Ion beam technology plays an increasingly vital role in semiconductor processing. Here Dr. Michael Zeuner, Mattias Nestler and Dr. Dietmar Roth, Roth & Rau discuss a novel approach to ion beam trimming technology.
IonScan 800 - Ultra-precise Wafer trimming Technology
Many applications in semiconductor technology are characterised by extreme requirements in terms of film thickness homogeneity. Here Dr. Michael Zeuner, Matthias Nestler and Dr. Dietmar Roth introduce a new method of local film thickness trimming and its technical implementation.
Over the past years, ion beam technologies have increasingly found their way into material processing in optics and semiconductor technology. The reason for this success is based on the characteristics of the ion beam processes outbalancing alternative technologies in terms of quality. In ion beam methods, the incident angle of the ion beam may be adjusted in a defined manner. However, the process is characterised by a narrow ion energy distribution, controllability of the ion beam composition, as well as a high time and spatial constancy of the ion flow. Consequently, ion beam methods are mostly used for large area milling processes whose removal depth accuracies get close to the atomic scale. These procedures enable homogeneous removal or structuring with outstanding anisotropy characteristics across the whole substrate surface.
Ion beam trimming technology
Ion beam technologies not only allow a homogeneous substrate removal, but also locally resolved etching by controlling the local ion dose. Upon this dose, it is possible to correct inhomogeneities of particular characteristics. When correcting film thickness or step height values of a structure, an error function gets etched down to the required function. The terms “ion beam trimming“or “ion beam figuring” were introduced for this technique.
Ion beam trimming can be performed with either an apertureor a residence time method. In the aperture method, a large area ion beam gets shaped with a shutter system or masks in its temporal progression. The local ion dose is controlled in a defined way by variable aperture windows of different size which are chronologically consecutive. However, the technical effort implementing the aperture method is notably high. At the same time, the process rates are low due to blanking a large share of the ion beam. Consequently, the aperture method is normally out of the question for use in a production environment.
It is much easier to control the local removal characteristics by means of the residence time method. The residence time method uses a focused ion beam, which is moved in relation to the substrate to be corrected according to a defined motional strategy. It is possible to calculate the required residence time values at the corresponding positions and the appropriate motional mode being aware of the static etch profile of the ion beam. The basic process arrangement of the residence time method is shown in Fig. 1.
The residence time method does not require any additional aperture or shutter systems. It always utilises the ion beam to its full extent for etching, and small-sized and economic ion sources are sufficient. For these reasons, the residence time method is commonly superior to the aperture method, both under technological and economic aspects. However, using the residence time method demands a sufficiently low width of the ion beam versus the local wavelength of the surface errors to be corrected.
A 2-axis system is required to implement the residence time method in order to carry out the necessary relative motion between the ion source and the surface. The layout of the axis system mainly depends on the motional strategy. Present default is to scan the surface following a meander-shaped course (Fig. 1). In this case, the performance of both axes should be clearly different, since for one axis a high performance is required whereas the other has only a linefeed function. Specific process variations may necessitate more complex axis systems. In the majority of cases, another axis in the substrate surface normal direction is used to adjust the ion source. This axis cannot only be used for feeding, but also for optimal adjustment of the ion beam focus.
In addition to milling removal, the ion beam can also be used for smoothing the surface and reducing the micro roughness values. To carry out these processes, it is necessary to adjust a defined ion angle of incidence and thus to tilt the substrate. For application in optics, to obtain the surface radii of curvature, it may be required to track the ion source along the surface normal and to control the etching distance. An axis system for such applications should have 2 tilt and 3 linear axes.
IonScan 800 system layout
The IonScan 800 system is designed for wafer based film thickness trimming in semiconductor technology. With the handler and the process module, it is possible to create a cluster layout of the entire system, which is able to integrate both two load-locks and up to three process modules (Fig. 2).
The system components for ion beam trimming are housed in the process chamber (Fig. 2). The chamber size is about 0.80 m x 0.80 m x 0.50 m. Approximately 5 x 10 7 mbar residual gas pressure is feasible with the turbomolecular pump set (2300 l/s). All door flanges are fitted with double Viton O-rings and are pumped differentially.
An additional chamber at the front door houses the ion beam source to be accessed for maintenance activities using a separate lid. A filament-free ion beam source cyberis 40-i made by Roth & Rau is used in the IonScan system 7. The compact source with 190 mm length and 100 mm diameter is mounted completely in the vacuum with discharge chamber and impedance matching.
The plasma excitation consists of a primary cylindrical coil supporting the discharge housing in the middle. According to the ICP principle, radio frequency power (13.65 MHz) is transferred inductively to the gas discharge. The full RF impedance matching is integrated in the rear part of the source housing. A hot filament or an RF neutralizer is used to neutralize the ion charge during processing of isolating substrates.
Different focusing multi-aperture grid systems made of graphite are available for the source. Each system consists of 3 individual grids of different geometry, which enable intentional control of both the ion flow and the focus characteristics. With the grid systems, it is possible to achieve a maximal total ion flow to 100 mA, as well as up to 2 keV ion energy. Typically, the processes on the IonScan 800 are run at a current ranging from 30 mA to 50 mA and an ion energy from 1.2 keV to 1.5 keV.
In most of the processes, the ion beam source is run with inert gases (Ar, Xe). The discharge chamber of the source is completely made of aluminum oxide, so that fluorine-containing process gases can be used without any constraint, too.
At the right of Fig. 3, the axis system with the wafer chuck is shown with the opened chamber door. The axis system is already configured to machine wafers up to 300 mm. Handling and wafer chucks are available in versions with 4”, 5”, 150 mm and 200 mm, both for wafers with flat and with notch.
The wafer chuck is equipped with a clamping and transfer mechanism actuated by compressed air. The handler places the aligned wafer on 4 lift-off pins. The pins and the clamping ring are pneumatically operated and press the wafer against the body of the wafer chuck. A helium back side cooling is used for efficient heat transfer from the wafer to the water cooled chuck body. With this cooling principle, a power input of typically 100 W may be deduced efficiently out of the ion beam.
As a rule, the resultant temperature at the wafer front side is below 100 oC, so that it is possible to process even wafers with photoresist or SAW substrates (LiTa, LiNb, and Quartz) without any problem.
The IonScan 800 system is equipped with a Faraday array consisting of 2 x 8 current probes. The probes and the wafer holder are mounted on the axis system. With the Faraday array, it is possible to run a complete current density profile of the ion beam within a few seconds. The array is used for routine check of the ion beam stability and to determine the exact focus position of the ion beam related to the wafer after maintenance operation.
All IonScan 800 components and functions are controlled by a PC system. The system environment is fitted with various modes for manual and automatic wafer processing, recipe administration, an MS SQL data base to log the system operation data, as well as an SECS/GEM interface for the process control system.
Process flow and calculation of residence time
To fulfil the high homogeneity requirements in the IonScan applications, each wafer has to be processed in a specific way. Before ion beam trimming, it is required to measure the film thickness or the frequency error of each wafer separately. This measurement is regularly carried out by an appropriate metrology (RF probes, Ellipsometry).
For the first step, it is necessary to calculate the residence time for a known etch profile of the ion beam. The mathematical representation of the problem leads to a convolution between the residence time t(x, y) to be found and the etch function R(x, y) of the ion beam, which has to comply with the film thickness error z0(x, y) (Fig. 5). The two-dimensional etch function of the ion beam has to be found with static and dynamical test etching operations, which are carried out specifically for each material and for each parameter set of the ion beam source. /1.1/
In the frequency domain, convolution operations can easily be executed as multiplications of the Fourier transformed functions. /1.2/
As a result, the inverse problem turns out to be in the frequency domain as follows: /1.3/
Inverse problems are generally known as sophisticated subjects in mathematical and numerical techniques, and are mostly used for applications in image processing. 7 Real problems according to /1.4/, as a rule, can not be solved exactly; they can only be solved as approximations. Approximate solutions for t (x', y') may be found by iterative methods when predefining special objectives or target criteria. In any case it is necessary to do additional arrangements in order to make these methods numerically stabile.
When executing the iteration in the frequency domain, transformation back into the space domain is carried out after each iteration step i, and residual error f of the calculation is determined: /1.4/
Based on the error function, the new residence time matrix t(i+1) is calculated with an damping factor. The iteration is aborted either after achieving a predefined cycle number or if dropping below a residual error of the iteration.
The residence time matrix provides the wafer specific data for the axis system control. Finally, they are transformed into local velocity and acceleration data.
For the process control, not only the specific wafer residence time data is incorporated, but also recipe data specific to each material to be trimmed. These recipe data include the twodimensional removal function of the ion beam, the settings of the ion beam source and neutralizer, wafer geometry, as well as data for helium cooling (Fig. 5).
The wafer is machined with these input data, without additional feedback of the process.
In the IonScan 800 system, special software IonTrim is available for residence time calculation according to the above described method. IonTrim was engineered particularly for this technique. Fig. 6 illustrates the user interface enabling access to various functionalities like inter- and extrapolation, filtering, residence time calculation, process error analysis and calculation of the axis control data. For modelling and optimization of layer thickness trimming, IonTrim can be not only installed at the IonScan 800 system, but also any other PC.
Frequency trimming of Bulk Acoustic Wave (BAW) devices
High-frequency components for the mobile radio technology increasingly use Bulk Acoustic Wave (BAW) rather than the Surface Acoustic Wave (SAW) components. The reasons for this change results from several advantages like enhanced product characteristics, smaller device size, less sensitivity against influences from the outside, such as temperature or electrostatic discharge, as well as the lower production costs based on as wide as possible standard CMOS technologies, thus avoiding special materials for substrates.
The main item of each BAW device (Fig. 7) is a piezoelectric film regularly made of aluminium nitride and contacted by two electrodes. To generate an acoustic resonator, the thickness of the piezoelectric film has to be Ï/2 of the wavelength of the transversal acoustic wave.
The resonator has to be sufficiently acoustically isolated from the substrate material. Free Bulk Acoustic Resonator (FBAR) arrangements obtained this by building the resonator on a cavity. The Solid Mounted Resonator (SMR) principle (s. Fig. 7) achieves with insulation by an acoustic mirror made of alternating Ï/4 layers with high and low acoustic impedance. Depending on the impedance differences, such as between tungsten and silicon oxide, it may be possible to achieve an excellent acoustic isolation even with only a few films.
The frequency is finally tuned with a low additional mass, which is deposited onto the upper electrode as another film, mostly silicon nitride.
The operation that makes the production of BAW resonators very demanding is exact adjustment of the required film thickness values, in order to keep the low frequency tolerance range of about 0.1 %. It is also necessary to guarantee an adequate accuracy of the film thickness values across the whole wafer, which cannot be obtained in these narrow tolerances with standard semiconductor technology equipment.
The IonScan 800 is a system suitable to manufacture these components. The IonScan 800 is capable of adequately trimming of all films in a BAW stack. In addition to the film thickness trimming of the mass load, IonScan can also be applied for trimming of the piezo-resonator and the acoustic mirror. With this step like trimming strategy not only the final variation of the device frequency is better met but also other device parameters like the Q-Factor gets clearly improved.
Fig. 8 demonstrates the thickness distribution of a 150 mm Si3N4 film, measured by ellipsometry, before and after ion beam trimming. With the IonScan 800 system, it is possible to correct film thickness errors arbitrarily distributed across the wafer. The local resolution of the technique is significantly determined by the standard deviation of the ion beam profile. In the example demonstrated, the ion beam was run with argon. For Si3N4, in the focus of the ion beam a removal rate of 20.0 nm/s and a volume rate of 6.1 x 10 3 mm3/s are achieved. Under these working conditions, base etching at all wafer positions is only 1.7 nm.
Typical rates for materials to be processed range from approximately 10 to 30 nm/s for argon processing. With reactive gases, one may increase the rates three- or fourfold, depending on each material. Due to the reserves in the axis parameters, the IonScan 800 system is capable of handling such high milling rates without any problem.
In the example demonstrated in Fig. 8, the average error is reduced by a factor of about 150, and the standard deviation of the film thickness error by about 30. After processing, there remains only a 0.46 nm deviation from the nominal film thickness at a standard deviation of 0.57 nm across the whole wafer. Fig. 9 represents the film thickness distribution before (red) and after trimming (blue). The process time for the wafer was less than 5 min.
A slight processing error appeared towards the wafer edge. These marginal effects result from the calculation and the extrapolation procedures used. These deviations may be compensated in the software when defining a locally variable milling rate.
Other applications of IonScan technology
Beside the trimming of BAW devices IonScan can also be applied for trimming surface acoustic wave (SAW) devices. The structure of a SAW device is quite simple compared to the film stack of a BAW. A series of geometrically defined contact fingers generates in a piezoelectric substrate (Quartz, LiTa, and LiNb) a surface acoustic wave which is picked up by another series of contact elements. Different combinations of contact materials and substrate materials are used due to the device specification.
Trimming of SAW devices requires a certain selectivity between the substrate and the contact material. Such a selectivity might be adjusted with the IonScan for a wide range by adapting the ion energy, the process gas composition and the incident angle.
Beside the selectivity adjustment, trimming of SAW devices requires control of a very low materiel thickness of only a few nm to be trimmed exactly. The low base etching of the Ion-Scan enables this tool to be very favourable for this application.
Another application of IonScan is based on an unsatisfactory performance of chemical mechanical polishing (CMP) for some challenging applications. Using the IonScan technology a Post- CMP processing is able to satisfy this demand or even to reduce the number of polishing steps.
Favourable film homogeneity is only one issue addressed by the trimming technology. Fig. 10 demonstrates the system performance on a 200 mm tungsten wafer adjusting the film thickness with an exactness of 0.6 nm and a remaining RMS of 2.1 nm. A similar performance is demonstrated for a variety off different metals and dielectrics.
Beside the film homogeneity, for several applications selectivity between different materials has to be controlled. For processing materials synchronously, no selectivity and identical processing rates are required. IonScan may also be used to reduce step heights between different materials produced in CMP processing. For this application a high selectivity between the materials has to be adjusted.
Beside ion energy and process gas, the tilt capability of the wafer stage allows a control and variation of the selectivity between certain materials. Fig. 11 demonstrates a continuous variation of the process rate and selectivity between a metal and a dielectric by variation of the ion incident angle.
The last application which IonScan has been successfully designed for is the ion beam figuring of optical surfaces. A slightly different system layout of the IonScan 1200 allows handling and processing of heavier optical samples.
By means of the residence time controlled ion beam the surface of the optical elements gets etched down to the required optimum contour. Ion beam figuring is used for example for the finishing of stepper lenses and astro-optics.
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