Lecture 5

INTEGRATED SENSORS

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Outline

1st Half

• Need for integrated sensors

• Examples of sensor-focused applications

• Thermal sensing

– The infrared spectrum

– Microbolometers

– Photodetectors

2nd Half

• Distance sensing

– Acconeer

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Sensor-focused applications

Personalized HealthPersonal Assistant

Autonomous Driving

Security

Your mood is

a bit sour.

Why not have

a snack?

Low-power

Low-cost

Highly integrated

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Personal Health: Tricorder x-prize

• To diagnose: Anemia, Atrial Fibrillation (AFib), Chronic Obstructive Pulmonary Disease (COPD),

Diabetes, Leukocytosis, Pneumonia, Otitis Media, Sleep Apnea, Urinary Tract Infection, Absence of

condition.

• Vital signs: Blood Pressure, Heart Rate, Oxygen Saturation, Respiratory Rate, Temperature.

http://tricorder.xprize.org/

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Useful types of integrated sensors

Chemical sensors

Atmospheric gas sensors

Pressure sensors

Thermal sensors

Dis

tance m

appin

g

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Infrared spectrum

• H2O absorption limits spectrum to three regions

• SWIR (1-2.5 µm)

• MWIR (3-5 µm)

– NOx/CO2 gas detection

• LWIR (8-14 µm)

– O3 gas detection

Light frequency Vibrational modes

LWIRMWIRSWIR

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Thermal spectrum

• All things that have a temperature radiate...

– Spontaneous radiative distribution of entropy

• Plancks law of radiation

– 𝐼 𝜆, 𝑇 =8𝜋ℎ𝑐

𝜆51

𝑒ℎ𝑐𝜆𝑘𝑇−1

• Stefan-Boltzmann law:

– 𝑃 = 𝜎𝑇4

• Lower temperature longer wavelength

• Lower temperature less intensity

• Bonfire 1100 °C

• Magnesium burning 3100 °C

• Room temperature (300 K) LWIR...VLWIR

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Thermal imaging

Finding heat leaks

Finding missing people/intruders

Night vision

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Thermal sensing in traffic control

• Thermal imaging instead of visible light

• Superiour under many circumstances

• Autonomous driving

Glare

Headlights

Shadows

Long-range

night vision

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Thermal sensor

• Long wavelength infrared photons (8-14 µm) converted to electronic signal

• Bandwidth f = reciprocal of twice the integration time.

• Noise equivalent power (NEP): signal power giving S/N=1 at 1 Hz bandwidth. [W/Hz1/2]

• Detectivity: 𝐷∗ =𝐴𝑓

𝑁𝐸𝑃[cm Hz1/2/W].

• Responsivity [V/W]:

– How efficiently conversion of incoming power to electrical signal V (or A) is done

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The microbolometer

• Absorption of LWIR light heating of detector

change of electrical resistance

• No need for cooling

• Temperature-sensitive material

– Thermally isolated membrane

– Resting on two pillars that connect to

CMOS Readout Circuit

• Reflector underneath improves signal

• Fabricated in MEMS foundry

• Up to 1024x768 pixel arrays currently

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Bolometer metrics

• Temperature coefficient of resistance (TCR): [%/K]

– ”How large the resistance change per degree is”

– Typically 2-6 %/K

• Low resistivity important

– Modulation of a low resistance less noise

• Typical D* ~ 108-9 cmHz1/2/W

• Materials: VO2, amorphous Si

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Vanadium oxide

• Mott insulator

– Strong Coulomb-Coulomb interaction binds electrons

together

– Only above a threshold T are they free to conduct current

• Massive change in resistance with temperature

• Different oxygen content gives different transition temperature

Kim et al New J Phys. 2004

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• VO2 and V2O5 have TCR > 4%/K

– VO2 expensive to purify

– V2O5 has high resistance at RT

• V2O3 metallic at RT

• Suitable phase mixture for maximizing TCR and conductivity

• |TCR| between 1.5-2.5%/C usually obtained.

• Detectivity of roughly 108 cm W-1 Hz1/2

• Nanostructuring VOx can improve TCR and D*

– TCR = -6.5%/K

– D* ~ 109 cm W-1 Hz1/2

• But VOx is not compatible with Si process lines...

Vanadium oxide for microbolometers

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Amorphous Si in microbolometers

• a-Si:H has theoretical TCR of -10 to -12 %/K

– But too high resistance

• B-doped a-Si:H TCR = -2.8%/K

• a-SiGe with nanocrystals TCR = -6.6%/K

– 200°C PECVD

– Nanocrystals improve mobility

• Detectivity: 2 x 109 cmHz1/2/W

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Benchmarking

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Time response of microbolometers

• Frame rate limit limited by heat diffusion

– Thermal time constant 𝜏𝑡ℎ =𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑚𝑎𝑠𝑠

𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦

– Thin layers, small pixel size help

– Typically limited to 50-100 Hz.

time

Heat signal Detector response

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Photodetectors for LWIR

• Optical detection electrical signal

• For LWIR needs very small band gap (< 150 meV)

• Dark currents are a severe concern

• Cooling < 150K is usually needed

• High operation speeds (1-10MHz)

– Limited by junction capacitance

and detectivity

• High detectivity (> 1011-12 cmHz1/2/W)

at 150 K...

• HgCdTe

– Not very environmentally friendly

– Hard to tune energy gap

• In(As)Sb

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Superlattices

• GaSb/InAs based superlattices

• Creates minibands to achieve LWIR

– 150 meV (8 µm) 42/42Å

or 80 meV (15.5 µm) 20ML/20ML

• Thickness and interface control is crucial

InAs GaSb InAs GaSbInAs GaSb InAs GaSb

12 µm

Image from 1024x1024 SLS FPA at 77K

Qmagiq SPIE 2012

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Reducing dark current

• Introducing a barrier to reduce dark current

– nBn, pBp

– pBn diodes

• Experimental results still few nBn

pBn

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Summary

• Sensor-focused applications on the rise

• Requires low-power, highly integrated sensors

Thermal sensors

• Microbolometers

– Resistive change with temperature change

– No cooling

– Easily reach sensitivity at >10µm

– Medium sensitivity

– Slow (50-100 Hz)

• Photodiodes

– High dark currents Needs cooling

– But much faster and more sensitive

– Heterostructure devices for optimization