# US 10,768,224 B2 · High frequency lock-in thermography using single photon detectors

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- Number: US 10,768,224 B2
- Document ID: US10768224B2
- Status: Granted
- Kind: B2
- Application number: 15/390,096
- Filed: 2016-12-23
- Published: 2018-06-28
- Granted: 2020-09-08
- Earliest claimed: 2016-12-23
- Assignees: FEI Company
- Applicants: FEI Company
- Inventors: Euan Ramsay; Theodore R. Lundquist
- Web page: https://patentgrep.com/patent/US10768224B2

## CPC

- G01R 31/308: using non-ionising electromagnetic radiation, e.g. optical radiation {(investigating or analysing materials by the use of optical means G01N21/00; image analysis G06T7/00)}
- G01J 5/0096: {for measuring wires, electrical contacts or electronic systems}
- G01N 25/72: Investigating presence of flaws
- G01R 31/311: of integrated circuits {(G01R31/31728 takes precedence)}
- H02S 50/15: using optical means, e.g. using electroluminescence
- Y02E 10/50: Photovoltaic [PV] energy
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- H02S: GENERATION OF ELECTRIC POWER BY CONVERSION OF INFRARED RADIATION, VISIBLE LIGHT OR ULTRAVIOLET LIGHT, e.g. USING PHOTOVOLTAIC [PV] MODULES (obtaining electrical energy from radioactive sources G21H1/12; light sensitive inorganic semiconductor devices H10F; light sensitive organic semiconductor devices H10K30/00; thermoelectric devices H10N10/00; pyroelectric devices H10N15/00)
- Y02E: REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION

## Abstract

Systems, methods, and computer readable media to improve the operation of thermographic imaging systems are described. Techniques are disclosed for generating thermograms using single low-noise photon detectors. More particularly, an array of single low-noise photon detectors operating in the Geiger mode may be used to accurately identify the time delay between the application of a periodic power stimulus to a device under test and the generation of photons resulting from that stimulus. In one embodiment an array of single photon detectors may be used to time-tag each detected photon. Thereafter, a high-speed counting circuit can correlate the detected photons to the applied stimulus. When operating at the frequencies possible in the Geiger mode, such measurements permit a higher degree of spatial resolution (e.g., in the x, y and z axes) of thermal hot-spots within the device under test than prior art approaches.

## Claims

### Claim 1

A method comprising:
applying an electrical test signal to a device under test, wherein the electrical test signal comprises a plurality of periods;
detecting individual photons emitted from the device under test in response to the application of the electrical test signal, wherein the detected individual photons comprise thermal photons;
for at least some of the detected individual thermal photons, determining time differences between times the individual thermal photons are detected and a time associated with at least one period of the electrical test signal; and
generating an image based on the time differences between times the individual thermal photons are detected and the time associated with at least one period of the electrical test signal.

### Claim 2 (depends on 1)

The method of claim 1, wherein the electrical test signal comprises a sinusoidal electrical test signal.

### Claim 3 (depends on 1)

The method of claim 1, further comprising measuring phase differences for at least some of the detected individual thermal photons with respect to the electrical test signal.

### Claim 4 (depends on 1)

The method of claim 1, wherein the individual thermal photons are detected using a plurality of single photodiodes operating in a Geiger mode.

### Claim 5 (depends on 4)

The method of claim 4, wherein the photodiodes comprise superconducting single-photon detectors.

### Claim 6 (depends on 1)

The method of claim 1, wherein:
the electrical test signal comprises a square wave, wherein the time associated with at least one period of the electrical test signal corresponds to at least one rising edge of the square wave in the at least one period.

### Claim 7 (depends on 1)

The method of claim 1, further comprising determining at least one histogram for each of a plurality of periods of the electrical test signal, each histogram corresponding to a distribution of the determined time differences in a respective period of the electrical test signal.

### Claim 8

A system, comprising:
an excitation source configured to supply an electrical test signal to a device;
a defect detection circuit configured to:
detect individual photons emitted from the device in response to the electrical test signal from the excitation source, wherein the photons comprise thermal photons, and
for at least some of the detected individual thermal photons, determine time differences between times the individual thermal photons are detected and a time associated with at least one period of the electrical test signal; and
an output module configured to generate an image based on the time differences between the times the individual thermal photons emitted from the device are detected and the time associated with at least one period of the electrical test signal.

### Claim 9 (depends on 8)

The system of claim 8, wherein the electrical test signal comprises a sinusoidal electrical test signal.

### Claim 10 (depends on 8)

The system of claim 8, wherein the defect detection circuit is further configured to establish phase differences for at least some of the detected individual thermal photons based on the electrical test signal.

### Claim 11 (depends on 8)

The system of claim 8, wherein the defect detection circuit comprises a plurality of single photodiodes operating in a Geiger mode.

### Claim 12 (depends on 11)

The system of claim 11, wherein the photodiodes comprise superconducting single-photon detectors.

### Claim 13 (depends on 8)

The system of claim 8, wherein:
the electrical test signal comprises a square wave, wherein the time associated with at least one period of the electrical test signal corresponds to a rising edge of the square wave in the at least one period.

### Claim 14 (depends on 8)

The system of claim 8, wherein the defect detection circuit is further configured to generate at least one histogram for each of a plurality of periods of the electrical test signal, each histogram associated with a plurality of determined time differences in a respective period of the plurality of periods.

### Claim 15 (depends on 8)

The system of claim 8, further comprising a lock-in circuit configured to:
receive an output from the defect detection circuit;
receive a reference signal having a frequency corresponding to a frequency of the electrical test signal; and
generate a synchronized input to the output module, wherein the synchronized input is synchronized to the reference frequency.

### Claim 16 (depends on 15)

The system of claim 15, wherein the output from the defect detection unit comprises a plurality of histograms, each histogram corresponding to a distribution of a plurality of time differences determined in a respective period of the electrical test signal.

### Claim 17 (depends on 8)

The system of claim 8, wherein the image comprises a phase image associated with phase differences for at least some of the detected individual thermal photons with respect to the electrical test signal, d.

### Claim 18 (depends on 1)

The method of claim 1, wherein the image comprises a phase image based on phase differences for at least some of the detected individual thermal photons with respect to the electrical test signal, wherein the phase image is indicative of a depth of a defect within the device under test.

### Claim 19 (depends on 1)

The method of claim 1, wherein the detected individual thermal photons have wavelengths between 2 μm and 5 μm.

### Claim 20 (depends on 8)

The system of claim 8, wherein the detected individual thermal photons have wavelengths between 2 μm and 5 μm.

### Claim 21 (depends on 1)

The method of claim 1, wherein the individual thermal photons are detected with a photodiode array operating in a Geiger mode.

### Claim 22 (depends on 1)

The method of claim 1, wherein the individual thermal photons are detected using at least one photodetector operating in a Geiger mode.

### Claim 23 (depends on 8)

The system of claim 8, wherein the defect detection circuit comprises at least one photodetector operating in a Geiger mode.

### Claim 24 (depends on 11)

The system of claim 11, wherein the plurality of single photodiodes operating in a Geiger mode comprises a photodiode array.