A Guide to BEPE Series
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Ⅰ．Introduction
Ⅱ． What kind of machine is a BEPE?
●The Basic Idea of BEPE Series is Simple.Ⅲ． Let's Try a BEPE
●Appearance of a BEPE●How To Use QR120KⅣ．Future of BEPE series
●Options of BEPE series●Plans for BEPE SeriesⅠ．Introduction
What is a 2-dimension Ellipsometer?
The name, '2-dimension Ellipsometer', which may not sound familiar to many of you is a new type of ellipsometer which is designed with a revolutionary idea.
Surface of the solid has been especially researched as a active phase mainly by the semiconductor industry. Ellipsometer has been used widely as one of the most sensitive film thickness measuring device which can also measure the refractive index simultaneously.
When a plane polarized light of some azimuth angles are irradiated to the sample, the valence electron vibrates, and a new electric field is generated through the dipole moment which was created by this vibration. This dipole moment is proportional to the external electric field (plane polarized light) and to the overlapped electric field with other dipole moment. The electric field which passed the first layer would similarly vibrate electrons of the second layer (i.e. substrate). Electric fields, because of dipole moments are generated in the first and second layers, and the superpose of them are observed as reflected elliptic polarized light. It is the principle of the ellipsometry to measure the conversion matrix of this injected plane polarized light and the reflected elliptic polarization.
The refractive index of the solid is a function of the density and the polarization rate of the atom. By measuring the refractive index, data on the uniformity of a substrate or a thin film, and on defects in the internal structure can be obtained.
A New Type of Ellipsometer is Wanted.
Ellipsometer have been playing an important role for the development of the surface technology. However, with the development, needs for the measurement of a thinner, larger sample with finer resolution are growing. There is a limit to the ability of the conventional ellipsometer, which we call 'single ellipsometer'. Thus it can not cope with such growing needs. A new break-through idea on the ellipsometer is wanted.
In single ellipsometer, the limit of film thickness for measurement is set thicker than its principle ability. There is a reason for this. When the beam is set narrower to observe in more detail, the error in the expanding angle of the light beam grows through diffraction. That is, the aberration in the angle of incident light grows. But if a wider beam is used so that the error is reduced, accurate measurement can not be made on the details of irregularities of the plane of the sample. Especially, error grows with thin film in initial state of formation because it has a Island Structure.
Ⅱ． What kind of machine is a BEPE?
The Basic Idea of BEPE Series is Simple.
First, a parallel beam with a wide area which can cover the entire sample is radiated on the sample surface. This wide parallel beam is a bundle of enormous number of narrow beams of single ellipsometers in parallel. The light which is reflected from the sample is observed by the imager with high amount of pixels. In other words, this is a device that put several 100,000 photo diodes of single ellipsometers together into parallel: a 2-dimension Ellipsometer. Therefore, BEPE series are 2-dimension Ellipsometers of which their beams are expanded and several 100,000 single ellipsometer is put together into one machine. From this simple basic idea, a machine was created which solves many difficulties of a single ellipsometer at once . Because there is no need to scan the sample, there would be no error in the angle of incidence from vibration, pitching, or rolling of the scanner. Therefore, high quality measurement with high speed and high resolution of the sample is possible now.
The hardware of a BEPE is designed to widen the range of measurement for the area where the ΔΨchart is concentrated.
Simple Explanation on the Optical System of BEPE Series
Our 2-dimension Ellipsometer consists of two arms: an incident-side arm and a reflection-side arm. The incident-side arm consists of the light-source module, the polarizer module, and the collimator module. The reflection-side arm consists of the objective module, the analyzer module, and the imaging module.
The light from the laser diode is collimated. Next, it is transformed into a plane polarized light by Glan-Taylor prism. Light which passed the polarizer is irradiated on the sample surface.
The light which is reflected from the sample passes the analyzer which is arranged to a suitable azimuth angle. The crystal wave plate can be inserted in front of the analyzer. Light which passed the analyzer is imaged by the imaging system on CCD camera.
Two Sample Models of Our Ellipsometer
The refractive index of the film thickness is calculated by assuming the real part of the refractive index as an independent variable and the imaginary part of the film thickness as an error parameter and using the quasi-Newton Method so that the error is approximate to zero.
The usual calculation of refractive index and film thickness is Smooth Surface Model. Calculation time is about 10 minutes for every 120,000 points.
Ellipsometer measures the ruggedness of a thin film by setting the standard coordinate on the substrate; unlike STM and AFM in which the device measures the ruggedness on the surface by putting the standard coordinate on outside of the sample.
The calculation in which the angle of incidence is assumed as constant is insufficient in case of a film formed on a rough surface which has dispersion. We recommend to calculate with Rough Surface Model at such case. The calculation time with Rough Surface Model would be about four times longer. However, its effectiveness is proven for certain kinds of films.
The optical system of BEPE series is designed to satisfy the imaging condition. Thus when measuring small parts of the sample, independent results for each measurement is obtained. In case of dispersed surface, the measured value of each pixel of the imager corresponds to the reflected light from each small part of the sample. In such case we recommend to use Rough Surface Model.
Two Principles of Measurement of BEPE Series
When measuring a multilayer, it is measured with several angles of incidence so that the measuring parameter is increased. However, it would not be a measurement of a same location on the sample surface when different angles of oblique incident light are used. It is necessary to control the angle of incidence for the improvement of the accuracy of measurement and the adjustment of resolution in the side of the sample.
BEPE measures a multilayer film with newly developed algorithm.
And the measurement of Interferometry became possible with BEPE series. Merits of a BEPE's interferometry compared to the usual method, are: the refractive index of film is measured, etc.
The two principles of measurement, the measurement of Ellipsometry and Interferometry method, can be selected according to characteristics of each film.
A World through the 2-dimension Ellipsometer
The figure is distribution of the film thickness and the refractive index when SiO2 on Si substrate is measured by microscopic module. It can be seen that the mountain of the film thickness gradually heightens and the valley gradually lowers. It is a world which became possible to be seen for the first time through the 2-dimension Ellipsometer.
The resolution of the measurement and the display can be freely set with the software. It is possible with our software to have a measurement with the resolution rough and leveled, to reduce measuring points while keeping the resolution delicate, or to change the method of measurement according to the parts of the sample.
With 120,000 points, maximum calculation time is 10-40 minutes. If the measuring point is reduced, measuring time shortens proportionally. However, when the resolution of the measurement is made rough, the microscopic structure can not be observed even with our 2-dimension Ellipsometer, and you would get a similar film thickness as with a conventional machine.
Dramatic Improvement of Efficiency in Development
The measurement time of the entire sample with QR120K is about 30 seconds. It would take about 1 minute to finish the calculation of ΔΨ(120,000points). The calculation of the film thickness and the refractive index would take 10-40 minutes.
You can select 'In Situ' Mode which does only the measurement and make the calculation later collectively, Data Strage Mode which can calculate up to ΔΨ, or Calculation Mode which can calculate upto the film thickness and refractive index.
It is possible to make the calculation only with QR120K, or forward the data outside to other computer for calculation, or share the calculation process between QR120K and outside computer.
With a BEPE, high-speed measurement means high accuracy.
2-dimension Ellipsometer And High Technology
What made the practical use of 2-dimension Ellipsometer possible is modern high-technology.
When the beam is expanded the resolution lowers accordingly. Developing a 2-dimension Ellipsometer would not have been possible without a high-output laser which have a stable wavelength and wave surface characteristics; also, without a CCD device with high pixels, high sensitivity, and low noise.
In addition, a computer that have a huge memory and can process huge graphic data at high-speed came into existence.
BEPE series is supported by such high-technologies. It is clear that our machine fits with the trend of future technology.
We hope that BEPE series can contribute to the further development of technology such as semiconductor.
Ⅲ． Let's Try a BEPE
Because we put importance on the performance, we did not try to reduce the size which could jeopardize its accuracy. Thus the size is rather large, but its functions are far more larger.
In this size, the optical system and the control system such as computers are unified in the 2-dimension Ellipsometer. The machine is completely covered so that the turbulence light would not be a bother and it can be easily operated while sitting on a chair.
The sample stage has the size to be able to measure 8" diameter sample and the smoothness of the surface is 1μ or less. The sample is put on the stage horizontally and is adsorbed by vacuum. The stage can be moved in the direction of the right angle to the optic axis 150mm by pulse motor.
We will respond to your request regarding the color of the machine and the shape of the cover so that it can give a nice atmosphere into your laboratory like a furniture.
We plan to downsize our machine carefully without jeopardizing its performance in the future.
With a single ellipsometer, you have to set the conditions such as refractive index of the substrate and imaginary parts of the film's refractive index before the measurement. This is one of the reasons which made an ellipsometer difficult to operate. With our 2-dimension Ellipsometer, there are also conditions that have to be set. However, we developed Operator Mode which can make it much easier to operate. By using Operator Mode, you can operate by pushing a key button like a copy machine.
When the power switch on the side is put on, it will automatically measure the constants of the system and adjust it accordingly. This will take about 2 minutes, and then it will be ready for measurement.
Next, number of measurement is inputted. Number of measurement can limited (up to 999) or can be endless.
When the start button is pushed, the door is opened and the loader moves forward. Please put the orientation flat of the sample on the stage adjusting it by the flat main position drawn on the stage as a rough standard.
When the start button is pushed again, the door closes, and the loader moves to the standard position and begins measuring.
With Engineer Mode, setting the condition, more complex measurement, or the display are possible.
QR120K's keyboard is minimalized to functions sufficient for the operation.
Ⅳ．Future of BEPE series
The optical system of our 2-dimension Ellipsometer is divided to several modules by its functions. It is possible to upgrade by simply changing one of these modules as a component.
The first of the option which is currently under preparation is a module for imaging system. The resolution of microscopic module, which can measure the surface more in more detail, is 0.05×<0.15mm. Zoom module which can control the resolution continuously, and 1:1 module which adjust the difference of resolutions of XY to 1:1 are under preparation.
Second of the option under plan is a polarizer module for high-speed measurement. If it is exchanged with the polarizer at the incident side, measurement of the entire sample can be done within about 5 seconds.
Third of the option under plan is a multi-wave module. By making a large change in the incident light source a multi-wave ellipsometer with multiple wavelength will be possible.
First, we plan to make the loading of sample automatic. Total size of machine will be reduced with this option.
New possibilities will develop with the emergence of high-resolution camera for HDTV.
Measurement with multiple wavelength is also our current theme. (patent under application)
On the other hand, we would like our 2-dimension Ellipsometer's high-speed processing to become more effective in the manufacturing line of semiconductor.
