Application of Scanning Hall Probe Microscopy Technique at Room Temperature 300 K .

Active areas of bismuth Hall Probe sensors in the range (0.1 – 1) μm have been fabricated on Si/SiO2 with GaAs substrates at thickness of bismuth from (40, 60 and 70) nm by Electron Beam Lithography (EBL) and lift-off process. Scanning Hall probe microscopy (SHPM) technique at room temperature (300) K used to study Hall voltage, characterization of the noise figures and minimum detectable fields. Results are presented for both 0.4μm sensor, which is found minimum detectable fields (Bmin) 1.1 G/ Hz 0.5 with dc currents about 5μA. But minimum detectable fields for HP size 0.6 μm at dc currents 20μA is Bmin 0.6 G/Hz 0.5 . The performance of our Hall probe devices at 300K could be improved still further are discussed.


Introduction
Scanning Probe Microscopy technique (SPM) [1,2] is a new technique at room temperature must operate effectively.Scanning Hall probe microscopy is best part of it which has high spatial resolution of sensors as well as nanoscale sizes are fabricated and worked in close to surface of active area.Bismuth is a semi-metal with low effective mass, low carrier density, and long mean free path .The carrier density with mobility of Bismuth are very important to find the figure of merit [3].Bismuth is much, or suited for room temperature operation than GaAs/AlGaAs .Also one-over-of noise is a fluctuation in the conductivity which leads to fluctuation in the resistance then leads to slow fluctuations in the power density of the thermal noise [4].But it has disadvantage which that the carrier density rely on a figure of features such as thickness of film, quality of substrate material and deposition technique.In practice, fabrication by scanning bismuth active areas have been broadly examined.Nanoscale of active areas about ~50 nm but Focussed Ion Beam (FIB) technique used for larger hall probe.Currently, we are used lift-off fabrication in order to get very low noise in bismuth hall devices to improved figures of merit such as minimum detectable fields, Hall coefficient (low carrier density), Johnson noise, 1/f noise and low offset resistances [5].
In this work was extended here to use EBL technique with scanning bismuth Hall probe devices which active areas have range (0.1 to 1) µm.

Experiment
This fabrication of Bi-Hall Probe sensors was performed at Bath University Nanofabrication Lab -clean room facility‖.Si/SiO 2 and GaAs substrates wafers were scribed and cleaved into (6.5 x6.5) mm squares ‗chips' using a diamond scribe.The chosen size was considered suitable for package and it is enough for Ohmic contact size.Once the devices were fabricated, the Si/SiO 2 and GaAs substrate (6×6) mm squares were scribed into bigger (7 ×7) mm or (6.5×6.5)mm chip.Samples were cleaned in an ultrasonic bath in solutions of trichloroethylene, acetone and isopropanol correspondingly.The samples are soaked in chlorobenzene for four minutes so as to make the top layer.The sample and mask (using a Karl Suss MJB3 mask aligner) must be very clean before starting.Best exposure time about 12-22 seconds.Throughout-the procedure care must be taken to ensure that there is a gap between sample and mask.The researcher should look for coloured interference fringes between the sample and the mask where the spacing is roughly the order of the wavelength of light in magnitude.
Sometimes fringes are never seen -in which case resist blotches near the corners of the chip or signs that the chrome mask is being deformed upwards due to contact with the chip near the chip corners.If this is seen, the sample must not be raised any closer to the mask.This mask is brought into close contact with the coated substrate and illuminated with UV light, which passes through the transparent areas and weakens the bound in the resist.In order to get high quality samples, it is required that any contamination is removed.Contamination is caused by particles from several things such as air and previous processing steps [4].The exposed samples are developed using 351 developer (in the ratio of H 2 O: Developer 3.5:1) for a duration of 30-50 seconds.After this period above immediately soak them in beaker of water for a period of around 10-15 seconds, in order to rinse and dry then using a nitrogen gun.
EBL technique is excellent for making best designs required by the up-to-date with electronics devices for integrated circuits.It has very small spot size of the electrons, as well as the quality of resolution in lithography material is best determination by the wavelength of exposure light device.See Fig. 1.

Results and Discussion
The main goal in this article is the study of Bismuth micro-Hall probe sensors with a wide range of sizes (0.1 -1) µm at room temperature (300K).All results obtain for some size of Bi-Hall probe such as 0.6µm and 0.   of spectrum noise is ranged between (0 -100)Hz with (100 rms).The noise level in Fig. 3 decreased at -74 ×10 4 (dBV) gradually with frequency until it reaches to corner of spectrum noise which is called (one-over-of) noise which is the minimum noise threshold of this 0.6µm Bi-HP device.The frequency semi stable at range (20-100) Hz then it called white noise.But for the 0.4µm Bi-HP sample, the noise level decreases from -70 ×10 4 (dBV) and range of white noise is around (30-100) Hz as shown in Fig. 4. The reason for using a Bi-Hall probe at room temperatures (300K) rather than a Hall-probe GaAs/ AlGaAs heterostructure is that the signal noise ratio of GaAs/ AlGaAs is poor.
Where, for a given field, temperature and Hall probe dimensions (width/length) the signal noise ratio (SNR) are three parameters such as current Hall probe (I Hall ), mobility ( ) and carrier density (n)for two dimensions in which mobility does not change much with temperature for Bi.Nevertheless, changes many orders of magnitude with temperature for GaAs 2DEGs and the (n2d) for Bi is given by ( n3d d ), where is the thickness of the Bi film.Table 1 shows an answer to the question as to why (Bi) is better than GaAs/AlGaAs at 300K.For Bi-Hall probe is four times bigger than GaAs/AlGaAs.The maximum value of (I Hall ) for Bi is probably >10 µA at all temperatures, while for GaAs it is 1 µA at 300K and (10-40) µA at 77K.The minimum detectable field ( B min ,) is important figures of merit for hall active areas and defined approximated by: [6 ] Where I H is the applied Hall current, R H is the Hall coefficient B is the magnetic field.

Fig. 1 :
Fig. 1: The Electron Beam Lithography (EBL) with Bi deposition.Shows many stages of Bi-HP fabrication as follows: (a) -The samples are covered by photo resist S1813 and are exposed by UV light.(b) -Ohmic Contact of samples deposited by 10nm Cr / 50nm Au.(c)-EBL for Bi-HP design and deposited by 70 nm of Bi.(d)-deposited especially Ohmic Contact by 20 nm Cr / 200 nm Au.

Fig. 2 :
Fig. 2: Images for SEM technique of 0.1 µm Hall probe size with 70 nm thick Bi-Hall probe device.

Fig. 4 .
Fig.4.Shows 0.4µm Bi-Hall probes at 300K with same Hall current.The data displays a 1.9Ω offset when B=0 with 14.1 nv/ Hz 0.5 for Noise floor and minimum detectable fields B min 1.1 G/ Hz 0.5 .The Hall coefficient (R H ) is 6.2 Ω /G and series resistance (Rs) of the Bi-HP around 12kΩ.The noise data is taken one hundred times for each sampling.It is noticeable that, the bandwidth of both Bi-HP 0.4µm and 0.6 µm is 954.85 mHz and frequency figure indicates the high frequency Johnson noise floor, corresponding to a voltage lead pair resistance of 3.1 k, nearly with the value 3.4 k .Obviously the little frequency noise produces rapidly with the 1/f shoulder and Hall current.Fig. 5 shows shifts of higher frequency, increasing above our frequencies measurement at I H =30 µA.

Fig. 5 :
Fig. 5: Noise spectral concentration, V n , shows a frequency function of 0.1um bismuth Hall probe size at room temperature.

=Fig. 6 :
Fig. 6: Minimum detectable fields as a function of Bi-HP sensors fabricated in 40nm and 60nm thick.

Table 1 :
Shows comparison between Bi Hall probe with GaAs/AlGaAs hetero structure at 300K.