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In-Situ thickness measurement methods and Anti Reflection CoatingsBy:Eshaan Gupta2009PH10716

INTRODUCTIONThe thickness of thin lms grown or deposited onto a substrate are crucial to the performance of the end product in the semiconductor industry.

Better metrology equipment is needed to increase yield and lower production time.

In-situ measurement of the thickness of lms would eliminate post-growth/deposition metrology time for devices.

Since the thickness is continuously monitored , precise control over the film thickness is gained.

Thin Film Measurement TechniquesContactNon-ContactMethods include techniques like Atomic Force Microscopy , profilometers, etc.Cannot be used for in-situ measurement.Methods include gravimetric, eddy currents and optical methods.Used extensively for in-situ measurement.

Quartz Crystal Monitor (QCM) QUARTZ

Has piezoelectric properties: develop an electric potential on the application of mechanical stress.

Use in crystal oscillator : resonant frequency of quartz crystal oscillator changed by mechanically loading it principle used for very accurate measurements of very small mass changes in quartz crystal microbalance and thin film thickness monitors.

Quartz Crystal Monitor (QCM) Measures mass per unit area by measuring the change in frequency of quartz crystal ocillator.

Applying alternating current to the quartz crystal will induce oscillations. With an alternating current between the electrodes of a properly cut crystal, a standingshear waveis generated. Resonance is disturbed by addition or removal of a small mass due to film deposition at the surface of the acoustic resonator.

Q factor ( ratio of frequency and bandwidth) can be as high as 106 . Such a narrow resonance leads to highly stable oscillators and high accuracy in determination of resonance frequency.

Quartz Crystal Monitor (QCM) QCM exploits this ease and precision for sensing.

Frequency of oscillation dependent on the thickness of the crystal.

During normal operation , keeping all other influencing variables constant ; change in thickness correlates directly to change in frequency.

As mass is deposited on the surface of the crystal, the thickness increases; consequently the frequency of oscillation decreases from the initial value.

With some simplifying assumptions, this frequency change can be quantified and correlated precisely to the mass change using Sauerbrey's equation.

Quartz Crystal Monitor (QCM)

Sauerbrey Equationf0Resonant frequency(Hz)f Frequency change (Hz)m Mass change (g)A Piezoelectricallyactive crystal area (Area between electrodes, cm2)qDensityof quartz (q= 2.648 g/cm3)qShear modulusof quartz for AT-cut crystal (q= 2.947x1011g/cm.s2)Assumption : Deposited mass must be evenly distributed.

Quartz Crystal Monitor (QCM)

Note : Sensor is not on substrate. Can you figure the disadvantage ?

Quartz Crystal Monitor (QCM)

Quartz resonators used in QCM , metalized with Gold electrodes.Typical Quartz Sensor

Quartz Crystal Monitor (QCM)DISADVANTAGES

Sensor cannot be placed in front of crystal because then shadowing will take place. So intensities of material deposited is not same on both sensor and substrate.

Measures the rate and thickness changes on the sensor crystal and not the substrates. Therefore, a successful application of QCM requires a consistent relationship between the deposition rate on crystal versus rate on substrates.

Life of sensor crystal is material limited meaning it will fail once a sufficient amount of material has been deposited.

In-Situ EllipsometryPrinciple of ellipsometry

Ellipsometer emits polarized light from its input unit , or light source, and reflects the light at one of several angles off the surface of the film.

The optical properties and thicknesses of the different layers in the film will change the polarization of the incident light that was emitted.

Changed light is reflected and then analyzed by detector and the analyzer of the ellipsometer.

Using a computer program the measured data for various samples and angles can be processed to generate thickness of the materials composing the film.

In-Situ EllipsometryLight- Electromagnetic Wave

Light is an electromagnetic wave that is described by Maxwells equation for electromagnetic fields. In a non-conducting, non-dispersive medium electric field is the electromagnetic plane wave :

In-Situ EllipsometryLight- Electromagnetic Wave (contd.)

If the imaginary part (k, or the extinction coefficient) of the complex index of refraction is nonzero, then the amplitude (the real part of electric field vector) changes :

Because of the negative sign in exponential amplitude decays as z increases. Implies that as the E-field travels through an absorbing medium , it will become weaker.Amplitude changes as electric field oscillates !

In-Situ EllipsometryLight- Electromagnetic Wave (contd.)

Penetration Depth : Distance the wave travels before it decays 1/e from its original amplitude.

Important concept in Ellipsometry : many materials exhibit large extinction coefficients (k) such that a light beam may only penetrate a few tens of nm or less !

No information can be gained unless the light penetrates it sufficiently and then is able to reflect back out of the sample.

This is why it is practically impossible to measure the thickness of films greater than about 50nm thick with an ellipsometer !

In-Situ EllipsometryPolarization

Convention has created two directions when describing the reflection and transmission of light relative to a surface :

p-polarized lying in the plane of incidence s-polarized perpendicular to the p-polarized and plane of incidence. Electric field can be defined using the p-direction and the s-direction since they form orthogonal vectors.

Ellipsometry ultimately compares these two components of the light reflected from the surface, to analyze the properties of the reflected surface.

In-Situ EllipsometryMeasurement

The sample film can be considered as an optical system that modifies the polarization state of the beam of light.

The actual values measured are expressed as psi ssss and delta . These values are related to the Fresnel reflection coefficients for p- and s- polarized light respectively.

From the above equation it can be seen that psi and delta correspond to the amplitude and phase of rho. By measuring these values and fitting a model to it , the optical properties and thickness can be deduced using a computer program.

In-Situ Ellipsometry

In-Situ Ellipsometry

In-Situ Ellipsometry

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In-Situ Ellipsometry

In-Situ Ellipsometry

In-Situ Ellipsometry ADVANTAGES

Very sensitive for thin film measurements: derived from determing the relative phase change of the beam of reflected polarized light.

Absolute intensity of the reflected light does not have to be measured. This allows ellipsometric measurements to be more accurate than simple intensity reflectance measurements.

From the equation it measures the two reflectance values

for p- and s- , it provides accurate and producible results unique to ellipsometers.

Since it measures relative intensity and phase rather than absolute values, it can be operated in any type of environment . So not necessary to maintain the ellipsometer in a dust free area.

In-Situ Ellipsometry RESTRICTIONS

Incident angles cannot be varied : would require changing the vacuum chamber ports (on which ellipsometer is mounted) means that ports have to be bent at varied angles , which is impossible for fixed stainless steel chambers.

Alignment and calibration must be done before the deopsition ,i.e, on substrate. When metal is deposited on the film , calibration may give erroneous values that can affect the accuracy of measurements.

Anti-Reflection Thin Films INTRODUCTION

When a light wave is incident on a surface , a part of it is reflected and a part of it is transmitted.

Purpose of AR films is to minimize the losses due to reflection.

Anti-Reflection Thin Films Anti Reflection Mechanism : Multi Reflection and Destructive interference.

Important parameters of the film Refractive index(n) Thickness (d)

n0n1n2

GlassFilmAir

n1dDestructive interferenceConstructive interferenceAdjust these parameters !

Anti-Reflection Thin Films FABRICATION OF THIN FILMS

Physical Vapor Deposition

Need of vacuum Approaches : (i) Resistive heating (ii) Electron Beam Controlling the thickness

Quartz crystal monitoring Optical methods for thickness monitoring

Single Layer Anti Reflection Coating

Single Layer Anti Reflection Coating

RExperimental = 0.7%

RTheoretical = 1.3%

Results of MgF2 Film

Single Layer Anti Reflection Coating Limitations of the approach :

Not always possible to find suitable material with suitable refractive index. (particularly in cases where bulk medium has low n ).

Works only in a limited bandwidth ( wavelength range).

Multiple Layer AR Coating

Reflectivity curve for a numerically optimized anti-reflection coating on a BK7 glass substrate for 1064nm and 532nm. Two layer pairs of TiO2and SiO2are used. If no suitable medium for single layer coating can be found , or if anti reflective properties are required for a very broad wavelength range, more complicated designs with multiple films are used.

Applications In solar cells , the deposition of anti reflective coating makes it possible to improve the conversion efficiency of solar radiation.

In laser systems the power of the output radiation, ensuring high laser strength of the materials.

Used by opticians in anti reflection lenses to produce less glare , which is particularly noticeable while driving at night and in front of computer screen.

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