# Patent

## Title
Measuring in-situ UV intensity in UV cure tool

## Abstract

Provided are improved apparatus and methods for radiative treatment. In some embodiments, a semiconductor processing apparatus for radiative cure includes a process chamber and a radiation assembly external to the process chamber. The radiation assembly transmits radiation into the chamber on a substrate holder through a chamber window. A radiation detector measures radiation intensity from time to time. The assembly includes a gas inlet and exhaust operable to flow a radiation-activatable cooling gas through the radiation assembly.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. patent application Ser. No. 12/008,149, filed Jan. 8, 2008, issued as U.S. Pat. No.7,935,940, which is incorporated by reference herein in its entirety.


FIELD OF THE INVENTION
This invention relates to apparatus and methods for radiation treatment, including radiation treatment of thin films.
BACKGROUND
Many layers of thin films are used to make an integrated circuit. IC manufacturing requires thin films to have certain properties in order for the circuit to function as designed. For example, there is a general need for materials with low dielectric constants (low-k). Using low-k materials as the intermetal dielectric (i.e., the layer of insulator separating consecutive levels of the conductive metal interconnects) reduces the delay in signal propagation due to capacitive effects, otherwise know as the RC delay. A dielectric material of low dielectric constant will have low capacitance, and hence the RC delay of an IC constructed with such a material will be lower as well.
As another example, there is a general need for materials with specific tensile or compressive stresses. Increasing shallow trench isolation (STI) film tensile stress increases transistor drain current and device performance because the electron and hole mobilities are higher. Other applications require dielectric films to have compressive stress. These and other properties may be met on the film as deposited, or after treatment.
One such treatment may be a thermal process in which the substrate is heated to a temperature for a time. A thermal treatment may remove unwanted particles from the film, or change its stresses and other properties. These thermal processes, however, have certain difficulties. In particular, substrate temperatures generally need to be high (i.e., greater than about 500 degrees Celsius) with exposure times typically on the order of hours. As is well known in the field, these conditions can damage copper containing devices, especially in the application wherein a low-k dielectric is being cured and the long exposure time may be unsuitable for mass manufacturing. Also, the use of temperature sensitive nickel silicide precludes inducing film stress by using temperatures above 400° C. while some SiN films have a cure temperature up to 480° C.
To overcome these disadvantages of thermal processing, another technique has been developed, which involves curing the film with UV radiation. Irradiation of the low-k or spacer nitride films permits modulation of desired film properties such as dielectric constant or film stress at lower temperatures. However, the use of UV radiation in such processes may result in deleterious side-effects unless special care is taken to deliver the precise amount of radiation and to maintain the substrate temperature at an optimal level. Delivery of the precise amount of radiation may be complicated when UV lamp intensity drifts over time and UV exposure results affect optical parts.
In other applications, radiation treatment may be performed using electromagnetic radiation from any part of the electromagnetic spectrum, including infra-red (IR), x-ray, and microwave radiation.
SUMMARY
Provided are improved apparatus and methods for radiative treatment. In some embodiments, a semiconductor processing apparatus for radiative cure includes a process chamber and a radiation assembly external to the process chamber. The radiation assembly transmits radiation into the chamber on a substrate holder through a chamber window. A radiation detector measures radiation intensity from time to time. The assembly includes a gas inlet and exhaust operable to flow a radiation-activatable cooling gas through the radiation assembly.
In one aspect, a semiconductor processing apparatus for UV cure includes a process chamber and a UV radiation assembly external to the process chamber. The UV radiation assembly transmits radiation into the chamber on a substrate holder through a chamber window. A UV detector measures UV intensity from time to time. The assembly includes a gas inlet and exhaust operable to flow a UV-activatable cooling gas through the radiation assembly.
In another aspect, the present invention pertains to a UV apparatus including a process chamber and a UV radiation assembly mounted on the process chamber. The UV radiation assembly includes one or more UV radiation sources, one or more reflectors operable to direct a portion of the UV radiation through the window towards the substrate holder, and a UV intensity detector. This detector is mounted below the UV lamps and external to the chamber. It is oriented toward the window and detects reflected light on a calibration substrate or a mirror placed above the window. A cover, e.g., a shutter or an iris, protects the detector from UV radiation when not in use. The UV intensity detector is able to withstand high UV energy. The UV detector may be a silicon carbide (SiC) photodiode or photodetector. It may also include a diffuser. The UV detector may also include a gas inlet and exhaust operable to flow a UV-activatable cooling gas through the radiation assembly.
The mirror is oriented toward the detector and reflects UV radiation toward the detector. It may be detachable from the UV radiation assembly or may be configured to not interfere with UV delivery to the chamber during substrate processing. The mirror may be coated with a high temperature UV coating or a metal coating, such as aluminum coating. The calibration substrate reflects UV light in the wavelengths of interest. The substrate may be a bare silicon wafer, a wafer coated with high temperature UV coating or another appropriate coating such as aluminum.
The apparatus may also include a controller configured to execute a set of instructions. The controller compensates for measured intensity by executing instructions to measure a UV intensity at a UV radiation power, calculate a deviation based on the measurement and a baseline intensity, and adjust the UV radiation power or exposure duration to compensate for the deviation. The controller may also direct the cover to move, by executing instruction to open the cover, e.g., shutter or iris, to expose a UV detector to the UV radiation and close a shutter or iris to isolate the UV detector from the UV radiation.
In another aspect, the present invention pertains to methods of calibrating a UV apparatus. The method may be simple compensation of measured UV intensity, issuance of alarm or a warning, or determination of maintenance or corrective action. The UV apparatus calibration may be performed while the pedestal remains hot and the chamber under vacuum, i.e., without shutting down or removing the system or chamber from service. The method includes receiving a calibration indication or determining that a calibration is required; positioning a calibration substrate on a substrate holder; setting a UV radiation power to a first power; measuring a UV detector output over a duration; and, removing the calibration substrate.
The calibration indication may be triggered in software by duration of exposure, total intensity, time lapse, number of wafers processed, etc or be triggered manually by an operator. A calibration substrate is provided to the chamber on a substrate holder. This calibration substrate may be kept in a front opening unified pod (FOUP) in the plant's substrate warehouse system or in a storage locker. The calibration substrate may be a bare silicon wafer with certain UV reflectivity or be coated. Appropriate coatings are those that can withstand periodic UV exposure with minimal change in UV reflectivity. Appropriate coatings may include high temperature UV coating and deposited metal, such as aluminum.
After the calibration substrate is positioned on the substrate holder, it is exposed to UV radiation at a first power, which may be chosen by the operator. The first power may be at 50%, 75% or 100% power. The UV radiation reflects off the calibration substrate back through the window and is measured by the UV detector over a duration of time. At the end of the measurement duration, the UV power may be turned off, the UV detector may be blocked from radiation, or the signal measurement may simply stop. The calibration substrate may be removed after the measurement duration.
The method may also include operations to open and close a cover that protects the UV detector from effects of continuous exposure to the UV radiation during normal processing. This cover may be actuated remotely through a controller to an open or a close position.
The method may also include calculating a UV intensity based on a the UV detector output measured and calculating a deviation based on the calculated UV intensity and a baseline intensity, and adjusting the UV radiation power or exposure time to compensate for the deviation. UV radiation generated by a lamp first passes through the chamber window to the substrate, a portion of the radiation reflects off the substrate then back through the window before it is detected by the UV detector. Thus the radiation intensity detected at the detector is not the same as that experienced by the substrate. A controller may be programmed to take into account the reflectivity of the substrate at various UV wavelengths to arrive at a UV intensity. This intensity only needs to be comparable from test to test. It may be a normalized value based on some maximum, it may be an absolute value in watts per centimeter squared (W/cm2). In some cases, the raw detector output may be used as indication of intensity. This measured UV intensity is compared to a desired value and corrective action taken to compensate for a deviation. For example, if the measured UV intensity is lower than desired, than UV power may be increased or exposure time may be increased if power cannot be further increased. If the measured UV intensity is more than desired, than UV power and exposure time may be correspondingly adjusted.
In certain embodiments, the method may also include positioning a mirror between a process chamber and the UV detector, measuring the reflected UV radiation, and determining whether the process window between the mirror and the substrate requires a cleaning. The difference between reflected UV radiation from the mirror and from the substrate is the path. The reflected UV radiation from the substrate passes through the chamber window twice, whereas the UV radiation from the window does not. Knowing the reflectivity of the mirror and the substrate, the controller calculates a difference that is attributable to the window. As discussed some UV curing results in gaseous byproducts that can deposit on the window and obscure it. If the difference attributable to the window is greater than a predetermined amount, then a signal to clean the window, either automatically through the system control or manually through an alarm or warning to the operator, is generated.
Other required maintenance or corrective activity may be determined. This determination may be in the form of an alarm or warning to the operator, or even logs associated with a batch of wafers processed. Particularly, the activity may be to replace UV lamps if the measured UV intensity too low to be compensated by lengthened process time. In addition to cleaning the window, the chamber itself may require cleaning or the detector may need to be replaced.
These and other features and advantages of the invention will be described in more detail below with reference to the associated drawings.
The apparatus may also include a controller configured to execute a set of instructions. The controller compensates for measured intensity by executing instructions to measure a UV intensity at a UV radiation power, calculate a deviation based on the measurement and a baseline intensity, and adjust the UV radiation power or exposure duration to compensate for the deviation. The controller may also direct the cover to move, by executing instruction to open the cover, e.g., shutter or iris, to expose a UV detector to the UV radiation and close a shutter or iris to isolate the UV detector from the UV radiation.
In another aspect, the present invention pertains to methods of calibrating a UV apparatus. The method may be simple compensation of measured UV intensity, issuance of alarm or a warning, or determination of maintenance or corrective action. The UV apparatus calibration may be performed while the pedestal remains hot and the chamber under vacuum, i.e., without shutting down or removing the system or chamber from service. The method includes receiving a calibration indication or determining that a calibration is required; positioning a calibration substrate on a substrate holder; setting a UV radiation power to a first power; measuring a UV detector output over a duration; and, removing the calibration substrate.
The calibration indication may be triggered in software by duration of exposure, total intensity, time lapse, number of wafers processed, etc or be triggered manually by an operator. A calibration substrate is provided to the chamber on a substrate holder. This calibration substrate may be kept in a front opening unified pod (FOUP) in the plant's substrate warehouse system or in a storage locker. The calibration substrate may be a bare silicon wafer with certain UV reflectivity or be coated. Appropriate coatings are those that can withstand periodic UV exposure with minimal change in UV reflectivity. Appropriate coatings may include high temperature UV coating and deposited metal, such as aluminum.
After the calibration substrate is positioned on the substrate holder, it is exposed to UV radiation at a first power, which may be chosen by the operator. The first power may be at 50%, 75% or 100% power. The UV radiation reflects off the calibration substrate back through the window and is measured by the UV detector over a duration of time. At the end of the measurement duration, the UV power may be turned off, the UV detector may be blocked from radiation, or the signal measurement may simply stop. The calibration substrate may be removed after the measurement duration.
The method may also include operations to open and close a cover that protects the UV detector from effects of continuous exposure to the UV radiation during normal processing. This cover may be actuated remotely through a controller to an open or a close position.
The method may also include calculating a UV intensity based on a the UV detector output measured and calculating a deviation based on the calculated UV intensity and a baseline intensity, and adjusting the UV radiation power or exposure time to compensate for the deviation. UV radiation generated by a lamp first passes through the chamber window to the substrate, a portion of the radiation reflects off the substrate then back through the window before it is detected by the UV detector. Thus the radiation intensity detected at the detector is not the same as that experienced by the substrate. A controller may be programmed to take into account the reflectivity of the substrate at various UV wavelengths to arrive at a UV intensity. This intensity only needs to be comparable from test to test. It may be a normalized value based on some maximum, it may be an absolute value in watts per centimeter squared (W/cm2). In some cases, the raw detector output may be used as indication of intensity. This measured UV intensity is compared to a desired value and corrective action taken to compensate for a deviation. For example, if the measured UV intensity is lower than desired, than UV power may be increased or exposure time may be increased if power cannot be further increased. If the measured UV intensity is more than desired, than UV power and exposure time may be correspondingly adjusted.
In certain embodiments, the method may also include positioning a mirror between a process chamber and the UV detector, measuring the reflected UV radiation, and determining whether the process window between the mirror and the substrate requires a cleaning. The difference between reflected UV radiation from the mirror and from the substrate is the path. The reflected UV radiation from the substrate passes through the chamber window twice, whereas the UV radiation from the window does not. Knowing the reflectivity of the mirror and the substrate, the controller calculates a difference that is attributable to the window. As discussed some UV curing results in gaseous byproducts that can deposit on the window and obscure it. If the difference attributable to the window is greater than a predetermined amount, then a signal to clean the window, either automatically through the system control or manually through an alarm or warning to the operator, is generated.
Other required maintenance or corrective activity may be determined. This determination may be in the form of an alarm or warning to the operator, or even logs associated with a batch of wafers processed. Particularly, the activity may be to replace UV lamps if the measured UV intensity too low to be compensated by lengthened process time. In addition to cleaning the window, the chamber itself may require cleaning or the detector may need to be replaced.
These and other features and advantages of the invention will be described in more detail below with reference to the associated drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1D are schematics of UV lamp assembly with in-situ UV detector assembly and mirror in various positions.
FIGS. 2A and 2B are process flow diagrams depicting a process in accordance with certain embodiments of the present invention.
FIGS. 3A-3D are schematics of UV detector assemblies in accordance with certain embodiments of the present invention
FIG. 4 is a cross-section schematic of a UV lamp assembly and chamber in accordance with certain embodiments of the present invention.
FIGS. 5A and 5B are schematics of a multi-station UV cure chamber in accordance with certain embodiments of the present invention.
FIG. 6 is a graph showing detected UV signal % over the course of usage of a lamp, comparing a clean N2-only coolant gas and an ozone-generating coolant gas.



DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Introduction
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail to not unnecessarily obscure the present invention. While the invention will be described in conjunction with the specific embodiments, it will be understood that it is not intended to limit the invention to the embodiments.
Reference will be made in detail to implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. In this application, the terms “work piece,” “wafer” and “substrate” will be used interchangeably. The following detailed description assumes the invention is implemented on a wafer. However, the invention is not so limited. The work piece may be of various shapes, sizes, and materials (e.g., displays of various sizes).
The present invention involves an ultraviolet (UV) irradiation apparatus for processing semiconductor substrates. UV radiation is used to treat thin films on substrates to achieve various property changes. UV radiation can break chemical bonds, change composition of material, cause chemical reactions to occur, affect density and stress, and otherwise provide energy to a substrate.
A typical implementation of UV irradiation apparatus has a number of UV lamps to illuminate a substrate. The UV lamps may be flood lamps, generating UV light in a broad range of wavelengths, or a high intensity discharge light that emits UV radiation in a smaller spectrum. Monochromatic sources of UV radiation, such as certain lasers, may also be used. Lamps may be powered through excitation by electrodes or microwave energy and may be inductively coupled.
In certain implementations, two individual linear mercury bulbs are used in two individual lamp assemblies. With a two-bulb configuration, the relative orientation of the bulbs to the circular substrate is varied over time to achieve a uniform exposure for each circular section. Various reflectors are also used in lamp assemblies to direct generated UV radiation toward the substrate. Linear bulbs generate light in all directions, but the substrate surface is only on one side of the bulb. Thus reflectors are used to direct UV radiation that would have otherwise escape from the lamp assembly toward the substrate. This way, lamp power is used more efficiently. In other implementations, the bulbs may be of different shapes, e.g., light bulb shape or toroidal.
In order to achieve the various film property changes on a substrate, the amount of curing that occurs as result of exposure to UV radiation must correspond to the amount of property changed desired. Thus for some property changes, a small amount of cure is required and for others, a larger amount of cure is required. The amount of UV cure is a function of UV radiation intensity, substrate temperature, and in some cases pressure and the gas present in the chamber. To ensure that the amount of film property change is constant from one substrate to another, the amount of cure must be controlled over time to ensure wafer to wafer uniformity.
Reliable methods are known to control substrate temperature, chamber pressure, and the gas present in the chamber. The substrate may be heated or cooled during the UV exposure. This heating or cooling may occur by controlling the temperature of the pedestal on which the substrate is positioned or by adding gas into the chamber at a certain temperature. Chamber pressure is controlled through vacuum pumps or common vacuum forelines and pressure gauges. The gas in the chamber may be controlled by first evacuating the chamber then adding whichever gas is desired. During some UV treatments, the gas in the chamber may actively participate in the changing of film properties by reacting with the film.
UV radiation intensity is also controlled. UV radiation intensity as experienced by the substrate depends on a number of factors. The power applied to the UV lamps determines how much radiation is generated at the source. The condition of the UV lamps determines how effective the power is used to generate UV radiation. The type of cooling gas in the UV lamp assembly determines how much radiation is absorbed. The design of the reflectors and the reflectivity determines how much radiation is directed through the chamber window. The type of material used for the chamber window and its cleanliness determines how much of the radiation directed at the window passes through. Finally the radiation reaches the substrate at a certain intensity.
The contribution of these factors changes over time in part because various parts of the UV lamp assembly and the chamber may become solarized over time. Solarization refers to a phenomenon where a material undergoes a change after being subjected to high energy electromagnetic radiation, such as ultraviolet light. Clear glass and many plastics will turn blue and/or degrade after long term solar exposure in the desert. It is believed that solarization is caused by the formation of internal defects, called color centers, which selectively absorb portions of the visible light spectrum. Solarization may also permanently degrade a material's physical or mechanical properties, and is one of the mechanisms involved in the breakdown of plastics within the environment. Prolonged exposure to high intensity UV light such as that used in UV cure damages chamber parts, especially those made from organic polymers and affects reflectivity over time. Some materials may be more resistant to UV-induced damage, such as Teflon, PTFE and PFA
Two major factors that affect UV intensity over time at the substrate are the condition of UV bulbs and the chamber window. UV lamp bulbs used for curing are subjected to high powers, e.g., for intensities up to 1 W/cm^2, and sustain high cycling. The bulb performance degrades over time. Given the same power input, the output intensity reduces over time. In order to maintain the same intensity output, bulbs may be subjected initially to a lower power that is slowly increased over time to compensate for the decrease in performance. At some point, the bulbs are replaced. The bulbs may be replaced on certain triggering events regardless of their performance. These events may be a number of cycles, total cure time, total time, the number of substrates cured, total power, etc. The bulbs may also be replaced based on performance, for example, if the bulb does not output an expected intensity. Efficient use of bulbs and system downtime, e.g., for bulb replacement, suggests that a performance based method is preferred. However, for planning purposes, having periodic replacements that are designed to be shorter than the useful lifetimes of the bulbs ensure functioning UV bulbs and reduces the number of unscheduled downtimes.
The chamber window also affects the UV intensity at the substrate. Some UV processes involve evolution of solvents or reactions that emit gaseous by-products. One example is porogen removal. Porogen is removed from as-deposited ultra low-k (ULK) layers with UV radiation and the gaseous by-product removed from the chamber. Sometimes the by-products, usually organic contaminants, deposit on chamber parts before they can be removed. If deposited on the chamber window, they can build up over time and obscure the window to reduce the UV radiation passing through. Efforts to reduce these unwanted deposits, e.g., inert gas curtain below the window or higher purge gas flows, have not eradicated them. Thus, even if the UV bulbs are performing well, UV intensity at the substrate may still be reduced.
One attempt to address wafer to wafer cure uniformity is to measure the UV intensity at the substrate position. A lightpipe embedded in the pedestal or substrate support can measure the UV radiation from time to time. A light pipe collects radiation at the tip, usually made of sapphire, and transmits it through optical fibers to a photodetector. The change in the lightpipe measurement over time approximates that experienced by the substrate. However, the gaseous contaminants can deposit on the lightpipe and must be constantly and consistently cleaned. The constant and high powered UV exposure can solarize the lightpipe material and change its sensitivity. Embedded in the pedestal, the lightpipe is subjected to thermal cycles with the heated pedestal, which changes the transmitted value. The heated pedestal also emits infrared radiation which interferes with the light measured. Finally, the optical fiber connecting from the lightpipe has also been found solarized, which degraded the signal.
The lightpipe is periodically recalibrated as its measurements change over time. The lightpipe measures a radiance temperature that is affected by the chamber environment. Changes in the environment and the lightpipe over time can affect interpretation of its measurement unless it is periodically calibrated. As discussed above, knowing the UV intensity at the substrate position may not resolve the cause—a low UV intensity may mean bad bulb performance or dirty window or both. In one aspect, the present invention allows a periodic calibration of the lightpipe that is unaffected by the conditions inside the chamber.
In another aspect, the present invention is a method of calibrating a UV apparatus. The measurements from the in-situ UV detector of the present invention may be used to calibrate the UV apparatus independently of the lightpipe. The in-situ detection and detector assembly apparatus avoid many of the problems of the lightpipe. The in-situ detector is mounted outside of the chamber, thus it is immune to any gaseous contaminants depositing on the detector. In certain embodiments, it is protected from solarization effects by a shutter mechanism that only opens during measurement. It is protected from thermal cycling by nitrogen purge gas and shielded from pedestal-emitted infrared radiation emitted by the calibration substrate. In some embodiments, the measured value can be used to determine the exact remedial/maintenance action required, because the effects of a dirty window can be isolated. Reflecting UV radiation to an outside UV detector is advantageous because it can be done under normal UV cure operating conditions. A calibration substrate can withstand substrate support temperatures of 400° C. or greater and can be inserted automatically using system controls. A calibration can immediately follow UV processing, without waiting for the pedestal to cool or chamber to be opened. Since chamber vacuum and temperature are maintained, time required to return the chamber to service is also reduced.
FIG. 1A shows a cross section schematic of one implementation of two lamp assemblies using an embodiment of the present invention. The schematic of FIG. 1A shows a configuration while a UV measurement takes place. Lamp assemblies are mounted over a process chamber 112 over window 108. Linear bulbs 102 generate UV radiation in all directions. Some of the generated radiation directly irradiates the calibration substrate 110 which is supported by substrate support 114. Some of the radiation is reflected off various reflectors before irradiating the substrate. Overhead reflectors 104 surround the bulb and redirect radiation toward the substrate. The ring reflector 106 surrounds the space above the process chamber window 108. A UV detector 122 is positioned inside a detector assembly 120. As shown in FIG. 1A, light ray 116 from the bulb 102 is reflected off the calibration substrate 110 to the UV detector 120. A light ray may also be reflected first from a reflector before reaching the substrate, such as light ray 118. Light rays 116 and 118 are both detected by detector 122. Note that before reaching the detector, the light passes through the window 108 twice.
FIG. 1B shows a top view of the same lamp assembly as FIG. 1A. Lamp assembly housings 124 enclose tubular bulbs 102 and overhead reflectors 104 and are rectangular in shape. They are mounted over ring reflector 106 which sits over the process chamber 112. The detector 122 is situated in between the lamp assembly housings and below the bulbs.
FIG. 1C shows the detector assembly 120 in the shut position. Just like in FIG. 1A, UV light reflected from the calibration substrate 110 reaches the detector assembly, but note that the light does not reach the detector 122. The detector assembly 120 has a closed cover that protects the UV detector from reflected radiation during, for example, UV curing of substrates. As required when a measurement takes place, the cover can then open to expose the detector to the UV radiation.
In FIG. 1D, a mirror 126 is positioned on top of the process window to take another measurement. For this measurement, only UV light reflected from the mirror is measured, for example, light ray 124. This measurement removes the effect of window cleanliness from the result because light ray 124 does not pass through the window 108 at all before being detected. As an example, light ray 128 is prevented from reaching the detector because of the mirror 108 in its path. Thus the pass-through rate of the window is removed from this measurement.
Results from the measurement of FIG. 1A over time correspond to the UV intensity as seen by the calibration substrate. However one must note that this intensity differs from that of the light pipe described above because one more pass through the window is included. Results from the measurement of FIG. 1D over time correspond to a condition of the UV bulbs 102. Because the effects of the window deposits are removed, this measurement gives a good indication whether the bulbs perform as expected. The difference in deviation over time between, say, time A and B for the measurement of FIG. 1A and time A and B for the measurement of FIG. 1D indicates how much the chamber window reduces UV transmission. For example, if the measurement from FIG. 1A shows a 75% decrease over a time period, but the measurement from FIG. 1D shows only a 90% decrease over the same time period, then the additional 15% must come from passing through the window. A trigger can then be set below which the window must be cleaned.
Process
In one aspect, the present invention pertains to a method to calibrate a semiconductor UV cure apparatus. FIGS. 2A and 2B are process flow diagrams showing certain embodiments of the present invention. The calibration usually occurs after a triggering event. In operation 201, a calibration indication is received or that a calibration requirement is determined. A calibration indication may be received from an operator or the system from a downstream process that determined an unacceptable wafer-to-wafer uniformity. An indication from an operator may be initiated as part of preventive maintenance or as part of trouble-shooting to resolve UV intensity loss.
Alternatively, a calibration requirement may be determined, such as the control system determining that a triggering event has occurred. A triggering event may be based on accumulation of a consumption-based variable, a performance-based measurement such as UV intensity from a light pipe, or an absolute number of time or substrates processed. A variable indicating consumption of UV bulbs may be total power. UV bulbs may have life expectancies measured in terms of total power. An example of an accumulation of a variable event may be total power consumption from the lamp assemblies. The total power consumed may occur over many or a few substrates processed, over a long duration or very short amount of time. This kind of indication mirrors consumption of consumable life, but is harder to keep track of or calculate than other kinds of indications. Another kind of indication is a measurement that indicates performance. In this case the measurement may be UV intensity from a lightpipe. A change in lightpipe measurement within a few substrates processed or within a few hours may indicate a change in performance of the UV bulbs or the window transmission. As discussed above, other variables affect the lightpipe measurement so it alone cannot be used to determine remedial action. The last kind of indication may be passage of time or number of substrates processed. This kind of the indication is not directly related to the consumption of life expectancy, but may be used instead when a consumption-based variable is hard to measure or calculate. To illustrate, a calibration trigger may occur every 8 hours regardless whether any substrates were processed. A calibration trigger may also occur after every 100 substrates processed regardless of whether the processing was for 15 minutes or for 15 seconds.
The calibration is initiated by positioning a calibration substrate on a substrate holder, in operation 203. A calibration substrate reflects UV radiation into the detector during measurement. The reflectivity of a substrate at various UV wavelengths is known and changes little over time. The UV detector measurement from the same substrate can be compared and difference in UV intensities determined. The substrate may be a bare silicon wafer or a coated silicon wafer. The coating may be high temperature UV coating that is resistant to UV radiation and high temperatures, for example, metal or metal oxide. Dielectric coatings applied with ION beam assist may be able to withstand a 400° C. operating temperature and may be suitable for this application.
In one embodiment, more than one calibration substrates having different reflectivities in the UV wavelengths of interest are used. The intensity measured from two substrates may be compared to isolate the intensity measurement at the UV wavelength or wavelengths of interest. In a simple example, two otherwise identical substrates may be made, but one substrate may have a dielectric coating designed to absorb UV radiation at 250 nm (0 reflectivity). By comparing the intensity measured, the UV intensity at 250 nm may be isolated. This technique may be expanded to measure UV intensity at more than one wavelength of interest and may even involve more than two calibration substrates. In some UV cure processes, the desired reaction occurs with radiation at particular wavelengths. For those processes, knowing the intensity at a particular wavelength may be important.
When not in use, the calibration substrate may be kept in the semiconductor processing system or outside of the system. For example, the substrate may be stored in a front opening unified pod (FOUP) close to the semiconductor processing apparatus or in the fabrication plant's wafer storage system. Additionally, the apparatus may include a slot for storing the calibration substrate that is accessible to the system robot arms.
The UV radiation power is set to a first power value in operation 205. This first power may be 100% power. Preferably, the UV intensity measurements are taken at the same power value for ease of comparison. The UV lamp may be on or off when the calibration substrate is positioned, and in some cases the UV lamp may be already set to the first power value.
In certain embodiments, the UV detector is protected by an assembly that opens and closes a cover to sequentially allow UV detection and to isolate the detector from UV radiation. This cover may be a shutter or an iris. In optional operation 207, a shutter is opened to expose the UV detector to UV radiation. During the time the cover or shutter is open, the UV detector measures the radiation and provides an output, in operation 209. This output is read or measured over a duration of time. Once the measurement is complete, the shutter may be closed to isolate the UV detector from the UV radiation, in operation 211.
Operations 203 to 211 may be repeated during the calibration. As disclosed above, more than one calibration substrate may be used, thus requiring a repeat of the measurement operations. The measurement may also be repeated using a mirror between the UV detector and chamber window, as further explained below in operation 219 and 221.
FIG. 2A is continued on FIG. 2B. In operation 213, a UV intensity based on the detector output measured in operation 209 is calculated. The UV detector output may be a current or a voltage. The current or voltage is converted to an equivalent UV intensity, which may be watts per square centimeter or just a percentage of maximum. In operation 215, a deviation between the calculated UV intensity and a baseline intensity is calculated. The baseline intensity is the expected value for the power level. Based on the deviation, an adjustment of the UV cure process parameters may occur. If the deviation is very small or none, no adjustment needs to be made. However, if the deviation is in a range where adjusting the UV radiation power or exposure time may efficiently compensate for the deviation, then the adjustment may be made in operation 217. For example, if the measured intensity is higher than the expected baseline intensity, then the UV radiation power may be reduced to affect the same UV intensity. More commonly, the measured intensity is lower than the expected baseline intensity. The preferred adjustment may be to increase the UV radiation power, if the power is not already at the maximum. If the power is already at the maximum, then the exposure time may be increased to compensate for the deviation. Note that increasing exposure time decreases system throughput, which is undesirable. Thus this adjustment may be made deliberately if the resulting throughput is within desirable limits.
Optional measurements using a mirror to isolate the effects of chamber window deposits may be made before, at the same time or after the calculation operations and the calibration substrate measurement. A mirror may be positioned between a process chamber and a UV detector, in operation 219. In some embodiments, this mirror positioning may be performed mechanically by using a robot arm or a solenoid. In other embodiments, an operator may perform this mirror positioning while the UV bulb is off. For example, a door on the side of the lamp assembly may be opened and a mirror inserted into a slot. The mirror is highly reflective of UV light. In certain embodiments, it may be a portion of a calibration substrate that is the same as one used inside the chamber. It other embodiments, it may be a piece of metal or glass coated with reflective material.
In operation 221, a UV intensity measurement is taken to measure reflected UV radiation from the mirror. This operation may be performed by repeating operations 205 to 211. During the measurement, whether the calibration substrate is on the substrate holder is not important because light reflected from the substrate is not measured. After the UV intensity is measured, the mirror may be removed in operation 223. Similar to the positioning operation, the mirror may be removed mechanically or by operator and automatically or manually.
As disclosed above, the UV intensity reflected from the mirror may be used to determine a cleanliness of the process window. From the window cleanliness and the deviation calculated in operation 215, a number of maintenance activities may be performed. In operation 225, the activity to be performed is determined. For example, if the window cleanliness accounts for most of the deviation, then the window is cleaned, either through a remote plasma clean or dissembling the chamber and manually cleaning the inside. In another example, if the deviation is not caused by window cleanliness, then the UV bulbs may be replaced. Because dissembling the chamber requires shutting down the entire system and takes a lot of time, it is avoided if possible. Knowing the deviation and whether the window is dirty allows the operator or the system to determine the best maintenance activity that minimizes downtime.
Other operations may be performed in addition to those of FIGS. 2A and 2B. For example, a comparison of UV intensity measured with different calibration substrates, as discussed above, may be done to resolve UV intensity at particular wavelengths that is important to the process. Other ways to resolve UV intensity at particular wavelengths may include using wavelength specific detector and adding a filter between the detector and the window. One skilled in the art would be able to design various ways to measure UV intensity overall, at different bands, or at specific wavelengths using the concepts disclosed herein.
In some embodiments, methods and assemblies that prevent or reduce sensor contamination are provided. Contamination can accumulate on the face of a detector and/or window and obscure UV light. This causes a downward drift of the detected UV signal. The drift is an artifact of the sensor obscuration and not indicative of the UV light that reaches the sensor face. Contamination can be caused by UV-induced decomposition of organic impurities in, for example, an inert gas flow and/or outgassing from outgassing from potential contaminated surfaces in the proximity of the detector. UV light decomposes organic compounds exposed to it, causing hydrocarbon outgassing and condensation on optical surfaces of the detector. The accumulated condensed hydrocarbons absorb UV light, causing the downward drift of the detected signal.
Methods include flowing a gas mixture include molecular oxygen (O2) over the optical surfaces of the detector. For example, in certain embodiments, clean dry air (CDA) available from a facility source is supplied in a mixture with one more inert gases such as helium (He), argon (Ar) or nitrogen (N2). The molecular oxygen reacts in the presence of short wavelength (around 160-250 nm) UV to form ozone (O3). The ozone reacts with the organic contaminants on the optical surfaces, forming gaseous products that can be purged. In some embodiments, a constant flow of a coolant gas including CDO or other O2-containing gas is used through operation. In some embodiments, the coolant gas consists essentially of molecular oxygen and one or more inert gasses. FIG. 6 is a graph showing detected UV signal % over the course of usage of a lamp, comparing a clean N2-only coolant gas (data series 603) and an ozone-generating coolant gas (data series 601). The clean N2 flow contains some amount of organic contaminants as evidence by the drift in signal. The ozone purge shows no drift. The same UV light that causes the contamination works to constantly clean the contamination and eliminate its artifact.
The amount of molecular in the gas mixture is controlled to control the amount of ozone generated. If too much ozone is present in the detector assembly, it can absorb UV, providing inaccurate measurements of the UV intensity. It may also damage organic material in the vicinity of the detector (such as cables and printed circuit boards). In certain embodiments, the coolant gas is actively pumped out of the detector assembly to control the amount of ozone in the assembly. The addition of an inert gas (such as N2) may be used to dilute the O2-containing gas to reduce the total amount of ozone. In addition to the amount of O2-containing gas, the amount of ozone present is also affected by the removal rate of the exhausting hardware. Example flows are provided below, using ozone exhausting hardware capable of exhausting 40-50 SLM.












Total N2 and CDA
CDA Flow Contribution



Combined Flow (SLM)
(SLM)
O3 Level (ppm)

















150
50
=>22


100
100
=>22


50
50
  6









In some embodiments, an ozone level between about 5 and 15 ppm ozone is used. In some embodiments, O2 is about 5%-20% of the total gas flow. Accordingly, if air is used as the O2-containing gas, the air flow contribution between about 20% to 100% of the total gas flow. Lower O2 contributions (e.g., 1%-5%) may be used if necessary to lower the ozone level for the reasons described above.
Apparatus
FIGS. 3A and 3B shows more detail in one implementation of the detector assembly from different views. In FIG. 3A, the UV detector 301 is located inside the assembly 300. The detector 301 faces an opening or slot 305 through which reflected UV radiation may enter the assembly. A cover 303 may be moved to cover the opening or slot 305 to isolate the UV detector 301 from reflected UV radiation. During UV lamp operation, inert gases such as argon or nitrogen, at about 1-10 slm, flow into the assembly through inlet 307 to cool the assembly and to remove any particles that result from the moving cover. The inert gas contains nitrogen in some implementations, which may be at room temperature or cooled. The gas exits the assembly below the cover. As described further below, in some embodiments, an oxygen-containing gas is flown through the assembly in addition to or instead of the inert gas. For example, a mixture of nitrogen and clean dry air (CDA) may be flown through the assembly.
FIG. 3B shows the assembly of FIG. 3A in a side perspective. The UV detector 301 show also includes a diffuser 311 installed on its face. The diffuser integrates light from different directions and is known in the art. Inert gas injected from inlet 307 exits the assembly below the cover 303. As shown, a small gap exists between the cover and the bottom of the UV detector assembly.
In the embodiment shown on FIG. 3B, a solenoid 309 rotates the cover 303 in place to cover the opening 305 for isolating the detector 301 or out and away to expose the detector 301 to reflected UV radiation. In other embodiments, other mechanical means may be used to effectuate the covering and uncovering of the opening. For example, the cover or shutter may be moved by a lead screw or a stepper motor. The cover also may be an iris with multiple pieces that rotate at the same time. A suitable mechanism is remotely controllable either electronically or mechanically from outside of the lamp assembly.
FIG. 3C is a schematic of another implementation of a detector assembly. Detector 301 faces an opening or slot 305 through which UV radiation may enter the assembly. Gases enter at inlet 307 and flow over optical surfaces 315a, 315b and 317, exiting at outlet 319. Optical surfaces may be lenses, windows, filters and the like. An exhaust pump 321 connected to outlet 319 pumps the gases out. In some embodiments, multiple inlets and/or outlets may be arranged to establish a flow path over any optical surface or other surface on which contamination may occur. FIG. 3D is a schematic of a light pipe sensor configuration, including UV detector 301. UV light enters aperture 305 and is transmitted through light pipe 330 to detector 301. Gas enters at inlet 307 and exits at outlet 319.
FIG. 4 is a schematic of a process chamber suitable for practicing the present invention. UV lamp assemblies 401 are mounted on top of process chamber 413. Each lamp assembly 401 includes transformers 403 and magnetrons (not shown) that pump microwave energy into the lamp, UV bulb 405, and reflectors 407. As shown, the UV detector assembly 409 is mounted below and between two lamp assemblies 401. A chamber window 411 transmits UV radiation from the bulbs 405 to the substrate 417 below in the chamber. The substrate 417 sits on a substrate support 415, which may be heated, cooled or both. As shown, a heater coil 419 is embedded in the substrate support 415.
In some embodiments, the process chamber includes multiple stations each having a substrate support, chamber window, and UV lamp assemblies. FIGS. 5A and 5B are schematics of a multi-station chamber suitable for practicing the present invention. Chamber 501 includes four stations 503, 505, 507, and 509. Each station includes a substrate support 513. Embedded in the substrate support 513 are pins 519. Four carrier rings are connected to a spindle 511 that rotates about an axis. When a new substrate is introduced into a chamber, for example, at station 507, it is positioned on a robot arm. The carrier ring lowers the substrate to the substrate support, where it sits on top of the pins 519. After substrate processing is complete at one station, the carrier ring lifts the substrate and rotates 90 degrees to the next station, where it lowers the substrate for processing. Thus a substrate enters the chamber at station 507 sequentially moves through each of the four stations before exiting the chamber.
FIG. 5B shows a top view of the same multi-station chamber with UV lamp assemblies 515 on top of each station. The UV detector assembly 517 is positioned about the center between two lamp assemblies. Note that the lamp assemblies have a different relative orientation to the substrate as the substrate moves through each station. This different relative orientation ensures that each circular section of the substrate is exposed to the same amount of UV radiation. Because the UV bulb is tubular and substrate is round, in each station different parts of the substrate receive differing amounts of UV radiation. By changing the relative orientation of the bulbs to the substrate, these differences are overcome. Generally, having four stations of varying orientation is sufficient to ensure a relatively uniform UV irradiance. Of course, the relative orientations are not limited to the one shown in FIG. 5B—multiple orientations of lamp axis can be chosen to optimize wafer process uniformity. Uniform exposure may also be achieved by rotating the either the lamp assembly or the substrate during UV cure.
In certain embodiments, a system controller 521 is employed to control process conditions during UV cure, calibration, insert and remove substrates, etc. The controller will typically include one or more memory devices and one or more processors. The processor may include a CPU or computer, analog and/or digital input/output connections, stepper motor controller boards, etc.
In certain embodiments, the controller controls all of the activities of the apparatus. The system controller executes system control software including sets of instructions for controlling the timing, mixture of gases, gas injection rate, chamber pressure, chamber temperature, substrate temperature, UV power levels, UV cooling gas and flow, remote plasma clean, and other parameters of a particular process. Other computer programs stored on memory devices associated with the controller may be employed in some embodiments.
Typically there will be a user interface associated with controller 521. The user interface may include a display screen, graphical software displays of the apparatus and/or process conditions, and user input devices such as pointing devices, keyboards, touch screens, microphones, etc.
The computer program code for controlling the UV cure and calibration processes can be written in any conventional computer readable programming language: for example, assembly language, C, C++, Pascal, Fortran or others. Compiled object code or script is executed by the processor to perform the tasks identified in the program.
The controller parameters relate to process conditions such as, for example, UV power, UV cooling gas and flow rate, process duration, process gas composition and flow rates, temperature, pressure, and substrate temperature. These parameters are provided by the user in the form of a recipe, and may be entered utilizing the user interface. The controller parameters that relate to calibration conditions may also be entered utilizing the user interface, such as the duration of exposure, the UV power setting for measurement, and location of the calibration substrate. Additional parameters may be entered during setup or installation of the UV detector assembly, such as conversion formula of the UV detector output to UV intensity, deviation threshold for bulb replacement and window cleaning, and whether UV intensities at specific wavelengths are to be determined. These additional parameters may be changed from time to time during system maintenance.
Signals for monitoring the process may be provided by analog and/or digital input connections of the system controller. The signals for controlling the process are output on the analog and digital output connections of the UV cure apparatus.
The system software may be designed or configured in many different ways. For example, various chamber component subroutines or control objects may be written to control operation of the chamber components necessary to carry out the inventive UV intensity measurement and calibration processes. Examples of programs or sections of programs for this purpose include substrate positioning code, process gas control code, pressure control code, heater control code, and UV lamp control code.
A UV lamp control program may include code for setting power level to the UV lamps, initial ramping up of lamp power, and final ramping down of lamp power. A substrate positioning program may include program code for controlling chamber components that are used to load the substrate onto a pedestal or chuck. A process gas control program may include code for controlling gas composition and flow rates and optionally for flowing gas into the chamber prior to UV cure in order to stabilize the pressure in the chamber. A pressure control program may include code for controlling the pressure in the chamber by regulating, e.g., a throttle valve in the exhaust system of the chamber. A heater control program may include code for controlling the current to a heating unit that is used to heat the substrate. Alternatively, the heater control program may control delivery of a heat transfer gas such as helium to the wafer chuck.
Examples of chamber sensors that may be monitored during UV cure and calibration include mass flow controllers, pressure sensors such as manometers, thermocouples located in pedestal or chuck, and of course, UV detectors in the UV detector assembly and in the pedestal. Appropriately programmed feedback and control algorithms may be used with data from these sensors to maintain desired process conditions.
In one embodiment, the controller includes instructions for performing calibration, adjustment of process parameters, and determining maintenance activities according to methods described above. For example, the instructions can specify the parameters needed to perform the UV cure and calibration. The instructions may include calculating results based on measurement using user-provided formulas or conversion charts, integrating measurement results over time, e.g., integrating UV intensity measurement over the exposure time, and compare the result to a pre-determined threshold value. The result of the comparison may be used in logic sequences where an alarm or warning is generated, process parameter changed, or system is shutdown.
Suitable semiconductor processing tools may be configured with one or more UV process chambers. Suitable semiconductor processing tools include the SOLA and modified VECTOR available from Novellus Systems, Inc. of San Jose, Calif. Other suitable semiconductor processing tools include the Centura and Producer available from Applied Materials, Inc. of Santa Clara, Calif.
Details of the UV lamp assembly, reflectors, process chamber window, and temperature control on the substrate holder may be found in commonly assigned co-pending U.S. application Ser. No. 11/688,695, titled “Multi-Station Sequential Curing of Dielectric Films”, which is incorporated herein by reference in its entirety for all purposes.
For convenience, discussion of the UV lamp assembly and in-situ UV detector assembly focused on a two linear bulb configuration for the UV source; however, the present invention is not so limited. For example, the in-situ UV detector would work with any number of bulbs, e.g., 3, 4, or 5, in any kind of configuration, e.g., parallel or end-to-end. The in-situ UV detector would also work with other types of bulbs, not just linear bulbs. As UV bulbs evolve and techniques to build UV lamp assemblies advance, the use of an in-situ UV detector to monitor bulb performance continues to apply.
While this invention has been described in terms of several embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, modifications, permutations, and substitute equivalents as fall within the true spirit and scope of the present invention. The use of the singular in the claims does not mean “only one,” but rather “one or more,” unless otherwise stated in the claims.

## Claims

1. An ultraviolet (UV) apparatus, the apparatus comprising:
a process chamber comprising a substrate holder and a window;
a UV radiation assembly external to the process chamber, the UV radiation assembly operable to direct UV radiation through the window towards the substrate holder;
a detector assembly comprising a UV radiation detector, one or more gas inlets and a gas outlet arranged so that a gas flow from the one or more gas inlets to the gas outlet flows over one or more optical surfaces of the UV radiation detector; and
a controller comprising instructions for:
flowing a gas mixture comprising an inert gas and molecular oxygen into the one or more gas inlets during operation of the UV detector.

2. The UV apparatus of claim 1 wherein the gas mixture includes air.

3. The UV apparatus of claim 1 wherein the gas mixture includes air and N2.

4. The UV apparatus of claim 1 wherein the gas mixture consists essentially of air and an one or more inert gases.

5. The UV apparatus of claim 1 wherein molecular oxygen is between about 5% and 20% of the coolant gas.

6. The UV apparatus of claim 1 where air is between about 20% to 100% of the coolant gas.

7. The UV apparatus of claim 1 further comprising an exhaust pump operable to pump gas out of the detector assembly via the gas outlet.

8. The UV apparatus of claim 1 wherein the UV radiation assembly includes a UV source operable to emit UV radiation having wavelength of between about 160 nm and 200 nm.

9. An ultraviolet (UV) apparatus comprising:
(a) a process chamber comprising a substrate holder and a window; and,
(b) a UV radiation assembly external to the process chamber, the assembly comprising one or more UV lamps, one or more reflectors operable to direct a portion of the UV radiation from the at one or more UV lamps through the window towards the substrate holder, a UV intensity detector positioned to receive UV radiation from the one or more UV lamps reflected through the window, a gas inlet, and a gas outlet, wherein the gas inlet and gas outlet are positioned such that a gas flow path from the gas inlet to the gas outlet includes an optical surface of the UV intensity detector.

10. The ultraviolet (UV) apparatus of claim 9 further comprising a cover configured to isolate the UV detector from radiation from one or more UV lamps.

11. A method comprising:
directing UV radiation from a UV radiation source external to a process chamber through a process chamber window;
measuring UV radiation reflected from a substrate within the process chamber with a UV radiation detector located in a detector assembly external to the process chamber;
flowing a coolant gas through detector assembly while measuring UV radiation such that the coolant gas contacts optical surfaces of the UV radiation detector, wherein the coolant gas comprises molecular oxygen.

12. The method of claim 11 further comprising generating ozone in the detector assembly from the coolant gas.

13. The method of claim 12 comprising maintaining an ozone concentration of about 5-15 ppm in the detector assembly during measurement.

14. The method of claim 11 wherein molecular oxygen is between about 5% and 20% of the coolant gas.

15. The method of claim 11 wherein the coolant gas includes air.

16. The method of claim 15 where air is between about 20% to 100% of the coolant gas.

17. The method of claim 11 wherein flowing a coolant gas through the detector assembly includes pumping a gas mixture including ozone out of the detector assembly.

18. The method of claim 11 wherein the UV radiation includes UV radiation having a wavelength of between about 160 nm and 250 nm.