Airborne chemical contaminants (AMCs) in the lithography cell are responsible for yield-impacting issues such as scanner haze and wafer defects. Detecting and monitoring these chemicals in real-time is critical in IC fabs. Many technologies have been deployed to monitor AMCs, but many of them are not production-worthy solutions that meet the stringent sensitivity and throughput requirements driving Moore's law. Here, we demonstrate a fully integrated AMC monitoring system, Picarro SAM (Sample. Analyze. Monitor.), that detects a variety of chemicals in real-time from multiple locations in a lithography cell. Multiple scanners, track tools, and reticle stockers are monitored 24X7 for excursions of critical inorganic AMCs such as hydrochloric acid, hydrofluoric acid, ammonia, and sulfur dioxide, as well as volatile organic compounds such as acetic acid, PGMEA, NMP, siloxanes, etc. that impact the performance of the lithography process. The SAM system integrates sensors based on Cavity Ring-Down Spectroscopy (CRDS) with a high-performance sampling system. While CRDS is an established technology to detect critical inorganic AMCs, here we demonstrate the ability to measure several critical VOCs using a new technique called broadband CRDS. SAM monitors contaminants in various parts of the scanner and track with multiple sampling tubes extending to a hundred feet or more to accommodate remote areas of the process tool. The SAM system is a fast, easy-to-use, production-ready analytical tool to monitor trace VOC and inorganic contaminants in the parts per trillion to low ppb range within seconds that helps engineers take corrective actions much faster than ever before.
There is widespread recognition in the industry that as the design rule decreases, the number of airborne molecular compounds that can drive defect formation is increasing at an exponential rate [citation: IRDS 2020]. The vast majority of these new critically important AMCs are volatile organic compounds. These VOCs are difficult to measure in the gas phase at the parts-per-billion and parts-per-trillion levels that are needed for the tight process control requirements of advanced design rules. In this paper, we report on a novel AMC chemical metrology solution for measuring critical VOCs that are relevant to the photolithography cell. The heart of the system is a real-time laserbased analyzer based on a new analytical technique called Broad Band Cavity Ring Down Spectroscopy (BB-CRDS). The VOC monitoring system has several features which make it ideally suited to AMC measurements in the production environment: ultra-trace measurements at ppb levels without the need for calibration, 24/7 operation without user intervention, and negligible consumables. The analyzer was integrated into a state-of-the-art multiplexer to monitor ten VOC species in the photolithography cell in a modern semiconductor fab. We report on multiple observations made, including transient solvent leak events, multifunction chemical filter performance, and baseline characterization of AMCs inside track tools.
It has long been understood1 that in photolithographic mini-environments, acetic acid reacts with residual ammonia to form salts that cause haze formation on the scanner optics. It also induces cosmetic defects (satellite defect) on the surface of the photoresist. The acetic acid is generated in multi-function chemical filters, where PGMEA (a photoresist solvent) undergoes hydrolysis in the acidified media used to remove bases, producing PGME and acetic acid. Acetic acid is difficult to measure in the gas phase at parts-per-billion levels, requiring either costly and cumbersome mass spectrometers that are unsuitable for deployment in a production environment, or ion chromatography, a labor-intensive, off-line technique commonly performed off-site by a third-party analytical services lab. In this manuscript, we report on a novel AMC monitoring solution for measuring acetic acid, PGMEA, PGME, NH3, and other VOCs (volatile organic compounds). The heart of the system is a real-time laser-based analyzer based on a new analytical technique called Broad Band Cavity Ring Down Spectroscopy (BB-CRDS). The VOC monitoring system has several features which make it well-suited to AMC measurements in the production environment: ultra-trace measurements of a variety of VOC species; 24/7 operation without user intervention; little to no consumables; unparalleled accuracy without the need for calibration. The AMC system also consists of a high performance 8-port multiplexer, monitoring the photolithography bay at STMicroelectronics - Rousset fab. Here we report on acetic acid and PGMEA levels observed in several track tool and scanner minienvironments, over a period of several months.
In photolithography, haze prevention is of critical importance to integrated circuit chip manufacturers. Numerous
studies have established that the presence of ammonia in the photolithography tool can cause haze to form on
optical surfaces resulting in permanent damage to costly deep ultra-violet optics. Ammonia is emitted into wafer
fab air by various semiconductor processes including coating steps in the track and CMP. The workers in the
clean room also emit a significant amount of ammonia. Chemical filters are typically used to remove airborne
contamination from critical locations but their lifetime and coverage cannot offer complete protection.
Therefore, constant or periodic monitoring of airborne ammonia at parts-per-trillion (ppt) levels is critical to
insure the integrity of the lithography process. Real time monitoring can insure that an accidental ammonia
release in the clean room is detected before any optics is damaged.
We have developed a transportable, highly accurate, highly specific, real-time trace gas monitor that detects
ammonia using Cavity Ring-Down Spectroscopy (CRDS). The trace gas monitor requires no calibration gas
standards, and can measure ammonia with 200 ppt sensitivity in five minutes with little or no baseline drift. In
addition, the high spectral resolution of CRDS makes the analyzer less susceptible to interference from other
gases when compared to other detection methods.
In this paper we describe the monitor, focus on its performance, discuss the results of a careful comparison with
ion chromatography (IC), and present field data measured inside the aligner and the reticule stocker at a semiconductor fab.
In the semiconductor industry, control of ammonia at the parts-per-billion concentration level is critical to insure the integrity of the lithography process. Ammonia is emitted into wafer fab air by various semiconductor processes including CVD, wafer cleaning, coater tracks, and CMP, as well as from outdoor air. Exposure to even low parts-per-billion concentrations of ammonia during the photolithography process can lead to yield loss and unscheduled equipment downtime. Picarro, Inc. has developed a field-deployable, real time, ambient gas analyzer capable of continuously monitoring parts-per-trillion levels of ammonia in situ, and in real-time, thereby allowing a user to directly monitor ammonia levels in sensitive photo-lithography equipment.
We report on cavity-enhanced second-harmonic generation of 488 nm radiation in a 5 mm long periodically poled KTiOPO4 (PPKTP) crystal pumped by the output of a single-mode 976 nm semiconductor external cavity laser. At a pump laser output power of 660 mW, a mode-matching efficiency into an enhancement cavity of 65 % was observed. A maximum power of 156 mW at 488 nm was generated in the enhancement cavity of which 130 mW was coupled out. Under these pump laser conditions an overall optical conversion efficiency of 20 % and an overall electrical to optical efficiency of 9 % was measured. Both the spatial and spectral properties of the 488 nm beam are of very high quality. Typically, a near-diffraction-limited beam with M2<1.1 is produced with low astigmatism and little ellipticity.
The population lifetime of the amide I vibration (v10 fundamental, ca. 1650 cm-1) in the protein myoglobin in D2O has been determined by picosecond infrared pump- probe spectroscopy using the Stanford mid-infrared free electron laser to be 1.3 +/- 0.2 ps. In a glass forming mixture of deuterated glycerol and D2O, the vibrational lifetime was found to increase from 1.3 +/- 0.2 ps at 310 K to 1.8 +/- 0.2 ps at 10 K. In addition to determining the time-scale of vibrational relaxation, we also observed multi-level vibrational excitation which has implications regarding the anharmonicity and homogeneous linewidth of the mode.
The Stanford Free Electron Laser (FEL) is a source of high peak-power, short-pulsed radiation tunable throughout the mid- and far-infrared. This light source is ideal for the study of nonlinear spectroscopic processes such as the characterization of the vibrational dynamics of molecules or the nonlinear optical response of engineered quantum well structures. We have developed a flexible experimental apparatus for conducting these nonlinear experiments. We present here three examples of published work done at the Stanford FEL Center which relied heavily on the unique and flexible characteristics of the Stanford FEL: pump-probe measurements of the SD stretch mode realization in amorphous As2S3, photon echo measurements of CO in three systems, and measurements of second harmonic generation in InGaAs/AlAs quantum wells. These examples are indicatives of the quality and variety of experiments performed at the Stanford FEL Center in collaboration with outside users.
Infrared spectral hole burning studies have provided a wealth of information concerning site reorientation of defects in solids and vibrational relaxation dynamics. The most investigated systems appear to be impurities trapped in alkali halides. Limited studies on molecules trapped in noble gas matrices have demonstrated that these systems are good candidates for investigating persistent spectral holes. However, most infrared spectral hole burning studies have been limited by the tunability of commercially available infrared lasers which in turn restricts the spectral feature which can be burned. On the other hand, the tunability of Infrared Free Electron Lasers (IR-FELs) allows for targeting radiation into vibrational of the molecular system under study. We have used the Free Electron Laser-Fourier Transform Infrared Spectroscopy to investigate infrared hole burning of formic acid (HCOOD) isolated in an Ar matrix at a matrix/sample ratio of 4000/1. The results of the FEL radiation tuned to v2 mode of HCOOD are discussed together with matrix induced frequency shifts and matrix induced band splittings.
We have performed second harmonic generation (SHG) measurements in the 3 - 5 micrometers region on p-type stepped quantum wells (QWs) using the tunable, high peak power pulses generated by a free electron laser. The samples were grown by MBE on (100) GaAs wafers. The asymmetric QWs are made of m monolayers of GaAs and n monolayers of Al0.5Ga0.5As sandwiched between AlAs barriers. The QWs were characterized by x-ray diffraction and room temperature photoluminescence (PL). We measured an order of magnitude enhancement of the second order susceptibility over bulk GaAs. In contrast to n-type QWs, the dominant component is the (Chi) xyz(2) component. The results are explained by a full pseudopotential band structure calculation of (Chi) (2).
Conduction band intersubband absorption and second harmonic generation (SHG) are demonstrated in doubly resonant asymmetric step high indium content quantum wells (QW) grown on a GaAs substrate. Intersubband absorption peaks at 5.51 micrometers and 3.05 micrometers corresponding to the 1 to 2 and 1 to 3 transitions are measured. The susceptibility of SHG from the QW, (Chi) QW(2), is measured using a free electron laser by interference between the SHG fields generated from the QW and the GaAs substrate. A large asymmetry in the SHG power with rotation angle of the sample arising from (Chi) QW(2) is observed. The magnitude and phase of (Chi) QW(2) is measured in the 4.6 - 6.3 micrometers pump wavelength range. (Chi) QW(2) is of maximum amplitude at 6.0 micrometers with a value of 145 +/- 20 nm/V. A change in the sign of the phase of (Chi) QW(2) within the SHG resonance is demonstrated for the first time. Agreement of both the linear and nonlinear properties to a simple model assuming Lorentzian linewidths is discussed. SHG of 2.0 micrometers light is also demonstrated in coupled In0.60Ga0.40As/AlAs quantum wells.
Strong enhancement in the second harmonic generation signal is observed in the mid-infrared in ultra-narrow p-type asymmetric GaAs quantum wells. The experiments have been performed with the high power, tunable free electron laser located at Stanford University
A free-electron laser (FEL) microscope has been constructed to perform spatially and spectrally resolved pump/probe experiments in single living cells. Picosecond infrared FEL pulses are absorbed by the sample and rapidly converted to heat. Excitation of localized fluorescent reporter molecules using a UV/VIS probe beam leads to thermally induced alterations in the radiative signal. Fluorescence-detected infrared (FDIR) spectra are generated from regions proximal to reporter molecules by varying the FEL pump wavelength. Sub- wavelength spatial resolution is a composite function of media thermal properties and probe selectivity.
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