How Atomic Absorption Spectroscopy works

Thin film deposition by Physical Vapor Deposition or PVD is a proven technique for a diverse range of applications including multi-layer x-ray optics, coatings of aerospace, automotive and machine tool components, depositing solar materials or thin films in electronic, semi-conductor and medical device production.  The AtOMS (Atomic Optical Monitoring System) from Accustrata uses Atomic Absorption Spectroscopy or AAS and Optical Emission Spectroscopy (OES) to address the need for in-situ, real time monitoring of the deposition process to ensure film quality and yield.

In PVD, metals, alloys, compound semiconductors or other materials are vaporized, resulting in a cloud of atoms, which are precisely deposited onto a substrate to create a thin film.  The layer can be as thin as a few Angstroms.  AAS is a real time optical metrology technique to monitor this process as it happens. 

To achieve this unique capability, a probe beam of light at a wavelength, specific to the element it needs to monitor is passed through the plasma. Specific hollow cathode light sources are used that uniquely match the wavelength to the element under study.  The vaporized atoms absorb the probe light proportionally to their atomic concentration. By monitoring this absorption, Atoms provides a unique method of process monitoring.  Software is used to calculate the atomic concentration of the individual elements of interest and correlates the film deposition rate, thickness and chemical composition. 

HCL light sources can be selected to monitor over 60 elements and some HCLs can emit lines to monitor 2 elements, and with two such lamps 4 elements can be measured simultaneously. This provides for monitoring the chemical composition of the growing film in real time. In addition, the unique, fiber optic-based approach supports multi-sensor capabilities for mapping of the entire area of the substrate to control uniformity.

In many applications, Atomic Spectroscopy has proven to be the best choice compared to more traditional monitoring techniques. It can be applied to non-transparent films such as metals and alloys, extremely thin films and complex multi-layers, as well as films deposited on curved or moving substrates.

AtOMS Atomic Spectroscopy System

Fig 1. AtOMS Atomic Spectroscopy System

AccuStrata, Inc. is a provider of custom and high performance optical metrology tools that can be used to substantially improve product yield and product performance. The company’s platform technology is based on deploying a variety of fiber-optics-based spectroscopic techniques combined with deep learning, process modeling and predictive process control. AccuStrata’s products comprise fiber optics sensors, hardware modules and proprietary software for real-time spectroscopic monitoring.  This information addresses detailed mechanisms, metrology and performance specifications for the AtOMS – Atomic Absorption/Optical Emission Spectroscopy system. Figure 1 shows the external view of the AtOMS hardware module. The system and methods described in this document is protected under a pending patent.

AtOMS Measurement Mechanism

The AtOMS has been developed by AccuStrata under US Department of Energy program for in situ process control in the manufacturing of extreme UV (EUV) optical coatings for 13.5 nm lithography. AtOMS is a combined optical emission and atomic absorption system that is designed to monitor the plasma region in vacuum deposition/etching chambers, in close proximity to the surface of the substrate (3-10 mm). The technique is uniquely capable of monitoring metals, alloys, compound semiconductors, non-transparent materials, very thin films (<10nm), very thick coatings (>50 micron) and very high deposition/etching rates. Having these capabilities, AtOMS is able to overcome many of the deficiencies of the  legacy in situ process control systems used in vacuum thin film processing. Figure 2 shows the schematic of the measurement configuration.

Atomic Absorption Spectroscopy Schematic
Figure 2 Atomic Absorption Spectroscopy Schematic

AtOMS is a fiber-optics based system supporting multi-sensor and multi-beam applications. The optical sensors (illumination and receptacle optical collimators) are connected to the hardware module by flexible fiber optics cables. The system operates in multi-period duty cycles with frequencies that are application dependent. While the typical duty cycle frequency is 5-10 Hz, some applications may require larger, or smaller frequency depending on the characteristics of the controlled process. Each duty cycle comprises several periods. The periods are described below:

1) The system collects the plasma emission in the range 190 – 800 nm in real time and analyses the spectrum to detect specific emission peaks of some elements and radicals, that are specific for the deposition/etching process.  

2) The system launches a probe beam of light at a wavelength, specific to the element it needs to monitor (for example, Silicon emits and absorbs at 251.6 nm, Mo  – 313.3 nm). It uses specific hollow cathode light (HCL) sources (Hamamatsu, Agilent, Perkin Elmer), which emit the desired wavelength.

The atoms in the plasma absorb the probe light proportionally to their atomic concentration next to the substrate. The system measures the absorbed light at that wavelength. Since all the individual chemical elements have specific electron structure and excitation energy,  different HCL are required for each monitored element. HCL are available for monitoring over 60 individual chemical elements. Multi-element HCL sources are also available that can emit 2 or 3 element specific lines simultaneously.  AtOMS is designed to use 2 HCL sources and to monitor 4 elements simultaneously.  Additional element configurations can be created to address specific customer problems when required. 

3) Based on the atomic absorption and atomic emission AtOMS calculates the atomic concentration of the individual elements of interest and correlates each element concentration to the film deposition rate and/or film chemical composition. Advanced signal processing algorithms are used to calculate process parameters such as film chemical composition and the deposition rate of each of the individual components in the film.

The application of multiple fiber optics sensors provides several proprietary capabilities that are unique to the AtOMS system and are not matched by any competitive solution. AtOMS uses a combination of fiber-optics switches, which allow to use multiple probe beams simultaneously, allowing users to monitor processes in separate chambers or multi-probe beam configurations in a single chamber. Figure 3 illustrates the capability to launch several probe beams simultaneously and measure the plasma cloud uniformity over a single substrate. The probing of the plasma cloud at different directions allows the recovery of the plasma profile and ensures the uniformity of the deposition or etching process over the entire substrate area.

Atomic Spectroscopy Probe Beam Configurations in a Single Vacuum Chamber
Figure 3: Probe Beam Configurations in a Single Vacuum Chamber

The application of multiple fiber optics sensors provides several proprietary capabilities that are unique to the AtOMS system and are not matched by any competitive solution.

  1. AtOMS uses a combination of fiber-optics switches, which allow to use multiple probe beams simultaneously, allowing users to monitor processes in separate chambers or multi-probe beam configurations in a single chamber. Figure 3 illustrates the capability to launch several probe beams simultaneously and measure the plasma cloud uniformity over a single substrate. The probing of the plasma cloud at different directions allows the recovery of the plasma profile and ensures the uniformity of the deposition or etching process over the entire substrate area.
  2. Traditional film monitoring systems require substantial chamber redesign and installation of quartz viewports in order to launch the monitoring beam into the chamber and direct the reflected/transmitted beam outside the chamber.  AccuStrata’s fiber optics sensors can be installed inside the deposition area in the vicinity of the monitored substrate using metal-coated fibers and vacuum sealed feedthroughs (Note that in some applications this option may not be necessary).

Atomic Absorption fiber optic sensorsAccuStrata has been installing fiber optics sensors inside deposition chambers for many years and owns critical intellectual property (US patent #6,879,744 and #7,345,765) and know-how in the design, installation and protection from contamination. This expertise was fully utilized in the design and development of the AtOMS system. Figure 4 shows how 2 fiber optics sensors are installed inside a magnetron deposition chamber.

  1. Installation of sensors inside the chamber offers unique advantages. It allows the monitoring of only that portion of the plasma area that is exactly responsible for the film properties and eliminating the plasma areas that are away from the wafer area. It also allows users to adjust the position of the probe beam with respect to the substrate as well as correct the position when necessary. These two features increase the accuracy of the deposition rate and composition measurement during the process. By adjusting the position of the two sensors AccuStrata has demonstrated 0.01A/sec deposition rate accuracy for deposition of Al films and is working to achieve 0.005A/sec.

Note that the deposition/etching plasma in the processing chamber is an energetic process and has a natural spontaneous emission. One of the most important features of AtOMS is the ability to measure the optical emission of the plasma in the vicinity of the substrate. To achieve emission measurements, AtOMS closes (using an optical switch) the HCL source during each duty cycle. AtOMS typically uses 190 – 800 nm, but expansion of this range up to 2,200 nm can be supported. The spontaneous optical emission of the plasma is an important characteristics needed for precise determination of the atomic concentration of the selected elements. It is also required to achieve accurate deposition/etching rate measurements needed for the EUV optics and 13.5 nm lithography. AtOMS calculates the element concentration.

 

AtOMS-Atomic Absorption/Optical Emission Spectroscopy

Optical Emission and Atomic Absorption Spectroscopy CurvesFigure 6 illustrates the measured OEM spectrum and Atomic Absorption Spectroscopy spectrum during deposition of Al and Si using  dual Al/Si HCL source.

Comparison to Current Technology

Figure 7 is an overview of how the AtOMS-Atomic Absorption/Optical Emission Spectroscopy technology compares to the main market competitors: the spectroscopic optical monitoring through the substrate and the quartz crystal microbalance monitoring.  

Simultaneous multi-element deposition rate and composition monitoring

The optical spectroscopic monitoring technologies that directly monitor the wafer surface (e.g. please see AccuStrata’s product OM2) can only derive deposition rate and chemical composition of the deposited films by relating the films thickness, refractive index, bandgap and extinction coefficient to the composition of the material.  While for composition of simple 2-3-element compounds this can be achievable (see OM2), as the composition become more complex, determination of the composition based on refractive index becomes too complicated.

Comparison of Atomic Spectroscopy to Optical and Crystal Monitoring

Fig 7: AtOMS Atomic Spectroscopy Comparison

 

 

A second competing technology is the quartz crystal microbalance technology. Crystal monitoring technology relies on a mass balance with “tooling factors” to obtain deposition rate from the instrument. While this approach is fairly accurate for single elements, as composition of the deposited film becomes more complex, different phases of mixed element materials create prohibitively complex scenarios that the instrument cannot handle.  

Because AtOMS is monitoring each chemical element individually and derives its element concentration and deposition rate, determination of film chemical composition can be achieved even for the most complex systems. With the individual elemental calibration curves, the system can also estimate the deposition rate of each of the individual components as well as the total deposition rate and the attained thickness of the film.

Unique Element Monitoring

Because the AtOMS system uses elemental absorption rather than bulk property analysis to determine film properties, incredibly specific information about the deposition process can be obtained. This means that the system can be used for:

  1. Deposition rate and thickness monitoring
  2. Monitoring of etching and striping processes
  3. Monitoring for contaminants within the process

None of the other monitoring techniques have such versatility in monitoring capability and application space.

Versatility in Materials Monitoring

Optical spectroscopic monitoring techniques which involve direct monitoring of the substrate in reflection or transmission or both to obtain information about the thin film are limited by several requirements. First, the film must have a sufficient optical thickness to result in a noticeable interference pattern. Films and coatings that are too thick have too dense interference patterns and smear of the interference fringes which undermines the features that are needed to determine film thickness. Also, the films must be optically transmissive so that the probing beam can penetrate all the layers down to the bare substrate. If all of these criteria are not met, optical monitoring cannot be successfully implemented as a quality control measure.

Quartz crystal monitoring also has severe operational requirements, which limit its usefulness as a monitoring tool. First, the quartz crystal monitor measures the mass of the deposited material and does not carry any quality information about the film. Complex calibration processes involving several tooling factors are required to resolve the issue. Second, it requires direct line of sight to the depositing material. This means that either the monitoring cannot be carried out in real time (otherwise the detector would shadow the substrate) or the monitor must be offset from the depositing material, meaning the measured rate must be again calibrated to “estimate” the actual deposition on the substrate. Quartz crystal monitoring also requires an accurate knowledge of the “Z” parameter and the density of the depositing material. The parameters are materials specific and typically vary depending on the process and the system being used for the deposition. This can introduce significant error in the measurement and limits the technique to very simple compositions.

With the AtOMS-Atomic Absorption Spectroscopy monitoring approach, many of the limitations optical monitoring and crystal monitoring have are resolved. Because the system is not measuring the film directly, the properties of the film do not influence the overall measurement. Finally, because light is used as the probing sensor, direct information about the deposition can be determined directly in front of the substrate without shadowing or other interference.

Measurement Applications

Measurement Strategy

It is important to note that direct monitoring of the deposited film is not carried out in this metrology mechanism, but rather assumes that the composition of the deposition process is directly related to the final composition of film. Experimental results show that this assumption is very accurate when proper sensor geometry is employed, especially when the probe beams trace the plasma at a very small distance to the substrate . The approach also has several advantages:

  1. Shape and type (composition) of the substrate are irrelevant to AtOMS system performance.
  2. Shadow masking and geometrical masking as well as a variety of sample rotations can be carried out simultaneously with AtOMS measurements without added complexity.

The AtOMS system can be implemented wherever atomization processes are being used for materials processing. The only requirement is that a hollow cathode light source exist for the element of interest. Figure 13 (below) lists all of the hollow cathodes that are currently available and are supported by the AtOMS system. In addition, The AtOMS system is capable of monitoring emission spectra, which can also be used to monitor different processes, including gas emission lines. While emission monitoring systems exist, there is currently no system that combines BOTH absorption and emission capabilities in a single system.

While the system can technically support any hollow cathode lamp that is listed in Figure 13, it is important to note that different performance metrics such as sensitivity, signal to noise and accuracy vary for each individual element. For example, the AtOMS system is incredibly sensitive to elements such as Aluminum, Molybdenum and Cobalt but is less sensitive to Silicon and Tungsten. AccuStrata has evaluated sensitivity for several elements of current interest; however, not all elements have been sufficiently tested. Conceptually, there is no limit to the number of atomic species that can be monitored with the AtOMS system; however, practically, the current technology can only support up to 2 hollow cathode lamps and up to 4 elements simultaneously. This capability can be expanded if needed.

Real Data From Measuring Deposition Rate

Atomic Absorption as a function of deposition rateAtomic Absorption Spectroscopy process provides a method for determining the absolute concentration of atoms within the processing region; however, more data processing is required to achieve specific deposition rate and thickness profiles. AtOMS uses a calibration curve to relate atomic absorption to film deposition rate and thickness. Figure 8 shows an example of these calibration curves for both Aluminum and Silicon. As with traditional atomic absorption, the calibration curve is used to convert total absorption to deposition rate. The calibration curves are loaded into the AtOMS control software and interpretation of the deposition rate is automatically calculated and presented to the user.

Real Data From Measuring Chemical Composition

From the value of the atomic absorption and concentration of each individual monitored component, chemical composition can be directly measured. To measure chemical composition, one of two requirements must be met:

  1. Multiple hollow cathode sources within the same system
  2. A single hollow cathode source with multiple elemental constituents

The AtOMS-Atomic Absorption/Optical Emission Spectroscopy

system supports both methods of metrology. Once the atomic concentration is measured within the deposition region for each element, the ratio of the concentrations will give the total concentration of material immediately prior to deposition. Figure 9 is a plot of Aluminum and Silicon monitored simultaneously during a codeposition process. The X and Y axis are the deposition rate as measured by a quartz crystal monitor for Silicon and Aluminum respectively. The Z axis is the ratio of the Aluminum atoms to the Silicon atoms and is directly related to the relative concentration of atoms within the deposition region.

To interpret this plot, focus on a single deposition rate of (for example) Silicon. As the deposition rate of Aluminum is increased while the deposition rate of Silicon stays fixed, an increase in the Al/Si ratio is observed. The current AtOMS control software can automatically calculate this composition in real time.

Real data from detection of end point in a wafer stripping process

Endpoint Capabilities

One of the greatest strengths that the AtOMS system has over other competing technologies is its ability to support predictive endpoint control during film deposition or etching. This capability does not depend on the type or shape of the substrate, nor on the substrate rotation.

Figure 10 shows an example of emission endpoint data taken during a semiconductor strip process on Mattson Suprema equipment. Traditional, single wavelength endpoint monitoring only uses the emission line around 307 nm to determine the end of a process. With the AtOMS system, users are provided with substantially more information about the process at the transition point, enabling the implementation of  predictive analytics to determine the precise endpoint of the process before it occurs.

Software and Data Storage

Atomic Absorption/Optical Emission Spectroscopy endpoint detection

AccuStrata is a complete software and hardware development house, which accelerates productization and decrease time to market. Because of this, AtOMS has a complete control software based in the Java programming language. This customizable software suite allow full control over system settings and data acquisition profiles. All data is stored in two formats. First, all raw data is saved as four .csv files. This allows more in depth data processing. Second, the processed data is saved to a separate .csv file.

The onboard microcontroller for the AtOMS system is capable of providing software tie-ins from external system. This facilitates complete system integration for full endpoint capability.

Real Data for Tolerance, Resolution and Error

AtOMS stability measurementBecause the AtOMS system does not directly measure the thin film, all metrics for performance of the system are independent of surface roughness, rotating substrates and film type.

Figure 11 shows a simple example of monitoring. The data is plotted as normalized transmission versus time during a deposition process. Any deviation from a transmission value of 1 would be the result of absorption. The raw absorption data shown in Figure 11 combined with the calibration curve in Figure 8 provides a real time monitoring of the deposition rate. The variation within the data points is a good indication of the signal to noise within the system. Based on the Aluminum data collected by the AccuStrata team, the standard deviation of the metrology is ~0.009 A/s and for Silicon is ~0.011 A/s. It is important to note that these parameters are element specific.

To illustrate sensitivity, the team has monitored the deposition of Aluminum within a 5 element high entropy alloy containing Al, Ti, V, Fe, Ni and Zr. Figure 12 shows the results. Using the aluminum calibration curve, the deposition rate of the Aluminum within the 5 element high entropy alloy was measured. WDS determined the Al concentration of deposited films to be ~20%. Because no metrology method was available to validate the total deposition rate of the alloy material, profilometry measurements were used to estimate a deposition rate of ~0.1A/sec for the entire alloy. Figure 12 shows the Al transmission curves for the deposition of pure Al and the alloy material. The blue line show that, at a power of 50 Watts, transmission is 95% with a deposition rate of 0.3A/s. The red line corresponding to the results from the alloy target shows transmission of ~99.76%. Since the deposition rate is 3-5 times less for the alloy and the alloy is only ~20% Al, transmission of ~99.76% is within error of the expected 99.75%. This shows that the AtOMS system can reliable measure individual elements within a complex matrix to determine deposition information.

Signal to noise and accuracy can be improved with longer integration times with a trade of sampling rate. Therefore, system tuning is an import step in installation of the an AtOMS unit. In addition, sensor geometry and deposition configuration influence the calibration results for the deposition rate. Therefore, calibration the each material in each system is required for proper use. If composition is the only parameter of interest, the calibration is not required.

Hollow Cathode Tubes for Atomic Absorption/Optical Emission SpectroscopyThe process of data acquisition for the AtOMS system involves referencing of all components, including light source intensity, deposition on sensor optics and spectrometer performance,  during each measurement cycle. This ensures the highest achievable accuracy and repeatability for a given element.  

Limitations of the AtOMS System

Because the AtOMS system does not directly measure the thin film being deposited, it is not capable of directly determining mechanical or thermomechanical properties of the film, such as their stationary or thermally induced stress. It is also not capable of measuring materials properties that exist under depositing layers (subsurface properties).

Figure 13 is a copy of the commercially available HCL sources, including the single source, 2-source and 3-source HCLs.