Transcript chrispp
Preliminary Presentation Characterisation and Reliability testing of THz Schottky diodes. By Chris Price [email protected] Supervisor: Dr Byron Alderman December 2006 Contents • Problems with THz technology • What is a Schottky diode • What are Schottky diodes used for • The fabrication method • European situation • Characterisation • Reliability • Summarise current position • Future plans • Questions ??? Probe station (Pegasus s200) • THz frequencies are loosely defined in the range of 0.1-10 THz • There is an absence of low cost, miniaturised solid state power supplies Problems with THz Technology 10-3 10-2 10-1 1 Frequency (THz) 10 Problems with THz Technology • Transit Time Devices (up to 170 GHz) • Advanced electronics • Equivalent to commercially available devices with reduced dimensions • Maximum frequency determined by; • Time taken for a carrier to move a characteristic distance • And to a lesser extent the RC time constant Problems with THz Technology • Energy Transition Devices – Charge carriers undergo a transition between energy states – Frequency given by (E2 – E1)= h f • Problems; – Cannot get population inversion at room temperatures – Photon density of states α Frequency^2 E2 Photon Frequency, f E1 • • P-N diode is a combination to two oppositely doped semi-conductors What is a Schottky diode P- Type N- Type Recombination of electrons and holes occurs creating a depletion zone Energy Band diagram of P-N junction What is a Schottky diode • Combination of metal and a semi-conductor • This creates; – Lower junction voltage – Almost non-existent depletion width in metal • Another property of Schottky diodes; – Majority carrier • No slow random recombination Energy band diagram of Schottky junction Planar Schottky diode • Diode current voltage relationship is given by; qVd i I 0 (exp( ) 1) k B T i is the current I0 is reverse saturation current q is the charge on an electron Vd is voltage across the diode kb is the Boltzmann constant T is the temperature in kelvin η is the ideality Schottky Contact Air Bridge Ohmic • Two devices that use the non-linear properties are; • Multipliers (diode used as a varactor) – High frequency multipliers use the nonlinear 2 I (VB capacitance V ) a0property, a1V and a2afilter V toa3eliminate V 3 ... specific harmonics and produce an output that is a multiple of the input frequency What diodes are used for What diodes are used for •Mixers (diode used as a varistor) • A mixer is a non linear circuit that A1. A2 receives two and v1 .v2 [cos( 2 [input f1 f 2signals ]t ) cos( 2 [outputs f1 f 2 ]t )] 2 a signal equal to the difference and to the sum of the two input frequencies The creation of these diodes can be broken down into four stages: 1) Making the ohmic Fabrication Process 2) Making the Schottky contact 3) Making the finger 4) Making the air gap 3 4 2 2 1 1 3 4 Preparing the ohmic Annealer Thermal evaporator Schottky Contact E- beam evaporator Anode Finger Air Gap • Virginia Diodes Inc. (VDI) – Leading supplier of Schottky diodes – Concerns over future availability • University of Bath Supply Situation in Europe – Made limited progress • Darmstadt – Are developing a novel approach • UMS – Provide commercial diodes, but no influence over design • RAL – Produced leading mixer results at 200 GHz – Very much at the forefront of technology • Two methods: – Current Voltage (IV) • Measures DC characteristics • Useful for mixers Characterisation – Capacitance Voltage (CV) • Measures AC characteristics • Useful for multipliers Probe station (Pegasus S200) IV setup IVplot plotof ofp8.1 p8.12um 2um single single anode anode row row 44 IV Basic Circuit 1.00E-03 1.00E-03 9.00E-04 8.00E-04 1.00E-04 7.00E-04 Current (A) Current (A) 6.00E-04 1.00E-05 5.00E-04 4.00E-04 3.00E-04 1.00E-06 2.00E-04 1.00E-04 1.00E-07 0.00E+00 0.000 0.000 0.100 0.100 0.200 0.200 0.300 0.300 0.400 0.400 Voltage (V) (V) Voltage 0.500 0.500 0.600 0.600 0.700 0.700 0.800 0.800 Calculating Parameters • Parameters Interested in; – Spreading Resistance Rs, – Ideality η and – Reverse saturation current I0 • Two methods of calculations: – Standard formulae on current point measurements – Curve fitting the IV characteristics at arbitrary measurement points Calculating Parameters • Standard formula • ΔV = V – V •R = 1000 ((V – V ) – ΔV •η = q ΔV log (e) / (k T) •I = i / (exp ((q V ) /(η k T)) – 1) • Allows comparison between measurements • Good heritage for comparisons with previously used diodes but • If two or more points are missed comparison is no longer valid 3 4 5 s 4 10 0 1 B 1 B Calculating Parameters • Curve fitting qVd i I 0 (exp( ) 1) k B T – Derived from IV equation and – assumes there is a constant k bT ln( I 0 ) k BT c also a b Rs spreading resistance q q – Assuming that i >> I – Substituting in V = V – i R – V = a ln(i) + b i + c – Least square fits is performed to calculate the coefficients 0 d s Results Results Surface map Plot of ideality, spreading P8.1 resistance 1um single anode S10 S10 S9 S9 S8 S8 S7 S7 S6 S6 YYcoordinate coordinate S5 S5 1.40-1.50 16.00-20.00 1.30-1.40 S4 S4 12.00-16.00 1.20-1.30 1.10-1.20 8.00-12.00 S3 S3 1.00-1.10 0.90-1.00 4.00-8.00 S2 S2 Resistance (ohms) Ideality 11 22 33 44 55 66 XXcoordinate coordinate 77 88 99 S1 S1 10 10 IV setup • Problem; Scanning electron microscope (SEM) • Defined as how resistant it is to failure • Failure criteria; – A 10% deviation from original measurements. • Simple tests – Repeatability – Soldering – Thermal Cycling • Accelerated Life tests – Thermal – Humidity – Electrical Biasing Reliability Tests • Current Situation: – Understood how the IV characterisation calculations are Summary done and – how the Probe Station works Above: Diced diode – I have separated the diodes and from a wafer currently building the basic circuit Left: dicing saw for the reliability tests and – I am currently learning LabView • Remove bugs from the probe station and calibrate with new heads Future plans • Develop the software necessary • Build thermal experiment • Perform repeatability test • Perform soldering test • Analyse data collected Any Questions ???