Characterisation and Reliability testing of THz

Schottky diodes.

By Chris PriceC.M.Price@rl.ac.uk

Supervisor: Dr Byron Alderman

December 2006

Preliminary Presentation

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)

Problems with THz Technology

• 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

10-3 10-2 10-1 1 10Frequency (THz)

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

E1

PhotonFrequency, f

Recombination of electrons and holes occurs creating a depletion zone

What is a Schottky diode

• P-N diode• is a combination to two oppositely doped semi-conductors

P- Type N- Type

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

Air BridgeSchottky Contact

Ohmic

)1)(exp(0 Tk

qVIi

B

d

• Diode current voltage relationship is given by; i is the current

I0 is reverse saturation currentq is the charge on an electronVd is voltage across the diodekb is the Boltzmann constantT is the temperature in kelvinη is the ideality

What diodes are used for

• Two devices that use the non-linear properties are;

• Multipliers (diode used as a varactor)– High frequency multipliers use the

nonlinear capacitance property, and a filter to eliminate specific harmonics and produce an output that is a multiple of the input frequency

...)( 33

2210 VaVaVaaVVI B

•Mixers (diode used as a varistor)• A mixer is a non linear circuit that receives two input signals and outputs a signal equal to the difference and to the sum of the two input frequencies

What diodes are used for

)]][2cos()][2[cos(2

.. 2121

2121 tfftff

AAvv

The creation of these diodes can be broken down into four stages:

1) Making the ohmic

2) Making the Schottky contact

3) Making the finger

4) Making the air gap

12

3

4 12

3

4

Fabrication Process

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– 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

Supply Situation in Europe

• Two methods:– Current Voltage (IV)

• Measures DC characteristics • Useful for mixers

– Capacitance Voltage (CV)• Measures AC characteristics• Useful for multipliers

Probe station (Pegasus S200)

Characterisation

IV setup

Basic Circuit

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 = V3 – V4

• Rs = 1000 ((V5 – V4) – ΔV• η = q ΔV log10 (e) / (kB T)• I0 = i1 / (exp ((q V1) /(η kB 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

• Curve fitting– Derived from IV equation and – assumes there is a constant

spreading resistance also– Assuming that i >> I0

– Substituting in Vd = V – i Rs

– V = a ln(i) + b i + c– Least square fits is performed

to calculate the coefficients

Calculating Parameters

)1)(exp(0 Tk

qVIi

B

d

q

Tka B

sRb q

ITkc b )ln( 0

Results

1 2 3 4 5 6 7 8 9 10

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

Ideality

X coordinate

Y coordinate

Surface Plot of ideality, P8.1 1um single anode

1.40-1.50

1.30-1.40

1.20-1.30

1.10-1.20

1.00-1.10

0.90-1.00

1 2 3 4 5 6 7 8 9 10

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

Resistance (ohms)

X coordinate

Y coordinate

Surface map of spreading resistance

16.00-20.00

12.00-16.00

8.00-12.00

4.00-8.00

Results

Scanning electron microscope (SEM)

IV setup• Problem;

Reliability Tests

• 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

Summary

• Current Situation:– Understood how the IV

characterisation calculations are done and

– how the Probe Station works– I have separated the diodes and

currently building the basic circuit for the reliability tests and

– I am currently learning LabView

Above: Diced diode from a wafer

Left: dicing saw

Future plans

• Remove bugs from the probe station and calibrate with new heads

• Develop the software necessary

• Build thermal experiment• Perform repeatability test• Perform soldering test• Analyse data collected

Any Questions???