News Column

Patent Application Titled "Method of Timing Laser Beam Pulses to Regulate Extreme Ultraviolet Light Dosing" Published Online

July 31, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Crouch, James (San Diego, CA); Jacques, Robert (San Diego, CA); Graham, Matthew R. (San Diego, CA); Liu, Andrew (San Diego, CA), filed on January 10, 2013, was made available online on July 17, 2014.

The assignee for this patent application is Cymer, Inc.

Reporters obtained the following quote from the background information supplied by the inventors: "The present invention relates generally to laser technology for photolithography, and more particularly to EUV dose control during laser firing.

"The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet ('EUV') light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.

"Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma ('LPP'), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site.

"The line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam simultaneously to a desired irradiation site (e.g., a primary focal spot) within an LPP EUV source plasma chamber for plasma initiation presents certain timing and control challenges. Specifically, it is necessary for the laser beam to be focused on a position through which the target material will pass and timed so as to intersect the target material when it passes through that position in order to hit the target properly to obtain a good plasma, and thus, good EUV light.

"A droplet generator holds the target material and extrudes the target material as droplets which travel along an x-axis of the primary focal spot to intersect the laser beam traveling along a z-axis of the primary focal spot. Ideally, the droplets are targeted to pass through the primary focal spot. When the laser beam hits the droplets at the primary focal spot, EUV light output is theoretically maximized. In reality, however, achieving maximal EUV output light across bursts over time is very difficult because energy generated by irradiation of one droplet varies randomly from energy generated by irradiation of another droplet.

"Thus, maximal EUV light output might sometimes--but not always--be realized. This variability in output is a problem for downstream utilization of the EUV light. For example, if variable EUV light is used downstream in a lithography scanner, wafers can be non-uniformly processed, with resultant diminution of quality control of dies cut from the wafers. Thus, a tradeoff of non-maximal EUV for greater reliability may be desirable.

"A stroboscopic pattern produces EUV in short exposures throughout exposure of a wafer die. Although this pattern of bursts can be beneficial for control of the EUV energy dose, what is needed is a method to generate--with greater reliability--acceptable levels of EUV energy output for downstream purposes--that is, to more accurately control an EUV energy dose."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "In one embodiment is provided a method of regulating a dose of energy produced during stroboscopic firing of an EUV light source configured to generate an energy dose target within one or more packet comprising: (a) setting by a laser controller a dose servo value for a current packet; (b) timing by the laser controller a trigger to pulse a laser beam to irradiate a droplet during the current packet; sensing by a sensor EUV energy generated by irradiation of the droplet; (d) accumulating by the laser controller the sensed EUV energy with EUV energy generated by irradiation of one or more preceding droplet during the current packet; (e) repeating steps (b), , and (d) when the accumulated EUV energy within the current packet is less than an adjusted dose target based on the energy dose target and an accumulated dose error; and (f) mistiming by the laser controller the trigger to pulse the laser beam to not irradiate another droplet during the current packet.

"In another embodiment is the method further comprising: (g) calculating by the laser controller a dose error for the current packet; (h) accumulating by the laser controller the dose error for the current packet with a dose error for one or more preceding packet; (i) calculating by the laser controller a new adjusted dose target for a next packet based on the energy dose target and the accumulated dose error; and (j) calculating by the laser controller a new dose servo value for the next packet.

"In still another embodiment a system for regulating a dose of energy produced during stroboscopic burst-firing of an EUV light source configured to generate an energy dose target within one or more packet comprising: a drive laser configured to pulse a laser beam when a trigger is received; a sensor configured to sense EUV energy generated by irradiation of a droplet; and a controller configured to: (a) set a dose servo value for a current packet; (b) time the trigger to pulse the laser beam to irradiate a droplet during the current packet; accumulate sensed EUV energy generated by irradiation of the droplet with EUV energy generated by irradiation of one or more preceding droplet during the current packet; (d) repeat steps (b) and when the accumulated EUV energy within the current packet is less than an adjusted dose target based on the energy dose target and an accumulated dose error; and (e) mistime the trigger to pulse the laser beam to not irradiate another droplet during the current packet.

"In yet another embodiment is the system wherein the controller is further configured to: (f) calculate a dose error for the current packet; (g) accumulate the dose error for the current packet with a dose error for one or more preceding packet; (h) calculate a new adjusted dose target for a next packet based on the energy dose target and the accumulated dose error; and (i) calculate a new dose servo value for the next packet.

"A method of regulating a dose of energy produced during continuous burst mode of an EUV light source comprising: (a) beginning a burst having a predetermined energy dose target; (b) timing by the laser controller a trigger to pulse a laser beam to irradiate a droplet during the burst; sensing EUV energy generated by the droplet; (d) calculating by the laser controller a current dose error for the droplet based on the sensed EUV energy and the energy dose target; (e) accumulating by the laser controller a burst error based on the current dose error and a running burst error calculated for one or more preceding droplet during the burst; (e) repeating steps (b)-(e) for a next droplet when the burst is not finished and the accumulated burst error does not meet or exceed a threshold burst error; (f) mistiming by the laser controller the trigger to pulse the laser beam to not irradiate the next droplet when the burst is not finished and the accumulated burst error meets or exceeds the threshold burst error; and (g) repeating steps -(g) until the burst is finished.

"A system for regulating a dose of energy produced during continuous burst firing of an EUV light source configured to generate an energy dose target comprising: a drive laser configured to pulse a laser beam when a trigger is received; a sensor configured to sense EUV energy generated by irradiation of a droplet; and a controller configure to: (a) time the trigger to pulse a laser beam to irradiate a droplet during the burst; (b) calculate a current dose error for the droplet based on the sensed EUV energy and the energy dose target; accumulate a burst error based on the current dose error and a running burst error calculated for one or more preceding droplet during the burst; (d) repeat steps (a)- for a next droplet when the burst is not finished and the accumulated burst error does not meet or exceed a threshold burst error; (e) mistime the trigger to pulse the laser beam to not irradiate the next droplet when the burst is not finished and the accumulated burst error meets or exceeds the threshold burst error; and (f) repeat steps (b)-(e) until the burst is finished.

BRIEF DESCRIPTION OF DRAWINGS

"FIG. 1 is a schematic illustrating some of the components of a typical LPP EUV system,

"FIG. 2 is a schematic illustrating laser puking to irradiate a droplet.

"FIG. 3 is a schematic illustrating mistimed laser pulsing to avoid irradiating a droplet.

"FIG. 4 is a graph of energy generated over time during periods of laser pulsing to irradiate droplets and during periods of mistimed laser pulsing to avoid irradiating droplets according to one embodiment.

"FIG. 5 is a block diagram depicting EUV system components involved in dose control of EUV light according to one embodiment.

"FIG. 6 is a flowchart of a method to control stroboscopic EUV dose by laser beam pulse timing according to one embodiment.

"FIG. 7 is a data plot showing percent variation around an energy dose target achieved over a 2-second burst using laser beam pulse timing to control EUV dose according to one embodiment.

"FIG. 8 shows packet EUV energy (upper panel) and pulse count (lower panel) generated over the 2-second burst using laser beam pulse timing to control EUV dose according to one embodiment.

"FIG. 9 is a flowchart of a method of timing laser beam pulses to control EUV dose during continuous burst firing according to one embodiment.

"FIG. 10 shows EUV energy (upper panel) and energy dose (lower panel) generated during continuous burst firing using laser beam pulse timing to control EUV dose according to one embodiment."

For more information, see this patent application: Crouch, James; Jacques, Robert; Graham, Matthew R.; Liu, Andrew. Method of Timing Laser Beam Pulses to Regulate Extreme Ultraviolet Light Dosing. Filed January 10, 2013 and posted July 17, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=5050&p=101&f=G&l=50&d=PG01&S1=20140710.PD.&OS=PD/20140710&RS=PD/20140710

Keywords for this news article include: Cymer, Cymer Inc.

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Source: Politics & Government Week


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