# US 5,707,466 A · Method and apparatus for selectively annealing heterostructures using microwave

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- Number: US 5,707,466 A
- Document ID: US5707466A
- Status: Granted
- Kind: A
- Application number: 08/415,073
- Filed: 1995-03-31
- Granted: 1998-01-13
- Earliest claimed: 1995-03-31
- Assignees: California Institute of Technology
- Inventors: Harry A. Atwater; Ruth A. Brain; Martin B. Barmatz
- Web page: https://patentgrep.com/patent/US5707466A

## CPC

- H10P 95/06: {Planarisation of inorganic insulating materials}
- H10P 95/04: {Planarisation of conductive or resistive materials}
- H10P: GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10

## Abstract

The present invention discloses a process for selectively annealing heterostructures using microwaves. A heterostructure, comprised of a material having higher microwave absorption and a material having lower microwave absorption, is exposed to microwaves in the cavity. The higher microwave absorbing material absorbs the microwaves and selectively heats while the lower microwave absorbing material absorbs small amounts of microwaves and minimally heats. The higher microwave absorbing material is thereby annealed onto the less absorbing material which is thermally isolated.

## Claims

### Claim 1

A method for selectively annealing a first material having a high microwave absorption located in close proximity to a second material with a top surface having low microwave absorption less than said high microwave absorption, comprising covering the entire top surface of said second material with said first material and exposing said first material to pulsed microwave radiation sufficient only to cause selective annealing of said first material and insufficient to cause localized heating by said microwave radiation of said second material.

### Claim 2 (depends on 1)

The method of claim 1 further comprising:
suspending the exposure of said microwave radiation whenever said first material reaches a selected maximum temperature.

### Claim 3 (depends on 2)

The method of claim 2 further comprising performing repetitive cycles of said exposing and suspending steps until said first material has annealed to a desired degree.

### Claim 4 (depends on 2)

The method of claim 2 wherein said suspending is carried on for a duration sufficient to equilibrate temperatures of said first and second materials.

### Claim 5 (depends on 2)

The method of claim 2 wherein said selected maximum temperature is sufficiently high for said first material to reflow by capillary driven surface diffusion.

### Claim 6 (depends on 5)

The method of claim 5 wherein said selected temperature is sufficiently low to minimize heat transfer from said first material to said second material.

### Claim 7 (depends on 6)

The method of claim 6 wherein said selected maximum temperature is on the order of no greater than about 500 degrees Celsius.

### Claim 8 (depends on 7)

The method of claim 7 wherein said first material is copper and said second material is silicon dioxide and said selected maximum temperature is on the order of approximately 300 degrees Celsius.

### Claim 9 (depends on 1)

The method of claim 1 wherein said pulsed microwave radiation has a pulse width and duty cycle providing a minimum heat transfer from said first material to said second material.

### Claim 10 (depends on 9)

The method of claim 9 wherein said pulse width and duty cycle are each on the order of approximately 1 second.

### Claim 11 (depends on 1)

The method of claim 1 wherein said exposing step comprises exposing said first material to a single mode microwave radiation field.

### Claim 12 (depends on 11)

The method of claim 11 wherein said exposing step comprises producing said single mode microwave radiation field in a high Q cavity while controlling a frequency of said microwave field to be a resonant mode of said cavity.

### Claim 13 (depends on 1)

The method of claim 1 further comprising recirculating a purified inert gas over said first material.

### Claim 14 (depends on 13)

The method of claim 13 wherein said gas comprises argon.

### Claim 15 (depends on 1)

The method of claim 1, wherein said top surface of said second material comprises patterned grooves and said first material is a physical vapor deposited metal film, and wherein said exposing step comprises placing said heterostructure in a microwave cavity with pulsed microwaves resonant with a mode of said cavity so that said metal film is heated by absorbing power from said microwaves sufficient to cause diffusion of said metal film within said patterned grooves, said pulsed microwaves having a characteristic pulse width and duty cycle tending to prevent localized heating of said lower microwave absorption material.

### Claim 16 (depends on 15)

The method of claim 15 further comprising interrupting exposure of said heterostructure to said pulsed microwaves whenever said metal film reaches a selected temperature for a sufficient amount of time to permit the heat in said metal layer to equilibrate with the remainder of said heterostructure.

### Claim 17 (depends on 16)

The method of claim 16 wherein said selected temperature is sufficiently high to cause reflowing by capillary driven surface diffusion of said metal layer and sufficiently low to minimize heat transfer from said metal layer to the remainder of said heterostructure.

### Claim 18 (depends on 15)

The method of claim 15 wherein said microwaves are resonant with only a single mode of said cavity.

### Claim 19

A method for processing a work piece by selectively annealing a first material having higher microwave absorption located in close proximity to a second material of said workpiece having lower microwave absorption inside a cavity and including a source producing microwaves, comprising the steps of:
covering an entire top surface of said second material with said first material;
selecting an appropriate resonant frequency of said cavity;
tracking continuously said appropriate resonant frequency of said cavity;
providing said cavity with a highly pure gas;
providing said cavity with microwaves sufficient only to cause annealing of said first material;
pulsing said microwaves from said cavity sufficient to prevent localized heating and limit absorption of microwave power by said second material.

### Claim 20 (depends on 19)

The invention as set forth in claim 19, wherein the providing said cavity with purified gas step further comprises the steps of:
providing argon gas to said cavity;
purifying said argon before said argon gas enters said cavity;
continuously recirculating said cavity with purified argon gas; and
purging argon gas from said cavity while purified argon gas is continuously provided to said cavity.

### Claim 21 (depends on 19)

The invention as set forth in claim 19, further comprising the step of changing said frequency in response to said tracking step so as to maximize power transfer from said microwave source to said higher absorption material.