KR100347737B1 - Apparatus and method for detecting a substance - Google Patents

Abstract

When the target membrane is removed by a process that selectively produces a chemical reaction product (eg, ammonia when polishing a wafer with nitride film in a KOH-containing slurry) with one of the stop membrane and the target membrane, and the target membrane is removed By monitoring the level of the chemical reaction product, the end point of removal of the target membrane over the stop membrane is detected. In addition, by extracting a substance present as a gas from the liquid and monitoring the gas to detect the substance, a very low concentration of substance in the liquid is detected.

Description

Apparatus and method for detecting substances {APPARATUS AND METHOD FOR DETECTING A SUBSTANCE}

TECHNICAL FIELD The present invention relates to semiconductor processing, and more particularly to the detection of the endpoint of removal of one film over another film.

In the semiconductor industry, an important step in the manufacture of integrated circuits is to selectively form and remove films (films) on underlying substrates. The membrane is made of a variety of materials and can be conductive (eg, metal or magnetic iron conductive material) or non-conductive (eg, insulator). The conductive film is usually used for wiring or wiring connection. Non-conductive or dielectric films are used in many applications, for example, as interlevel dielectrics between metal layers or as insulators between adjacent circuit elements.

Typical processing steps include (1) attaching a film, (2) patterning a region of the film using lithography and etching, (3) attaching a film filling the etched region, ( 4) planarizing the structure by etching or chemical mechanical polishing (CMP). The film is formed on the substrate by various known methods, for example by physical vapor deposition (PVD) by sputtering or deposition, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition (PECVD). The film is formed by any of a number of known methods, for example chemical mechanical polishing (also known as CMP), dry etching such as reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching and spray etching. Removed.

When removing the film, it is very important to stop the process when the correct thickness has been removed (when the end point is reached). According to chemical mechanical polishing, a film is a semiconductor wafer by rotating the wafer with respect to the polishing pad (or by rotating the pad with respect to the wafer or by rotating both) in a controlled amount of pressure in the presence of the slurry. Is optionally removed from. If the film is excessively polished (too much removed), the yield is reduced, and insufficiently polished (too little removed) expensive rework is required (the CMP process must be redone). Various methods have been used to detect when the desired end point of removal has been reached and the polishing should be stopped.

Prior art methods for detecting CMP endpoints suitable for all types of membranes include (1) simple timing measurement, (2) friction measurement by motor current, (3) capacity measurement, (4) optical measurement, ( 5) acoustic measurements and (6) conductivity measurements.

An exception to this is disclosed in US Pat. No. 5,399,234 to Yu et al, which describes the chemical reaction between potassium hydroxide and the layer to be polished in the polishing slurry. The end point of the polishing transmits sound waves through the slurry and changes the speed of sound when the concentration of the reaction product (which is considered silanol when polishing silicon dioxide) decreases from the layer being polished when reaching the lower polishing stop layer. It is monitored by detecting.

These prior art methods have inherent drawbacks, such as the inability to monitor in real time and the need to remove the wafer from the processing apparatus to check the completion of polishing (not in situ), or lack sensitivity.

This disadvantage has been overcome by in situ endpoint detection for conductive membranes, as disclosed in US Pat. No. 5,559,428 to Li et al. Entitled " In-situ monitoring of membrane thickness changes, " Detection has not yet been disclosed.

Thus, there is a need for an in situ real-time endpoint detection scheme suitable for use with all types of membranes. This approach should have high detection sensitivity and extremely fast response times, preferably less than 1 or 2 seconds.

It is therefore an object of the present invention to provide a method and apparatus for detecting the end point of the removal of any type of membrane on top of another membrane.

Another object of the present invention is to provide in-situ (ie real time) endpoint detection when the membrane is removed.

It is another object of the present invention to provide end point detection with high detection sensitivity and extremely fast response time.

According to the above and other objects, (a) removing a target film by a process of producing a chemical reaction product with a stopping film; (b) A method is disclosed for detecting the end point of removal of a target membrane on top of a stationary membrane by monitoring the level of chemical reaction product when the target membrane is removed. In addition, either the target membrane or the stop membrane comprises nitride, wherein (a) the target membrane is removed by a process that produces ammonia when the nitride is exposed to the process; (b) A method is disclosed for detecting the end point of removal of a target membrane over a stationary membrane by monitoring the level of ammonia when the target membrane is removed. (A) extracting the material present from the liquid as a gas, contacting the first side of the hydrophobic permeable membrane with a material containing liquid, contacting the second side of the hydrophobic membrane with a gas stream, An extraction step comprising allowing material to pass through said hydrophobic membrane as a gas and being loaded in a gas stream; (b) A method of detecting a very low concentration of a substance in a liquid is disclosed by monitoring the gas stream to detect the substance.

In addition, according to the object of the present invention, (a) removing the target membrane by a step of selectively producing a chemical reaction product of the stop membrane and one of the target membranes; (b) converting the chemical reaction product into a separate product; (c) A method is disclosed for detecting the end point of removal of a target membrane on top of a stationary membrane by monitoring the level of the separate product when the target membrane is removed. In addition, either the target membrane or the stop membrane comprises nitride, wherein (a) the target membrane is removed by a process that produces ammonia when the nitride is exposed to the process; (b) converting the ammonia into a separate product; (c) A method for detecting the end point of removal of a target membrane on top of a stationary membrane by monitoring the level of ammonia when the target membrane is removed is disclosed.

In addition, according to the object of the present invention, (a) removing the target membrane by a step of selectively producing a gaseous chemical reaction product of one of the stop membrane and the target membrane; (b) mixing the gaseous chemical reaction product present with a separate gas to form solid particles; (c) A method is disclosed for detecting the end point of removal of a target membrane over a stationary membrane by monitoring the amount of solid particles when the target membrane is removed. (A) extracting the material present from the liquid as a gas; (b) mixing the gas with other materials to form solid particles; (c) A method for detecting a very low concentration of substance in a liquid is disclosed by monitoring the amount of the solid particles to detect the substance.

In addition, according to the object of the present invention, (a) removing the target membrane by a process for producing a chemical reaction product in the slurry; (b) adding to the slurry a reagent which produces a characteristic result when reacting with said chemical reaction product; (c) A method for detecting the end point of removal of a target membrane on top of a stationary membrane by chemical mechanical polishing using the slurry by monitoring the slurry for characteristic results when the target membrane is removed is disclosed.

In addition, according to the object of the invention, (a) contacting a first side of a hydrophobic membrane with a solution; (b) contacting the second side of the hydrophobic membrane to the aqueous stream; (c) A method is disclosed for reducing the amount of gaseous product in a solution by allowing the gaseous product to pass through a hydrophobic membrane as a gas and being loaded in an aqueous stream.

In addition, according to the object of the present invention, (a) removing the target membrane by a step of selectively producing a chemical reaction product of the stop membrane and one of the target membranes; (b) converting the chemical reaction product into a separate product; (c) generating excited species from said separate product; (d) A method is disclosed for detecting the end point of removal of a target film on top of a stationary film by monitoring the level of light emitted from the excited species when the target film is removed.

According to the present invention, there is also provided a method of removing a target membrane by (a) selectively producing a gaseous chemical reaction product of one of the stop membrane and the target membrane; (b) A method is disclosed for detecting the end point of removal of a target membrane on top of a stationary membrane by monitoring the level of the chemical reaction product by critical photoionization mass spectrometry when the target membrane is removed.

These and other features and advantages will be more readily apparent and better understood from the following detailed description of the invention.

1 is a cross-sectional view of a target membrane to be chemically mechanically polished according to the present invention;

1A is an enlarged view of a portion 1A of FIG. 1,

2 is a cross-sectional view of an ammonia scrubber for reducing ammonia concentration before polishing in a slurry according to the present invention;

3 is a cross-sectional view of an extraction unit for extracting ammonia gas from a slurry according to the present invention;

4 is a schematic diagram of an apparatus for chemical detection using Fourier transform microwave spectroscopy according to the present invention,

5 is a schematic diagram of an apparatus for chemical detection using critical photoionization mass spectrometry according to the present invention;

6 is a schematic diagram of an apparatus for converting ammonia gas into NO and detecting using mass spectrometry according to the present invention,

7 is a schematic diagram of an apparatus for sampling NO using resonance enhanced multiphoton ionization analysis according to the present invention;

8 is a schematic diagram of an apparatus for detecting ammonia gas using light scattering according to the present invention;

9 is an enlarged view of the monitoring portion from FIG. 6;

10 is a schematic diagram showing an apparatus for sampling a slurry and detecting ammonia using a reagent according to the present invention;

11 is a schematic diagram of an apparatus for converting ammonia gas into NO and detecting using chemiluminescence according to the present invention,

12 is a schematic diagram showing an output trajectory from a computer for monitoring polishing of a semiconductor wafer according to the present invention.

Explanation of symbols for the main parts of the drawings

100 substrate 102 stop film

104 target membrane 200 contactor

300: extraction unit (extractor) 302: fiber

400: Fourier Transform Microwave Spectrometer

402, 502, 606, 702: bypass valve

404: pulse valve 406: microwave source

408: transducer / receiver 410: detector

506: krypton resonance lamp 508: vacuum chamber

510 quadrupole mass spectrometer

600: catalytic converter 604: mass spectrometer

806: laser beam 808: chopper

902: photomultiplier tube 904: mirror focusing system

1002: storage container 1010: color sensor

1106, 1108: Flow Limiter 1100: Converter

1112: signal conditioner 1114: computer

The present invention is described by way of specific examples only in the context of chemical mechanical polishing, and is not intended to limit the applicability of the present invention to semiconductor technology.

Those skilled in the art will appreciate that the present invention removes a target membrane by (a) producing a chemical reaction product with the stop membrane; (b) It will be appreciated that by monitoring the level of chemical reaction product when the target membrane is removed, detecting the end point of the removal of the target membrane on top of the stop membrane can be widely applied to all desired processes. In addition, the present invention is that either the target membrane or the stop membrane comprises a nitride, (a) removing the target membrane by a process for producing ammonia when in contact with the nitride; (b) By monitoring the level of ammonia when the target membrane is removed, detecting the end point of the removal of the target membrane on top of the stationary membrane can be widely applied to any process desired. The present invention also provides for (a) extracting a substance present from a liquid as a gas, contacting a first side of a hydrophobic permeable membrane with a material containing liquid and contacting a second side of the hydrophobic membrane with a gas stream. An extraction step comprising allowing the material to pass through the hydrophobic membrane as a gas to be loaded into a gas stream; (b) By monitoring the gas stream to detect the substance, the detection of very low concentrations of substance in the liquid can be widely applied to any process desired.

In addition, those skilled in the art will appreciate that the present invention provides a method for removing a target membrane by (a) selectively producing a chemical reaction product of a stop membrane with one of the target membranes; (b) converting the chemical reaction product present into a separate product; (c) By monitoring the level of the separate product when the target membrane is removed, it will be understood that detecting the end point of the removal of the target membrane on top of the stop membrane can be widely applied to all desired processes.

In addition, the present invention is that any one of the target membrane and the stop membrane comprises a nitride, (a) removing the target membrane by a process that produces ammonia when the nitride is exposed to the process; (b) converting the ammonia present into a separate product; (c) By monitoring the level of ammonia when the target membrane is removed, detecting the end point of the removal of the target membrane on top of the stationary membrane can be widely applied to any process desired.

In addition, those skilled in the art will appreciate that the present invention provides a method for removing a target membrane by (a) selectively producing a gaseous chemical reaction product of a stop membrane with one of the target membranes; (b) mixing the gaseous chemical reaction product present with a separate gas to form solid particles; (c) By monitoring the amount of solid particles when the target membrane is removed, it will be understood that detecting the end point of the removal of the target membrane on top of the stationary membrane can be widely applied to all desired processes. In addition, the present invention provides a process for the preparation of (a) extracting a substance present from a liquid as gas; (b) mixing the gas with other materials to form solid particles; (c) By detecting the substance by monitoring the amount of the solid particles, the detection of very low concentrations of substance in the liquid can be widely applied to any desired process.

In addition, those skilled in the art will appreciate that the present invention includes the steps of (a) removing the target membrane by a process that produces a chemical reaction product in the slurry; (b) adding to the slurry a reagent which produces a characteristic result when reacting with said chemical reaction product; (c) by monitoring the slurry for characteristic results when the target membrane is removed, it is widely applicable to all processes where it is desired to detect the end point of the removal of the target membrane on top of the stationary membrane by chemical mechanical polishing with the slurry. I will understand.

In addition, those skilled in the art will appreciate that the present invention comprises the steps of (a) contacting a first side of a hydrophobic membrane with a solution; (b) contacting the second side of the hydrophobic membrane to the aqueous stream; (c) It will be appreciated that by allowing the gaseous product to pass through the hydrophobic membrane as a gas and being loaded into the aqueous stream, reducing the amount of gaseous product in solution can be widely applied to all desired processes.

In addition, those skilled in the art will appreciate that the present invention provides a method for removing a target membrane by (a) selectively producing a chemical reaction product of one of the stop membrane and the target membrane; (b) converting the chemical reaction product into a separate product; (c) producing excited species from said separate product; (d) It will be appreciated that by monitoring the level of light emitted from the excited species when the target film is removed, detecting the end point of the removal of the target film on top of the stationary film can be widely applied to any desired process. .

In addition, those skilled in the art will appreciate that the present invention provides a method for removing a target membrane by (a) selectively producing a gaseous chemical reaction product of a stop membrane with one of the target membranes; (b) by monitoring the level of the chemical reaction product by critical photoionization mass spectrometry when the target membrane is removed, detecting that the end point of the removal of the target membrane on top of the stationary membrane can be widely applied to any desired process. I will understand.

As shown in FIG. 1, the inventors have contained potassium hydroxide (KOH) in a substrate 100 having a target film 104 of oxide (SiO ₂ ) over a stop film 102 of nitride (Si ₃ N ₄ ). It has been found that when chemical mechanical polishing with one slurry, a chemical reaction occurs upon reaching the interface 106, resulting in ammonia (NH ₃ ). In more detail, the slurry used is a mixture of fumed silica, water and KOH, having a pH of about 10.5. When polishing an oxide, the following reactions occur.

SiO ₂ + 2KOH + H ₂ O → K ₂ SiO ₃ + 2H ₂ O

When polishing the nitride, the following reaction occurs.

Si ₃ N ₄ + 6KOH + 3H ₂ O → 3K ₂ SiO ₃ + 4NH ₃

The resulting ammonia is dissolved in the slurry, and due to the relatively high pH, ammonia is mainly present in the form of NH ₃ rather than NH ₄ ⁺ . Thus, a change in the ammonia level in the slurry indicates that a lower nitride film has been reached, and the end point of removal of the oxide film can be determined by monitoring the level of ammonia in the slurry. When the end point is reached, polishing stops.

More specifically, the end point of the removal of any non-nitride film on top of the nitride containing film can be detected by monitoring the level of ammonia in the slurry. In contrast, the end point of the removal of the nitride-containing film on top of the non-nitride film can be detected in a similar manner, in which case ammonia is initially present, and a significant reduction in the ammonia amount represents the end point.

More specifically, the endpoint of the removal of any membrane on top of the other membrane can be detected by monitoring the level of chemical reaction product in the slurry as the components of the slurry selectively react with one of the membranes (top or bottom membrane). . The above reaction to produce ammonia will be described below, but is not intended to limit the scope of the invention to the specific examples. In order to make the above discovery related to the production of ammonia in an environment suitable for manufacturing, recovery and sampling of the slurry in situ (ie, while the wafer is being polished) is required. Preferably, the recovery and sampling of the slurry provides a fast and high sensitivity response (against ammonia) and minimizes the effects of interference from the slurry and other materials in the ambient air.

Unfortunately, the slurry described above already contains ammonia before it is used for polishing. The concentration of ammonia varies from as low as 5.0x10 ^-6 M to as high as 5.0x10 ^-5 M. The ammonia produced in the slurry when polishing a blanket (i.e. uniform) layer of nitride is about 1.0x10 ^-4 M at room temperature and has a nitride layer occupying 15% of the wafer area (the remaining oxide For typical low pattern factor fabricated wafers, polishing at the oxide / (oxide + nitride) interface produces a concentration of 1.5 × 10 ⁻⁵ M. In this case, the desired concentration change will be indistinguishable from the variation in the ammonia concentration before prepolish during polishing. Thus, to achieve the desired sensitivity, the ammonia concentration must be reduced before polishing this type of wafer.

Reduction of ammonia concentration before polishing can be achieved by using an ammonia scrubber if necessary. In this case, the scrubber reduced the concentration to about 2.5 × 10 ⁻⁶ M. The main element of the scrubber is the Liquid-Cel Membrane Contactor 200 (Model Extra-Flow 4x28 manufactured by Hoechst Celanese) shown in Fig. 2. The contactor is an aqueous solution of fiber membrane. A Celgard (TM) microporous polypropylene fiber 202 is hydrophobic, which does not allow passage through, but permits gas exchange.

Slurry from the reservoir enters the contactor 200 at 204 and flows through the contactor 200 on the outside of the fiber (also called outside the cell), whereby ammonia exits 206 and flows to 204. Permeate into the fiber (also called the lumenside) before being recycled. To facilitate the removal of ammonia in the slurry, an aqueous HCl solution having a pH of about 3 from other reservoirs is circulated on the lumen side, flowed in (208) and exited in (210) before being recycled to (208). do. The ammonia gas from the slurry across this HCl stream is immediately converted to NH ₄ ⁺ by a moderately high concentration of protons, so that any detectable amount of NH ₃ inside the scrubber tube is possible. Accumulation can be prevented effectively. In that case, the recycled aqueous HCl stream and reservoir may be a sink of a large amount of ammonia from the slurry.

The ammonia level was 30 by using an aqueous HCl reservoir of about 10 liters of water adjusted to 3.5 pH using 0.1 M HCl solution and passing 10 liters of a 1.0x10 ^-4 M ammonia solution at 10.7 pH to the contactor described above. Within minutes it was reduced to the desired 2.5 × 10 ⁻⁶ M. This time can be shortened by increasing the size of the contactor, by using several contactors in series, by slowly heating the slurry to increase the volatility of ammonia, or by any combination of these three. The desired target ammonia level in the slurry can be measured by commercially available detectors such as ammonia specific ion selective electrodes (ISE).

Once the slurry has reached the desired target ammonia level, the slurry is used for polishing the wafer. The slurry is recovered from the polishing pad for sampling during the polishing process.

Sampling method of ammonia dissolved in slurry

Once the slurry is recovered from the polishing pad, various sampling methods can be used. Representative methods are as follows.

Ion selective electrodes

One method of detecting liquid ammonia is to use commercially available ion selective electrodes. In a method using such an ion selective electrode, the internal electrode solution having a predetermined pH is contacted with the solution through a semipermeable hydrophobic membrane. This membrane allows the ammonia gas from the slurry to pass through, changing the pH of the internal electrode solution. This pH is monitored electrochemically. This method can detect up to 5x10 ^-7 moles of ammonia and takes less than 30 seconds to detect at high concentrations, but takes as long as 1-3 minutes to detect at low concentrations. This is suitable for many applications but not as desired for this particular CMP endpoint detection.

Fluorescence Analysis

Another method of detecting ammonia in liquid phases is to use complex reagents that generate fluorescent species when in contact with ammonia. The reagents used contain O-phthaldialdehyde (OPA) and sulfite, which act as reducing agents (Genfa et al., 1989 Anal. "See Fluorescence Measurements of Aqueous Ammonia Ions in a Flow Injection System," Chem., Vol. 61, page 408. The reaction product is highly fluorescent and can be detected at concentrations down to 2x10 ^-8 moles using emission spectroscopy. However, this method can take up to 10 minutes to reach termination, which is also not suitable for CMP applications.

Chemiluminescence

In addition, using a photomultiplier with an amplifier, the measurement of chemiluminescence due to the reaction between ammonia and hypobromite in alkaline solution was carried out (Hu et al. 1993 Anal. Chem. Vol. 65, page 3489, "Measurement of Ammonia Ions in Rainwater and Fogwater by Flow Injection Analysis by Chemiluminescence Detection". However, this method did not have the sensitivity required to monitor the ammonia produced in this application while grinding the nitride with CMP. The response time of this method is too slow for real in-situ real time process control.

Extraction of Ammonia Gas from Slurries

In order to enable a method such as mass spectrometry by detecting and monitoring gaseous forms of ammonia, the slurry from the polishing apparatus (not shown) is pumped through the ammonia extraction unit 300 shown in FIG. The extraction unit 300 is made of polypropylene microporous hollow fiber 302 obtained from a disassembled liquid cell contactor (model 2.5x8 manufactured by Hoechst Celanese). Fiber 302 allows gas to pass from the outside to the inside of the fiber but not the liquid.

The slurry is pumped on the outside of the fiber 302 inside through the extraction unit (extractor) 300 at 304 and discharged at 306. Dry clean gas (dry air) (from a dryer with ammonia filter, NO _x removal filter) is pumped in through the inside of the fiber in 308 and in a stream containing ammonia gas in 310 ( 312). Carrier gas (dry air) is pumped to a reduced pressure of about 30 Torr to facilitate the transfer of ammonia from the slurry through the fibers into the gas stream. In addition, as the pressure is reduced, the overall flow rate is increased to help reduce the change in ammonia concentration or the response time for measuring the level of ammonia.

Analysis method of ammonia gas

The ammonia containing gas stream 312 is then analyzed and monitored from the extractor 300 for endpoint detection of removal of the target membrane. Meteorological chemical analysis, such as standard mass spectrometry, can have the high sensitivity and fast response time desired for endpoint detection. However, slurry sampling shows significant interference from water vapor that is as high as 1 atomic mass unit (AMU) and present in large quantities. During electron impact ionization, water of mass 18 may lose hydrogen, resulting in OH ions of mass 17 having the same mass as NH ₃ ⁺ . Therefore, the ammonia signal from the slurry can be masked (interrupted) very effectively, and the detection of the end point becomes impossible. The following technique solves the masking problem.

Fourier transform microwave spectroscopy (FTMS)

Fourier transform microwave spectroscopy using pulsed vapor sample injection has extremely high detection sensitivity and chemical selectivity, enables accurate measurement of contaminants at sub-ppb levels, and overcomes the masking problems associated with mass spectrometry [Harmony] 1995 Rev. Sci. Instrum. Vol. 66, page 5196, "Small Hot Nozzle Fourier Transform Microwave Spectrometer," and Suenram, 1989, J. Mol. Spectrosc. Vol.137, pp. 127, "Effects of Tunneling Motion on Hydazine's Quadrupole Ultrafine Structure".

From the extractor 300 (FIG. 3), the ammonia containing gas stream 312 enters the Fourier transform microwave spectrometer 400 shown in FIG. 4. Bypass valve 402 is used to regulate the flow of the stream into pulse valve 404. Bypass valve 402 allows significant overall flow through extraction unit 300, resulting in rapid measurement response time without overloading spectrometer 400 with sampling gas from the incoming ammonia containing stream. .

Pulse valve 404 injects sample gas from stream 312 into the spectrometer vacuum chamber at a repetition rate of about 20 Hz and a valve opening time of 1 ms. This pulsed gas enters the chamber and expands at supersonic speed, cooling to about 10 ° K. In addition, high intensity microwave radiation (23.786 GHz for ammonia) is pulsed at 20 Hz (in time to match the pulsed gas) from the microwave source 406 to the transmitter / receiver 408 in the chamber. Ammonia molecules absorb radiation and re-radiate in the free induced attenuation process. The attenuation is then digitized and converted by the detector 410 to obtain a frequency domain signal. The strength of the signal is monitored to determine the amount of ammonia present in the slurry.

Critical Photoionization Mass Spectrometry

In this method, as shown in Fig. 5, detection by a quadrupole mass spectrometer is performed in the critical photoionization of ammonia. The ammonia containing gas stream 312 exiting the extractor 300 shown in FIG. 3 enters the photoionization mass spectrometer 500 (eg, Syagen Technology Model TPMS-100S) through the bypass valve 502. . Sample region 504 at a pressure of about 1 Torr is irradiated by a krypton resonance lamp 506 with two primary vacuum ultraviolet rays of 10.0 eV and 10.6 eV. The ultraviolet of 10.6 eV is slightly higher than the ammonia ionization potential (IP) of 10.10 eV and sufficiently lower than the ionization potential of water of 12.6 eV. Ions generated by irradiation are directed through a small orifice into the high vacuum chamber 508 and detected by a modified quadrupole mass spectrometer 510. Mass spectrometry is used to distinguish interference from hydrocarbon species with low ionization potentials that can be easily ionized by krypton lamps. Detection sensitivity better than 10 ppb is expected.

Another advantage of using critical photoionization mass spectrometry in place of more conventional electron impact ionization is that water cleavage that generates partial ions (OH) at mass 17, the same mass as ammonia, is prevented. In addition, the photon energy is lower than the ionization potential of other gaseous molecules that may be present, including N ₂ , O ₂ , CO ₂ , CO and most importantly H ₂ O.

It should be noted that the critical photoionization mass spectrometry and Fourier transform microwave spectroscopy and apparatus described above are not limited to those used to monitor the end point of CMP. When the upper film is removed from the lower film by etching, for example dry etching (e.g. reactive ion etching), the lower film (i.e. the etch stop) may not generate the display chemical reaction product upon contact with the etching agent. It can also be selected as. The reaction product of the etching process can be sampled by any of these methods to monitor the level of the displayed chemical reaction product.

In addition, critical photoionization mass spectrometry and Fourier transform microwave spectroscopy may be performed by extracting a chemical present in gaseous form from a liquid and detecting ammonia in gaseous form using the steps described above. For example, 1.0x10 ^-5 M or less).

Conversion to nitric oxide

Interference by component OH in water is accomplished by converting ammonia in the ammonia containing gas stream from extraction unit 300 (shown in FIG. 3) into a separate product having a different mass than OH and detecting that separate product. It can be prevented by indirectly detecting ammonia. In further detail, the ammonia in the ammonia containing gas stream from the extraction unit 300 is converted to a separate product, which can indirectly detect ammonia by detecting this separate product. One example is the conversion of ammonia to nitric oxide (NO) using a catalytic converter 600 as shown in FIG.

The ammonia containing gas stream 312 enters the catalytic converter 600, for example, a series of stainless steel tubes that are heated to 800 ° C. with a surface area large enough to convert a sufficient amount of gas. The conversion, known as the Oswald process, is explained by the following reaction.

4NH ₃ (g) + 5O ₂ (g) → 4NO (g) + 6H ₂ O (g)

The carrier gas of the stream entering the catalytic converter preferably contains oxygen (O ₂ ) rather than air because the air has a mass equal to the mass of NO 30 and about 15 ppm nitrogen isotope ( ₇ N ¹⁵ ) ₂ This is because it contains.

To achieve fast response time and minimization of background NO, the carrier gas must be dry and clean. Conversion to NO and measurement of NO by chemiluminescence can detect sub-ppb levels and avoid interference from other chemicals.

Mass spectrometry

Gas stream 602 exiting catalytic converter 600 contains NO and enters mass spectrometer 604. Bypass valve 606 is used to regulate the flow of the stream and also allows significant overall flow through extraction unit 300 and catalytic converter 600, resulting in sampling gas from the incoming NO containing gas stream. This provides a quick measurement response time without overloading the mass spectrometer 604. The mass spectrometer 604 consists of a vacuum chamber 608 containing a quadrupole mass spectrometer 610 with an electron impact ionization source, such as the Stanford Research Systems Residual Gas Analyzer Model RGA 200. . Such an analyzer has a dynamic range of eight orders of magnitude and detects NO down to about 10 ppb levels. The analyzer is gate controlled to detect ions only at mass / charge 30, and a signal for real-time monitoring can be obtained by a computer.

REMPI-TOF Mass Spectrometry

Another method of mass spectrometry is a resonance-enhanced multiphoton ionization (REMPI) assay that is configured to easily and sensitively detect the presence of nitrogen oxides (DC Jacobs et al. 1986 J. Chem. Phys. Vol. 85, page 5469, "Improvement of 1 + 1 Resonance for Population Distribution, Reduction of MPI Spectrum: Application to NO A ² ∑ ⁺ -X ² II System"]. In FIG. 7, the NO-containing gas stream 402 exiting the catalytic converter 600 is time-of-flight (TOF) mass spectrometer 700 via a bypass valve 702 similar to that described above. Flows into. Pulsed molecular beam valve 704 converts stream 602 into pulse stream 706. The pressure of the gas stream 602 is about 30 Torr, which is sufficient to form a beam when entering the chamber of the analyzer but does not require low temperature supersonic expansion.

Pulse valve 704 is timed to match pulse laser beam 708 that selectively ionizes NO from pulsed gas stream 706. Laser beam 708 may be the output of a laser system with a frequency doubling Q switch Nd: YAG laser operating at 20 Hz, for example, used to pump adjustable lasers of titanium and sapphire (Ti: S). . This Ti: S laser is tuned to 904 nm and its output is frequency doubled twice using BBO nonlinear crystals. The second harmonic is 452 nm, and the fourth harmonic is the desired wavelength of 226 nm, or the wavelength of one photon excitation of NO from the ground state. The ionization scheme described above has been shown to be extremely efficient with a total detection sensitivity down to 1.0 × ^{10 −11} Torr (M. Asscher et al., 1982 Phys. Rev. Lett. Book 49 Page 76 "Vibration and rotational energy distribution of NO dispersed from the Pt (111) crystal surface: see detection by two-photon ionization".

The pulsed ionized gas stream 706 is then accelerated by the fields generated using the electrode 710. Ions migrate to the detector 714 along the length of the drift tube 712. The movement time of the ions is proportional to their mass and the time gating of the arrival signal is used to measure only the signal from NO. The resulting signal is averaged at Boxcar averager 716 (model SR250 of the Stanford system) and directed to a processing computer (not shown). Based on static gas measurements, this technique is expected to have a sensitivity of 10 ppb with a response time of a few seconds.

Light scattering

In another embodiment of the present invention, after the ammonia gas is extracted from the slurry, the ammonia gas can be detected by mixing it with the hydrochloric acid vapor and monitoring the formation of solid particles of ammonium chloride according to the following formula.

NH ₃ (g) + HCl (g) + nH ₂ O → NH ₄ Cl (H ₂ O) n (s)

An apparatus for such a monitoring scheme is shown in FIG. 8. From FIG. 3, the ammonia containing gas stream 312 is mixed with an air stream 802 containing gas phase HCl at a known concentration and well controlled. Once mixed, ammonia and hydrochloric acid vapors react immediately to form solid ammonium chloride particles 804. The amount of ammonium chloride particles 804 produced is measured by monitoring the macroscopic scattering from the particles 804 using, for example, a laser beam 806 from a helium neon laser.

9 illustrates a method of monitoring scattering. Scattered photons generated by the laser beam 806 striking the particles 804 are detected by a photomultiplier 902. It is desirable to use a mirror collection system 904 to improve the light collection efficiency. The sensitivity of this technique is based on the mechanical chopper 808 of FIG. 8 (e.g., model SR450 from Stanford Research Systems) and a lock-in amplifier synchronized with the chopper for sensitive detection of phase. Not shown, for example, by using a model SR810 from Stanford Research Systems. The amount of scattered light is proportional to the ammonia concentration in the slurry as the wafer is polished.

The reaction to form solid particles is rapid because it is a simple acid group reaction and is selective for ammonia because very few gas molecules act like bases. The reaction occurs in less than 1 second, which is well suited for the endpoint detection scheme as described above.

The light scattering methods and apparatus described above are not limited to those used to monitor the end point of the CMP. Once the top film is removed from the bottom film by etching, e.g. dry etching (e.g. reactive ion etching), the bottom film (i.e., etch stop) may be selected to form a display chemical reaction product upon contact with the etchant. It may be. The reaction product of this etching process can be sampled by the described method to monitor the level of the displayed chemical reaction product.

The light scattering method also involves extracting a chemical present in gaseous form from a liquid, mixing the gas with another material to form solid particles, and monitoring the amount of solid particles by, for example, scattered light. It may also be used to detect substances such as ammonia at very low concentrations (eg 1.0 × 10 ⁻⁵ M or less) in the liquid.

Detection by reagent

The slurry can be analyzed using appropriate reagents to indicate the presence of chemical reaction products.

To detect liquid ammonia in the slurry (milky white), a suitable choice is to use a Nessler Reagent (alkaline solution of potassium tetraiodo (II) potassium), which reacts with ammonia to produce a characteristic brown color. Change. After polishing the wafer, the slurry 1000 from a predetermined point on the polishing pad 108 (see FIG. 1) is continuously pumped through the inlet valve 1001 to the reservoir 1002. The Nessler reagent 1004 is supplied to the reservoir 1002 in a pulsed manner. Opening 1006 in reservoir 1002 is used to discharge the slurry / reagent mixture into a waste container (not shown). The valve 1001 and the opening 1006 are adjusted so that the dynamic balance of the slurry in the reservoir 1002 is reached and maintained. More specifically, the slurry can flow constantly into the reservoir and be discharged constantly, but enough fresh slurry remains for the detection of ammonia.

Storage vessel 1002 has at least one transparent wall or portion of transparent wall 1008. A color sensor 1010 (commercially available) is disposed outside the transparent portion 1008 of the reservoir 1002 and detects discoloration when the Nessler reagent reacts with ammonia in the slurry. This method is capable of detecting ammonia at concentrations as low as 5 × 10 ⁻⁵ mol (about 100 ppb), and discoloration occurs in less than 1 second.

Once characteristic discoloration is observed, the nitride layer is exposed and polishing stops as desired (polishing may continue for some time known in the art as "overpolishing"). Computers may be installed to control the process, receive signals from the color sensors, and send signals to the polisher to stop polishing.

In addition, other reagents for endpoint detection may also be used, depending on the chemical reaction product produced during the membrane removal process. In general, the reagents added to the slurry will cause reactions that give characteristic results such as discoloration, photons (ie, emitted light) or precipitation.

An exemplary apparatus used to detect characteristic results in case of discoloration has been described. In order to detect photons, suitable sensors such as conventional photomultipliers or infrared sensors are used. In order to detect precipitation, a light scattering device may be used.

Chemiluminescence Detection (CLD)

When the excited molecules generated from the chemical reaction emit light (light) when they are switched from the excited state to the allowed low state, the phenomenon of chemiluminescence occurs. Using chemiluminescence, in this case it is possible to detect the presence of ammonia (indirectly by detection of nitric oxide) according to the following equation.

NO (g) + O ₃ (g) → NO ₂ ^* (g) + O ₂

NO ₂ ^* (g) → NO ₂ (g) + hν

Here, the emitted light (photons) is detected by the highly sensitive photomultiplier tube (PMT).

An apparatus for chemiluminescent detection (CLD) is shown in FIG. 11. The NO containing gas stream 1102 exiting the converter 1100 enters the chemiluminescence detector 1104 through a flow restrictor 1106 and mixes with ozone in the reaction chamber. Ozone is produced by a conventional ozone generator having a flow restrictor 1108 as shown. In the reaction chamber, NO and O ₃ react as indicated above to produce excited NO ₂ ^* molecules, which emit broadband light centered at 1200 nm. High sensitivity PMT 1110 (e.g., detectors of ECO physics, Zurich, Switzerland) detects luminescence through color filters used to block photons with undesired wavelengths. The PMT 1110 operates in photon counting mode and is controlled and processed by the signal conditioner 1112. The signal is then sent to computer 1114, which monitors the NO ₂ ^* chemiluminescence signal and controls the polishing process using an output connection to a polishing apparatus (not shown).

FIG. 12 shows the output from a system used to monitor the polishing of a semiconductor wafer with a layer of oxide (SiO ₂ ) over nitride (Si ₃ N ₄ ). Thus, the target membrane to be removed is an oxide layer and the stop membrane is a nitride layer. The trace 1200 before 41 seconds corresponds to the polishing of oxide only, and the trace after 41 seconds indicates that the nitride is polished and ammonia is being produced. The point at which nitride polishing was stopped was very clear, and levels of less than 1 ppb were detected with a response time of 1 second or less.

Note that the aforementioned chemiluminescent methods and apparatus are not limited to use in connection with endpoint detection for CMP. If the top film is removed from the bottom film by etching, for example dry etching (eg, reactive ion etching), then the bottom film (ie, etch stop) that generates the display chemical reaction product upon contact with the etchant may be selected. have. The reaction product of the etching process can be sampled by this method to monitor the level of the displayed chemical reaction product.

In summary, the method and apparatus of the present invention described above makes it possible to detect the end point of removal of all types of membranes on top of other membranes. The present invention provides in-situ endpoint detection with high detection sensitivity and extremely fast response time when the membrane is removed.

While the invention has been described in terms of specific embodiments, it will be apparent to those skilled in the art that many modifications, variations and variations are possible in light of the above teaching. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the invention and the appended claims.

The method and apparatus of the present invention make it possible to detect the end point of removal of all types of membranes on top of other membranes, provide in-situ endpoint detection when the membrane is removed, high detection sensitivity and very fast response time.

Claims (11)

A device for detecting very low concentrations of substances in liquids, (1) extracting the material present from the liquid as a gas, the means for contacting a first side of a hydrophobic permeable membrane with the substance containing liquid and a means for contacting a second side of the hydrophobic permeable membrane with a gas stream An extractor having means for allowing said material to pass through said hydrophobic permeable membrane as a gas and being loaded into said gas stream, and a contactor having a microporous fiber; A monitor for monitoring the gas stream to detect the substance; Device for detecting substances. The method of claim 1, The concentration is 1.0x10 ^-5 mol or less Device for detecting substances. delete The method of claim 1, The monitor comprises a Fourier transform microwave spectrometer Device for detecting substances. The method of claim 1, The monitor includes a critical photoionization mass spectrometer Substance detection device. In a method for detecting very low concentrations of substances in a liquid, ① extracting the substance present from the liquid as a gas; (2) mixing the gas with other materials to form solid particles; (3) monitoring the amount of solid particles to detect the substance, wherein the monitoring includes contacting the solid particles with a laser beam to generate light scattering and measuring the light scattering Method of Detection of Substances. The method of claim 6, The concentration is 1.0x10 ^-5 mol or less Method of Detection of Substances. The method of claim 6, The extraction step (1) contacting said first side of a hydrophobic permeable membrane with said substance containing liquid, Contacting the second side of the hydrophobic permeable membrane with a gas stream; (3) allowing the material to pass through the hydrophobic permeable membrane as a gas to be loaded into the gas stream; Method of Detection of Substances. The method of claim 6, The laser beam is generated by a helium neon laser Method of Detection of Substances. The method of claim 6, And mechanically chopping the scattered light Method of Detection of Substances. The method of claim 10, Lock-in amplifying the scattered light; Method of Detection of Substances.