Lithography system with non-invasive monitoring and methods

1 . A method, comprising:

forming a mask layer on a semiconductor wafer;

forming a tin droplet, including:

supplying tin to a high-pressure reservoir from a low-pressure reservoir;

monitoring a level of tin in the high-pressure reservoir by at least two electrodes attached to the high-pressure reservoir;

in response to the level of the tin exceeding a threshold value, supplying the tin to a droplet generator from the high-pressure reservoir; and

forming the tin droplet by the droplet generator using the tin supplied from the high-pressure reservoir;

generating light by the tin droplet; and

patterning the mask layer by the light.

2 . The method of claim 1 , wherein the monitoring a level of tin includes:

measuring a capacitance value by an electric meter coupled to the at least two electrodes.

3 . The method of claim 2 , wherein the measuring a capacitance includes:

applying an alternating current (AC) signal to the at least two electrodes, the AC signal having frequency in a range of about 1 kilohertz to about 1 megahertz.

4 . The method of claim 2 , wherein the monitoring a level includes applying a signal to at least two electrodes that include:

a first electrode attached to a sidewall of the high-pressure reservoir at a first vertical level; and

a second electrode attached to the sidewall at a second vertical level offset from the first vertical level.

5 . The method of claim 4 , wherein the first and second electrodes are on opposing sides of the high-pressure reservoir along a horizontal direction.

6 . The method of claim 2 , wherein the monitoring a level includes applying a signal to at least two electrodes that include:

a first electrode that extends vertically along a sidewall of the high-pressure reservoir; and

a second electrode that extends horizontally along a base of the high-pressure reservoir.

7 . The method of claim 2 , wherein the monitoring a level includes applying a signal to at least two electrodes that include:

a first electrode attached to a base of the high-pressure reservoir; and

at least two second electrodes that extend vertically along a sidewall of the high-pressure reservoir.

8 . The method of claim 7 , wherein:

the first electrode includes an inner portion and at least two extension portions that extend outward from the inner portion; and

the at least two second electrodes are positioned in gaps between the at least two extension portions.

9 . The method of claim 2 , wherein the monitoring a level includes applying a signal to at least two electrodes that include:

a first electrode attached to a base of the high-pressure reservoir; and

a second electrode that wraps entirely around a sidewall of the high-pressure reservoir.

10 . A method, comprising:

forming a tin droplet, including:

monitoring a level of tin in a reservoir by at least two electrodes attached to the reservoir;

in response to the level of the tin exceeding a threshold value, supplying the tin to a droplet generator from the reservoir; and

forming the tin droplet by the droplet generator using the tin supplied from the reservoir; and

generating extreme ultraviolet (EUV) light by the tin droplet.

11 . The method of claim 10 , wherein the monitoring a level includes:

measuring a capacitance by an LCR meter connected to the at least two electrodes;

generating a digital capacitance by an analog-to-digital converter (ADC) connected to the LCR meter;

determining the level of the tin by a processor connected to the ADC; and

in response to the level of the tin being below the threshold value, generating a notification.

12 . The method of claim 11 , wherein the generating a notification includes generating an alarm.

13 . The method of claim 11 , wherein the generating a notification includes outputting real-time data associated with the level of the tin.

14 . The method of claim 13 , further comprising:

controlling supply of tin to the reservoir based on the real-time data.