strength of the magnetic field. For a transducer with a given width W between sense electrodes, the Hall electric field can be integrated over W, assuming it is uniform, then
WvB
VH = − (2.4)
Therefore, the Hall voltage is a linear function of：
(a) The carrier velocity in the body of the transducer (b) The applied magnetic field in the sensitive axis (c) The spatial separation o f the sense contacts
Consider the case of a piece of conductive material with a given cross section area of “A”. The carrier drift velocity can be determined by：
A
So we can derive an expression that describes sensitivity of a Hall transducer as a function of cross sectional dimensions, current, and carrier density.
d Where d is the thickness of the conductor.
Besides, the resistance of the Hall transducer is a function of the conductivity and the geometry, for a rectangular slab, the resistance can be calculated by：
d Where R is resistance, ρ is the resistivity, l is the length, w is the width, and d is the thickness. We suppose that the magnetic field is constant. From equation 2.1, if we want higher hall voltage, we must select higher biased current, material of lower implanted concentration and of thinner thickness. But higher biased current will cause higher consumption of the power and instable characteristic of device. From equation 2.2, material of lower implanted concentration and thinner thickness will cause higher resistance of material.
From equation 2.6 and 2.7, there is no sufficient information to select appropriate material and design structure of device. In our study, we employ traditional thin film transistor to carry out the hall sensor. We use inversion layer of MOS as sensing region and has to analyze the basic sensing mode of sensing region.
In general, hall effect sensor has two sensing modes which we can measure the hall
voltage to compute the intensity of magnetic field for longer structure of device and employ Lorentz current to compute the intensity of magnetic field for wider structure of device. The followings we specifically make a sample analysis from two different modes to obtain what information we need to design the structure of device.
2.2 Hall Effect in semiconductor
In n-type semiconductor materials, we can derive a equation when there is no magnetic field applied to the semiconductor：
Jn(0)=σnε +qDn∇n (2.8) where σn is conductivity, D is diffusion coefficient, n n is carrier concentration.
When a magnetic field is applied to semiconductor materials, the current density can be described by the equation：
1
If we consider the scattering factor r in materials, we will substitute Hall n mobility µn* for drift mobility r ： n
If we neglect the carrier concentration gradient, we will simplify the equation：
]
Lorentz force produced by the vertical electric field, and the vertical field is produced by our proposed Hall sensor using external magnetic filed and itself electric field.
Thus, we can get： Now we consider the Hall voltage mode of operation, let us consider the long and thin sample, and there is no current in y direction, Jny =0
Îεy =−µn*Bεx =RHJnxB (2.15) The Hall coefficient is the proportionality factor relating the Hall field or the Hall voltage to the current-magnetic field product in a Hall effect experiment. The value and stability of the Hall coefficient directly determines the magnitudes and the stability of the sensitivity of the sensors based on the Hall effect.
For a sensing layer with thickness t, we can derive the Hall voltage and relative sensitivity
Thus, we can find that the Hall voltage mode of operation is dependent of carrier concentration and thickness of the sensing layer but independent of carrier Hall mobility [1].
2.3
Recrystallization of Amorphous Si（a-Si）Thin FilmsGenerally, as the higher mobility in the channel, the thinner of the channel, and the lower concentration in the channel under gate voltage bias, the device has better performance in measuring the magnetic field. For LTPS TFTs, there are three kinds of low temperature crystallization methods and they are roughly reviewed.
2.3.1 Solid Phase Crystallization
Amorphous silicon thin films deposited by low-pressure chemical vapor deposition（LPCVD）below 600 annealed in furnace at 600 several hours℃ ℃ （~24 h）.
The films will be converted into polycrystalline form, and the grain sizes obtained by this method（SPC）is more larger and smoother morphology than as-deposited poly-Si films. However, due to the low deposition temperature used, long crystallization duration is necessary, and large defect density exists in crystallized poly-Si.
2.3.2 Excimer Laser Annealing Crystallization
Laser crystallization is a much faster process than others. Especially, excimer laser crystallization is by far most widely used method now [2]-[4]. Because excimer laser is the strong absorption of UV light in silicon, most of the laser energy is deposited close to the surface of the a-Si films. The laser process heats the a-Si films to the melting point in a very short duration（several nanoseconds）without damaging the glass substrate, and the silicon films will melt and recrystallize. Because ELA process has the highest annealing temperature among the other methods, we can obtain the higher quality poly-Si films.
2.3.3 Metal-Induced Lateral Crystallization
A certain metal, for example, Al [5], Cu [6], Ag [7], or Ni [8], are deposited on a-Si. By annealing in furnace they will transform to metal silicide. Considering the metal-Si eutectic temperature, an a-Si thin film can be crystallized below 500 . ℃ Consequently, the metal-induced crystallized （MIC）process is lower than SPC annealing temperature to get low temperature process. However, in spite of low crystallization temperature, metal contamination is a serious problem in MIC poly-Si.
To improve its property, metal-induced lateral crystallization（MILC）process has demonstrated that high performance LTPS TFTs can be fabricated using Ni-MILC.
2.4 The Geometrical Correction Factor
A Hall device does not have to have a rectangular shape such as the example shown in Fig. 2-3, or indeed any other regular shape. Actually, any finite plate, provided with a least three contacts, may be used as a Hall device. Using conformal mapping theory, Wick demonstrated the invariance of Hall plate electrical efficiency with respect to geometry. However, some of the shapes may have some technological or application advantages over the others. For example, a vertical Hall device is much easier to fabricate in IC technology if all contacts are situated on one side of the plate.
Alternatively, achieving a high value for the geometrical factor G in small-size devices is much easier in a cross-shaped configuration than in a retangular one.
Hall voltage of a Hall plate with an arbitrary shape can be expressed as
VH = GVH∞ (2.19)
Where G is a parameter called the geometrical correction factor and VH∞ denotes the Hall voltage in a corresponding infinitely long strip. ”Corresponding” means that the two devices have identical Hall coefficients and thicknesses, are biased by identical currents, and are exposed to identical homogeneous magnetic inductions. The geometrical correction factor summarily represents the diminution of the Hall voltage due to a non-perfect current confinement in a finite length Hall device. The geometrical correction factor is a number limited by 0 < G < 1. For a very long Hall device, G ~ 0.The geometrical correction factor G is an important parameter of real Hall devices. To determine the value of G for a particular Hall plate shape, one must somehow calculate the Hall voltage for a plate of this shape VH, and theHall voltage of the corresponding infinitely long device VH∞ . The geometrical correction factor is given by
G = VH / VH∞ (2.20) The Hall voltages for Hall plates of various shapes have been calculated using the following methods：conformal mapping techniques, boundary element methods, and finite difference or finite element approximations.
The influence of the shape of a Hall plate on the Hall voltage can be represented by the geometrical correction factor G. Briefly, this factor describes the diminution of the Hall voltage in a Hall device due to a non-perfect current confinement. The geometrical correction factor is defined by (2.20). The geometrical correction factor is a function of device geometry and the Hall angle. For a rectangular Hall plate with point sense contacts, such as that show in Fig.2-3 (a), the geometrical correction factor can be approximated as
3 )
For a relatively long Hall plate, with l/w > 1.5 and small sense contacts s/w < 0.18, it was found for small Hall angles that
)
2.5 Basic characteristics
The normal component of the magnetic induction is then the input signal, and the Hall voltage is the output signal. We shall define and discuss the basic coefficients of a Hall device characterizing its operation as a magnetic induction/voltage transducer [9].
(a) Sensitivity S：
Sensitivity is the most important parameter of a sensor. In a modulating transducer such as a Hall device, we may define absolute sensitivity and relative sensitivity.
(2) supply voltage related sensitivity：
L output even in the absence of a magnetic field. The offset voltage will limit the ability of the transducer to discriminate small steady-state magnetic fields.
Some effects conspire to create this offset voltage, including misalignment of the sense contacts, inhomogeneous in the material of the transducer, and nonuniform thickness of the film. Besides, the materials used to make Hall effect transducers is highly piezoresistive, meaning that the electrical resistance of the material changes in response to mechanical distortion. This may cause most Hall effect transducers to behave like strain gauges in response to mechanical stresses imposed on them by the packaging and mounting.
(c) Noise voltage V ： N
The fundamental and unavoidable of electrical noises is called Johnson noise, and it comes from the thermally induced motion of charge carriers in a conductive material. It is a function of the operating temperature and the resistance of the device. Johnson noise is described by：
kTRB
Vn = 4 (2.26) where k is Boltzmann constant, T is absolute temperature, R is resistance,
B is bandwidth
Besides, another noise called Flicker noise ( or 1/f noise) is found in many physics systems, and can be generated by many different and unrelated types of mechanisms. The flicker noise developed by a transducer is related to the specific materials and fabrication techniques used. Then, we must consider the two kinds of noise simultaneously and call them V . N where µn* is Hall mobility, ∆l is the geometrical offset of the sense contacts relative
to an equipotential plane. Generally, it is proportional to carrier mobility.
2.6 Structure design
We can know that hall voltage is proportion to the geometry factor for semiconductor magnetic sensor from equation 2.28 and 2.29 is the induced charge from channel [10], [11].
ch
We add two sensing electrodes on the structure of traditional thin film transistor.
When the device is operated at biased voltage and simultaneously affected by a
magnetic filed simultaneously, the potential will be measured by electrodes. Figure 2-2 shows the profile structure of Hall sensor. The potential will proportion to the applied magnetic filed bias. On the other hand, the potential will arise when we applied higher bias current at constant magnitude of magnetic field. The magnitude of potential will saturate when the bias current up to a degree. At this time the carrier of inversion layer will not change their direction with different applied magnetic field.
The size and location of electrodes will affect the magnitude of potential and sensitivity. Besides, the channel length and width will also have influence on the potential and sensitivity. In our study, we focus our study on the comparison of different channel length and width, the location of the sensing electrodes, and the geometry of structure. Figure 2-3 shows the different electrode designs of structure.
Reference
[1] George Caruntu , Ovidiu Dragomirescu, “ Consideration regarding the offset of the magnetic sensors＂ , IEEE, 2002
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[3] N. Kubo, N. Kusumoto, T. Inushima, and S. Yamazaki, “Characterization of polycrystalline-Si thin-film transistors fabricated by excimer laser annealing method,” IEEE Trans. Electron Devices, vol. 40, pp. 1876-1879, Oct. 1994.
[4] G. K. Giust and T. W. Sigmon, “High-Performance Laser-Processed Polysilicon Thin-Film Transistor,” IEEE Electron Device Lett., vol. 20, no. 2, pp. 77-79, Feb. 1999.
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5153-5155, 1991.
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[8] Yunosuke Kawazu, Hiroshi Kudo, Seinosuke Onari, and Toshihiro Arai,
“Low-temperature crystallization of hydrogenated amorphous silicon induced by nickel silicide formation,” Jpn. J. Appl. Phys. Part1, vol. 29, pp.
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London, 1989.
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Chapter 3
Experimental Procedure
3.1 The Fabrication Process Flow
The devices were fabricated on 4-inch-diameter p-type silicon wafer. Fig. 3-1 shows the process flow of the device. The 100nm undoped amorphous silicon (a-Si) films were initially deposited on 500nm thermally oxidized silicon (100) wafers by low-pressure chemical vapor deposition (LPCVD) system with silane (SiH4) gas at 550。C. The deposition pressure was 100 mtorr and the silane flow rate was 40 sccm.
Amorphous Si thin films anneal in furnace at 600℃ several hours (~24 hr) to convert into polycrystalline form. After defining the device active areas, a 60 nm-thick TEOS oxide film was deposited at 350。C to serve as the gate dielectric by PECVD. Then, a 300 nm thick poly-Si was deposited by LPCVD at 600。C with SiH4 for the gate electrode. Gate areas were patterned and the regions of source, drain, and gate electrode were doped by a self-aligned 5x1015 ions/cm2 phosphorus implantation with a He-diluted PH3 gas, at 50 KeV of acceleration voltage. The dopant were activated at 600。C in N2 ambient for 24 hr. Next, a 500nm TEOS oxide was deposited by PECVD at 350。C as a passivation layer, and contact lithography was carried out. After opening contact holes, a 500 nm Al was deposited by evaporation and the metal layer was patterned. Finally, the samples were sintered at 400。C for 30min in N2 gas ambient.
The conventional Hall sensors which use inversion layer as sensing layer has three disadvantages compared with bulk Hall sensor, including lower channel mobility, surface instability, and larger 1/f noise. In order to improve the performance of the device, we have to lower defects of the channel to add carrier mobility, avoid misalignment of the sense contacts to decrease offset voltage, and decrease contact
The conventional Hall sensors which use inversion layer as sensing layer has three disadvantages compared with bulk Hall sensor, including lower channel mobility, surface instability, and larger 1/f noise. In order to improve the performance of the device, we have to lower defects of the channel to add carrier mobility, avoid misalignment of the sense contacts to decrease offset voltage, and decrease contact