FET Based Sensors Lecture 8 U-Tokyo Special Lectures Biosensors - PowerPoint PPT Presentation

1 FET Based Sensors Lecture 8 U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

2 Summary • Ion Sensitive Field Effect Transistors • Theory of operation, pH sensitivity • Instrumentation and CMOS integration • The REFET and differential measurement • Weak inversion and charge trapping • Example of ISFET Sensor IC • Silicon nanowire biosensors U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

3 Ion Sensitive Electrodes • Potentiometric devices characterised by the Nernst equation: ✓ a i 1 ◆ E = E 0 + RT nF ln a i 2 • Common form is the glass electrode for measurement of pH • Difficult to miniaturise so look for a solid state equivalent? U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

4 Ion Sensitive Field Effect Transistor • First developed in early 1970’s by Bergveld at the Univ. of Twente • MOSFET without 
 Vref Reference a gate electrode Solution Electrode • Gate dielectric 
 Encapsulation SiO2 (SiO 2 ) becomes 
 n − Si n − Si ion sensitive 
 Source Drain membrane p − Si U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

5 MOSFET/ISFET • Drain current of both MOSFET and ISFET: ( V GS − V T ) V DS − V 2 ✓ ◆ W DS I DS = β β = µC ox 2 L • Threshold voltage ( V SB = 0 ): V T = V F B + Q B + 2 φ F C ox • Flatband voltage (MOSFET): V F B = Φ MS − Q f + Q ox C ox U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

6 V FB in an ISFET • ISFET gate voltage is set by RE but the solution will contribute to the VT • Expression for ISFET flatband voltage: V F B = E ref + Ψ 0 + χ sol − Φ Si − Q f + Q ox C ox • χ sol - Solution dipole potential, Ψ 0 - oxide surface potential (solution side) • All terms are constant except for Ψ 0 U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

7 Oxide Surface Reactions Solution Oxide • Hydroxyl groups neutral formed at SiO 2 surface OH Si site • Equilibrium reactions 
 O SiOH ⇔ SiO − + H + proton 
 O − Si B donor SiOH + 2 ⇔ SiOH + H + B O • Protons from bulk proton 
 Si OH 2+ solution bind to sites acceptor Surface U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

8 Surface Potential • Full derivation is beyond this course, but surface potential Ψ 0 depends on pH B δ Ψ 0 = − 2 . 3 kT q α δ pH B • α is a dimensionless sensitivity parameter and as it approaches unity the ISFET pH sensitivity becomes more Nernstian • If α =1 then ∆Ψ 0 = –59.2 mV/pH at 298K U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

9 ISFET Sensitivity α • Formula for ISFET sensitivity factor: 1 α = 2 . 3 kT C dl + 1 q 2 β int • We know k , T & q , so the important factors are C dl and β int • C dl - Electrical double layer capacitance • β int - Buffer capacity of the oxide surface U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

10 Double Layer Capacitance Helmholtz Layer • C dl made up of two parts, Diffuse Layer + + + − compact “Helmholtz layer” − Electrode Surface − + and the diffuse layer + − + − 1 1 + 1 − − + − = + − C dl C H C d − + + • Diffuse layer width + − + + − decreases with solution − concentration, increasing C d C H C d U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

11 Buffer Capacity • Ability of the dielectric surface to accept or donate protons from solution • The higher this is the closer α is to unity • SiO 2 hasn’t got a very high β int and sensitivity is around –30 mV/pH • Si 3 N 4 , Al 2 O 3 and Ta 2 O 5 are all better. Why? U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

12 Effects of Gate Dielectric • As β int increases the response becomes more Nernstian • Ta 2 O 5 has high β int and a sensitivity of 
 –58 mV pH –1 U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

13 Effects of Gate Dielectric • C dif varies with ionic concentration • This can seriously affect an ISFET • Ta 2 O 5 β int is so high that C dif doesn’t matter U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

14 ISFET Response • Surface potential Ψ 0 ∝ pH B and: V F B = E ref + Ψ 0 + χ sol − Φ Si − Q f + Q ox C ox • So the ISFET V T will change with Ψ 0 and: δ V T = − 2 . 3 kT q α δ pH B • Assuming other terms stay constant U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

15 Measuring ISFETs • ISFETs are often biased in the linear region. • Constant I DS and V DS with ( V GS − V T ) V DS − V 2 ✓ ◆ W DS I DS = β β = µC ox 2 L • V T controlled by pH of measured solution • Needs some electronic feedback U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

16 Source and Drain Follower Circuit V in R 2 V + = V R 2 = R 1 + R 2 R 3 Ref. electrode D V DS = V − = V + + − V R S in − 1 ISFET V + + V in R 1 V R 3 = V R 1 = R 2 R 1 + R 2 V out I DS = V R 3 R 3 U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

17 Source and Drain Follower Circuit V DS = V R 2 R 3 Ref. electrode I DS = V R 3 R 3 D + CONSTANTS − V R S in − 1 ISFET V + + δ V T = − 2 . 3 kT q α R δ pH B 2 δ V out = 2 . 3 kT V q α out δ pH B U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

18 Source and Drain Follower Circuit δ V out = 2 . 3 kT R q α 3 Ref. electrode δ pH B D + − V R S in − 1 ISFET V + + R 2 − 1 . 3V < V out < 3V V out U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

19 ISFET Amplifier Vdd R out V out = I f R out = δ V s I in V DS = I in R DS R S + R R 2 3 R out V out = − δ V T − R 1 R S Ref. electrode + D + R DS S − ISFET − V ref R out V out I DS controlled R 1 − by feedback R R + 2 3 I f R S δ V S = I f R S U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

20 ISFET Amplifier Vdd At pH=7 set V ref to give V out =0V I in V out ∝ δ pH + R R 2 3 Set R out for desired 
 − R 1 pH sensitivity Ref. electrode + D + R DS S − ISFET − V ref R out V out R 1 − R R + 2 3 I f R S U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

21 ISFET Fabrication • CMOS uses a “self aligned” process • Source/Drain implant requires the polysilicon or metal gate as a mask • Problem for e e integrated ISFET production e - self-aligned. te - self-aligned. U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

22 CMOS Compatible ISFET Structures • Use metallisation layers pH Sensing Region available in CMOS Silicon Nitride Membrane • Connect ISFET gates to Inter-Layer the surface of IC Dielectric • Top passivation on ICs Metallisation G G G is typically silicon Oxide S D S D nitride n-well • Ideal as a pH sensing p-type silicon substrate layer U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

23 CMOS Integration • Previous instrumentation example doesn’t work with n-ISFETs in CMOS • N-MOS/ISFET has common, grounded p- type substrate in most technologies • Body effect - Offset voltage between bulk and source. p 2 ε s qN a (2 φ F + V BS ) V T = V F B + 2 φ F + C ox U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

24 The REFET • One of the biggest problems in microscale electrochemical sensors is the lack of a solid-state reference electrode • These can be miniaturised (Ag/AgCl) but at the cost of performance. • The REFET isn’t a solid state reference electrode but a way to do without one! U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

25 REFET Concept • REFET - ISFET with pH sensing blocked • Allows the use of a pseudo-reference • This could be a simple platinum electrode ISFET V=f(pH,V PRE ) Pseudo Reference Electrode + Diff. Amp. V=f(pH) - V=f(V PRE ) REFET U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

26 REFET Problems • Ideally a REFET would be identical to the ISFET but with no pH sensitivity • In practice this is very difficult as the blocking layers can affect V T mismatch • This is a significant barrier to developing practical ISFET based sensors U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

27 ISFET Problems • ISFETs are temperature dependent so an integrated temperature sensor is useful • Drift is a huge issue, with a Si 3 N 4 sensing layer it can gradually change to SiO 2 or a silicon oxynitride in aqueous solutions • Good encapsulation of metal connections is essential for long-term stability U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith

28 Weak Inversion ISFET • When V GS < V T the drain current is: ✓ q ( V GS − V T ) ◆ h q ⇣ ⌘i I D = I D 0 exp 1 − exp − V DS nkT kT • If V DS > 4 kT / q ignore the last part • V T dependent on pH so:  � nkT ( V GS − γ − 2 . 3 α kT q I D = I D 0 exp q pH) • Where γ is a constant including the reference electrode potential U-Tokyo Special Lectures Biosensors and Instrumentation Stewart Smith