Chapter 2 Principles of ISFET-based biosensors: pH-ISFETs,
2.4 Biosensors
2.4.2 ISFET based biosensors
In recent years there has been great progress in applying FET-type biosensors for tion. Among them, the ISFET is one of the most popular appr
ET measurements owin
highly sensitive biological detec
oaches in electrical biosensing technology. One distinct merit of the ISFETs is the feasibility of miniaturization, thereby allowing its easy integration into the required electronics. Therefore an ISFET device of small size and low weight might be use in a portable monitoring system, i.e., a hand-held drug monitoring system.
Currently, various kinds of biological recognition materials for biological analysis such as DNA, proteins, enzymes, and cells are being applied to ISF
g to the unique electrical and biological properties, thereby elevating the sensitivity and specificity of detection [2.22, 2.23]. In the ISFET system based on different bio-contents for biological analysis, assorted concepts of biosensors like enzyme FETs, Immuno FETs, and DNA FETs that contain layers of immobilized enzymes, antibodies, and DNA strands respectively, have been reported in a large number of documents [2.23–2.28]. Nevertheless, although various types of ISFET-based biosensors have been developed, they still suffer from a variety of fundamental and technological problems such as rigid immobilization for biological materials, electrical properties and instability of functional groups in the sensing layer. To overcome these problems, interdisciplinary cooperation from various research fields such as chemistry, biology, electrics, and physics are essential.
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Figure 2-1 Properties of the electrode/electrolyte interface.
Figure 2-2 The site-binding model for the electrode/electrolyte interface.
(a)
(b)
(c)
Figure 2-3 Different approached for ISFET reference systems. (a) ISFET with pH buffer compartment (b) miniaturized Ag/AgCl reference electrode (c) ISFET with ion insensitive membrane. 1. pH buffer solution or KCl solution 2. glass capillary or hole 3. cover layer 4.
measuring electrode with metallic lead 5. chemically modified sensing layer
(a)
(b)
Fig. 2-4 Different measuring circuits for ISFETs. (a) One-ended m suring circuit (b) ISFET/REFET differential measuring circuit.
ea
Fig. 2-5 Schematic diagram of biosensor elements.
Chapter 3
Optimization of post-annealing process and characterization of ion selectivity for ZrO
2gated ISFETs
3.1 Introduction
ricated with ZrO2 gate as sensing membrane was reported [3.1]. To btain the best performances, such as pH sensitivity, drift and hysteresis, the optimization of pr
ntial capacitance. It is
tes of ZrO2 are
Annealing is a heat treatment wherein a material is altered, causing changes in its properties such as surface morphology and atom density. It is a process that produces conditions by heating to above the re-crystallization temperature and maintaining a suitable
The ISFETs fab o
ocesses and characterization of ion interference are essential.
According to Equations (2-16) and (2-17), the pH sensitivity has been extensively described in terms of the intrinsic buffer capacity and the differe
determined by the reactions in solid/electrolyte interface. On the other hand, the drift and hysteresis are effects of surface slow effect. The phenomenon and algorithms have been widely discussed by many research groups [3.2-3.8]. The reports indicated that the material and surface condition of sensing layer dominated the drift and hysteresis.
Eisenman’s theory of ion selectivity has shown that the selectivity is determined by the electrostatic field strength at the ion exchange sites [3.9]. Surface si
considered to have strong field strength and therefore should have pH sensitivity.
Meanwhile, due to the intrinsic mechanical stability of metal oxide, easy miniaturization and compatibility with CMOS processing, ZrO2 is a good candidate for ISFET development.
te erature, and then cooling. Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations.
2 3
mp
Recently, Pan et al. investigated the influence of thermal annealing on the structural characteristics of the Tm O sensing membrane for pH detection [3.10]. The results revealed that the sensitivity, drift, and hysteresis are annealing temperature dependent.
They were attributed to the formation of a well-crystallized Tm2O3 structure, a thinner low-k interfacial layer at the oxide/Si interface, and the higher surface roughness at optimal thermal annealing conditions. Lai et al. also reported that the pH sensitivity of 8 nm thick HfO2 without underlying SiO2 was increases from 46.2 to 58.3 mV/pH by 900°C post deposition annealing treatment [3.11].
Another important characteristic of pH-ISFETs is the membrane selectivity. The potentiometric selectivity coefficients are expressed according to the Nicolsky–Eisenman equation [3.12] as
day constant, R is the gas constant, T is the absolute temperature, a is the ion activity, i, j are the main and interfering ion indices respectively, z is the ion electrovalence and kij is the selectivity coefficient. While performing the detection for the main ions, e.g. hydrogen ions, other so-called interference ions, K+, Na+, for example, are also join to determine the potential change. As a result, the exact concentration of main ions can not be measured. Since it is unavoidable, the impact of the interference ions on a sensing membrane should be evaluated.
3.2 Experiment
3.2.1. Device Fabrication
The detailed fabrication procedures and fundamental characteristics of ZrO2 gate FETs has been reported [3.1]. Figure 3-1 shows the schematic diagram of fabricated
2 dielectric FETs were fabricated on p-type silicon wafers with (100
on dioxide by Buffered Oxide
30 nm)
(6) d Chemical Vapor Deposition (PECVD) deposition of silicon dioxide
oxide by BOE
(9) DC sputtering of ZrO2 (30 nm) and post annealing in N2 at 600, 700, 800 and 900 ºC for
ing of oxide by BOE IS
ZrO2 gate ISFET. The SiO
) orientation accordingly, and their source/drain areas were fabricated with phosphorus/boron ion implantation respectively. A 30-nm-thickness sensing layer of the ZrO2 membrane was deposited onto the SiO2 gate FET by DC sputtering with 4-inch diameter and 99.99% purity of Zirconium target in oxygen atmosphere The total sputtering pressure was 20 mTorr in the mixed gases Ar and O2 for 200 mins while the base pressure was 3 ×10-6 Torr, and the RF power was 200 W and the operating frequency 13.56 MHz.
Brief manufacturing process steps are addressed as follows:
(1) Standard RCA clean for 4-inchp-type silicon wafers (2) Wet oxidation growth for silicon dioxide (600 nm) (3) Defining of Source/Drain areas and wet etching of silic
Etching (BOE)
(4) Thermal growth of silicon dioxide as screen oxide (
(5) Phosphorus or boron ions implantation and post annealing at 950 ºC Plasma Enhance
as passivation layer
(7) Defining of contact hole and gate region and wet etching of silicon di (8) Dry oxidation of gate oxide (30 nm)
30 minutes, respectively
(10) Defining of gate region and wet etch
(11) Aluminum sputtering with hard contact mask (500 nm)
6A semiconductor parameter nalyzer was used for measuring the IDS–VGS characteristics for the ZrO2 gate ISFETs m R.D.H., Seelze, Germany). The source-drain volta
e immersed in the pH=7 buffer solution for 2 hours to keep the devices stable. The hysteresis curve was obtained by
3→7→11→7 and then the second sequency of pH =
lutions were prepared as follows:
3.2.2. Packaging and Measurement setup
Figure 3-2 shows the measurement setup. A HP415 a
soaked in pH buffer solutions (purchased fro
ge was kept constant at VDS = 2V. A container was bonded to the gate region of ISFET by using epoxy resin. All the measurements were performed based on a commercial Ag/AgCl glass reference electrode, which was connected to the gate of the device and the power supplier to provide stable bias potential for device operation. The measurements were performed at room temperature of 25 ºC, which was kept constant by a temperature control system, and all the setup was placed in a dark box.
3.2.3 Hysteresis and drift measurement
Prior to perform the measurement, the pH-ISFETs wer
measuring in the first sequence of pH = 7→
7→11→7→3→7, with loop time of 15 minutes. Three measuring points were obtained for each pH value in the duration of one minute.
The ISFET’s sensing layer was soaked in the buffer solution (pH=7) for 13 hours before drift measurement. 36 points were obtained during 10 minutes in the same pH value of aqueous solution.
3.2.4 Preparation of Na+ and K+ ion solutions The K+ and Na+ test so
(1) The 0.1M Na+ or K+ ion solutions were prepared by diluting 1ml 3M NaCl and KCl
(2) T ting 1ml 0.1M Na+ or K+ ion
of pNa and pK equal to 10.
.3.1 Optimization of post-annealing process
nsitivity and linearity of ZrO2 membrane annealed at ifferent temperatures. For the sample without any post annealing, the pH sensitivity was s. On the contrary, the pH sensitivity of 900
T
solution with 30ml of each pH buffer solution.
he 0.1M Na+ and K+ ion solutions were prepared by dilu solutions with 100ml of pH buffer solution.
(3) Repeating step (2) to obtain the ion solutions
3.3 Results and discussions
3
Figure 3-3 shows the pH se d
only 39.54 mV/pH, which is lower than other
°C annealed sample was 48.17 mV/pH, which is higher than 700 °C and 800°Cannealed samples, which few defects were found. Meanwhile, the results show that the ZrO2 layer annealed at 600°Cpresents the best sensitivity (54.5 mV/pH) and linearity (0.9996) performances.
o explain the post deposition annealing effects on sensitivity, the site-binding model was applied again as Equation 3-2.
(
pH pH)
surface charge switches from negative through zero to positive. k is the acidic equilibrium constant, kb is the basic equilibrium constant, k is the Boltzmann constant, T is themeasuring temperature, and β is a parameter, given by
a
the surface site density and diff is the differential double-laye
Figures 3-4 (a)-(e) show the scanning electron microscope (SEM) images of the ZrO2
films which annealed at 600, 700, 8 C
diff and
r capacitance. According to
Equation 3-2, the ka, kb, Ns, which determined by the electrolyte/solid interface
condition, domi ity.
00, 900 and not annealed, respectively. The sputt
e
[3.13], the ions diffuse from the surfa
C
nate the pH sensitiv
°C
er-deposit ZrO2 film without post deposition annealing seems to be amorphous. As a result, it demonstrated low sensitivity and low linearity performances, which represents the ka, kb, and Ns are smaller than others. For the films with post deposition annealing at 700 to 900°C, the grain sizes as well as surface roughness of films are proportional to the annealing temperatures, it is inferred that the Ns should be larger accordingly. As a result, the pH sensitivities were proportional to the annealing temperature. However, for the case of 600°Cannealing, the pH sensitivity was higher than other conditions. It could be explained that though the size of surface grains are smaller than others, but due to the tight arrangem nt, the effective Ns could be larger than others.
The hysteresis effect may induce the inaccurate measurement of pH-ISFET devices. The hysteresis is caused by the slow response of the pH-ISFET
ce of the sensing film into the buried site are very slow, and results in slow response. In addition, due to the different sizes of H+ and OH- ions, the diffusion speed of H+ ions into the buried site are faster than that of OH- ions, as described by Bousse et al. [3.14]. This causes the asymmetric hysteresis behavior of the pH-ISFET devices. Figures 3-5 (a)-(e) show hysteresis curves in pH loop 7→3→7→11→7 and Figures 3-6 (a)-(e) show hysteresis curves in another pH loop 7→11→7→3→7 for samples. The asymmetric hysteresis behavior can be observed in all samples. Table 3-1 summarized the overall performances for all samples with different annealing temperature. It is observed that the 600 C° sample demonstrates the smallest hysteresis, which can be attributed to the surface quality with dense atoms arrangement. The
similar results also happened in drift behaviors as shown in Figures 3-7 (a)-(e). According to Yule et al. [3.15] and Jamasb et al. [3.16], the drift is due to the hydration. The thickness of the hydrated layer is increased with time. Thus, the overall insulator capacitance would be decreased, results in the threshold voltage increases with time. Again, the film annealed at 600°C demonstrates the best drift performance than other. For the application of pH-ISFETs, the ZrO2 films with optimal post deposition annealing process at 600 emonstrated the best performance in pH sensitivity, linearity, hysteresis and drift.
3.3.2 Characterization of ion selectivity
°Cd
he
e
ry
T ISFET with ZrO2 sensing film which fabricated in 600 post deposition ensitivity property for hydrogen ions. However, sin
low to treat eve
°C annealing process demonstrated the high s
c the ISFETs are possible use in complex electrolyte environment, the selectivity to other ions, such as K+ and Na+, becomes important and requires further study.
Ions are charged so they interact in the solution attracting and repelling each other with coulomb forces. These interactions influence ions behavior and does not al
ion in the solution independently. Figures 3-8 (a) and (b) show the pK and pNa responses of the ZrO2 ISFET at pH=9, respectively. While the K+ concentration lower than pK=5 or the Na+ concentration lower than pNa=3 at pH=9, the potential differences caused by K+ or Na+ are very small which can be ignored. On the contrary, while the K+ or the Na+ concentration larger than the same values above, the sensitivities are estimated as 8.67 mV/pK and 7.5 mV/pNa, respectively. The ranges and sensitivities are influenced by the ionic strength of the buffer solutions. In chapter 2 we described the relationship between ion concentration, activity and activity coefficient as Equation 2-1. To obtain the precise ion activities, the most popular method used to calculate ions activities proposed by Debye and Hückel [3.17] in 1923 is incorporated. First step in calculations is calculation of so
called ionic strength, using following formula:
Summation is done for all charged molecules present in the solution. Second step is calculation of activity coefficients given by formula:
I
Activity coefficient for all ions bearing the same charge is ide + where γz denotes activity coefficient for z-charged ions.
Figures 3-9 (a) and (b) d
centration increased, where the ionic
r hand, with escribe the detection limits and sensitivities of K+ and Na+at the buffers of pH=3 to pH=11. As the hydrogen ion con
strength of H+ is also increased, the detection limits and sensitivities of K+ and Na+ are decreased due to the interference from the H+. In contrast, the activity of H+ is also interfered by that of K+ and Na+. The pH sensitivity is decreased significantly while the pK or pNa less than 5. Meanwhile, the maximal pH sensitivities of ZrO2 film are around 45 mV/pH and 39 mV/pH with K+ and Na+ adding in pH buffer solutions. It revealed that the selectivity coefficient γ(H+, K+)andγ(H+, Na+) are different for ZrO2 sensing film.
Figure 3-10 summarizes the multi-ion detecting characteristics of ZrO2 gated ISFET.
With the K+ and Na+ interference, the pH sensitivity was degraded. On the othe
the interference from different ionic strength of hydrogen ions, both the pK and pNa sensitivity curves are linear and lower than 10 mV/pX (X = K or Na).
3.4 Summary
The optimal post-annealing process of fabricating ZrO2 gated ISFETs are developed.
he results reveal that the post deposition annealing temperature of 600℃ for 30 minutes is es
T
sential, it improves the surface condition of the ZrO2 film. As a result, the high pH sensitivity and linearity, low hysteresis and drift characteristics are obtained. Based on the ISFETs with optimal annealing process, the characterization of ion selectivity and interference are investigated. Through the characterization of multi-ion detection, the ion sensitivity under different ionic strength conditions can be more precisely described. It is important for ISFET used in the biological applications.
References
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[3.12] Janicki, M. Napieralski, A. Department of mcroelectronics and cmputer sience, tchnical uiversity of Lodz, Al. Politechniki 11, 93-590 Lodz, Poland.