WO2007040530A1

WO2007040530A1 - Novel temperature insensitive low coherence based optical metrology for nondestructive characterization of physical characteristics of materials

Info

Publication number: WO2007040530A1
Application number: PCT/US2005/035417
Authority: WO
Inventors: Wojciech J. Walecki; Phuc Van
Original assignee: Koo, Ann
Priority date: 2005-09-30
Filing date: 2005-09-30
Publication date: 2007-04-12

Abstract

This invention is a device for measuring of absolute distances by means of low coherence optical interferometry. The proposed apparatus eliminates thermal of the conventional fiber optic interferometers caused by variation of the refractive index of the optical fiber material to change of the temperature.

Description

NOVEL TEMPERATURE INSENSITIVE LOW COHERENCE BASED

OPTICAL METROLOGY FOR NONDESTRUCTIVE CHARACTERIZATION

OF PHYSICAL CHARACTERISTICS OF MATERIALS

BACKGROUND OF THE INVENTION

The low coherence optical interferometry has been proven to be an effective tool for characterization of thin and ultrathin semiconductor wafers and other materials. It is particularly valuable for measurement of thickness of wafers thinner than 150 μm, or wafers mounted on dielectrics materials such as tapes or sapphire plates. For these applications standard well-established non-contact thickness gauges such as air pressure of capacitance gauges do not provide direct physical results, which meet industry process window or require introduction of additional experimental parameters While the bulk of effort was concentrated in the area of metrology for manufacturing of ultra thin Silicon wafers, other very promising areas include metrology of III-V materials mainly for opto- electronics and microwave applications and metrology of micro electro mechanical (MEM) structures.

It has been recognized that low coherence optical interferometry can be used to measure absolute distances between a probe and a wafer. The accurate distance ranging measurements are necessary when measuring physical characteristics of the wafer such as

bow and warp. In practice the absolute distance ranging measurements were not very

accurate due to thermal drift of the optical elements of the system. The present invention reduces this effect, and in particular eliminates the influence of the thermal drift of the fiber optic components on performance of the low coherence optical interferometer. The apparatus used in the measurement is a fiber optics interferometer shown in Figure 1, which represents a low coherence Michelson interferometer Light emitted by a low coherence source is split by means of beam-splitter into two beams first called later reference beam propagates in the reference arm of the interferometer, second portion of the beam later called signal beam propagates in the signal arm The polarization of the reference beam is controlled by means of polarization controller, and is collimated by means of lens on reflective element such as mirror or corner cube retro reflector Mirror

resides on delay stage such that the length of the optical path of the reference beam is

controlled by means of optical delay stage The reference beam is reflected from the reflective element, passes again through polarization controller and is partially transmitted by beam-splitter and directed to detector The signal beam is collimated by lens and impinges sample The reflected portion of the signal beam is directed by means of beam splitter towards detector

The intensity of the optical beam impinging detector surface I_d is given by

I_d =W_r +I {{K(t + τ)-E,{t))) (1)

where I_s and I_r are signal and reference beams, τ is delay equal to difference of the

optical paths of the signal and reference beams, / is time, E_r and E_s are electric fields of

reference and signal beams respectively, and angle ( ) bracket means averaging over t When optical paths of the signal and reference beams differ by much more than the coherence length of the source, the intensity detected by detector is simply equal to the first τ independent term in the Equation (2), however when the path of the reference

and signal beams are different within the coherence length than the second term becomes

comparable to the first term This phenomenon is well known and was applied in past for

distance ranging since the optical delay time is related to difference in length Δ/ between the reference and signal beams by simple formula

τ = 2-n- ΔI (2) where n is refractive index of the medium The Equation 2 implicitly assumes that medium is non-dispersive within the bandwidth of the light source

An example of the interferogram of light reflected from the surface of reflective (and nontransparent) sample is shown in Figure 6 and Figure 7

In principle the position of the center burst can be used directly for distance ranging Results of such measurement are presented in Figure 8 The result presented in Figure 8 reveals significant drift Experiments in which various elements of the low coherence interferometer shown in Figure 1 were heated indicated that change of the optical path of signal arm contributes the most to thermal drift observed in Figure 8 The

temperature coefficient of refractive index of glass is 20 ppm/°C

This means that in case of 2 m optical fiber change of the optical length of the

fiber is of the order of 40 microns/°C The change of the physical length of the fiber due

to physical thermal expansion is less significant is coefficient of thermal expansion is several times smaller then the temperature coefficient of refractive index BRIEF DESCRIPTION OF DRAWINGS

Figure 1 represents a conventional low coherence interferometer without reference plane

Figure 2 represents a low coherence interferometer with reference plane in collinear configuration

Figure 3 represents a low coherence interferometer with reference plane in collinear configuration including ray paths

Figure 4 represents a low coherence interferometer with reference plane in non-collinear configuration Figure 5 represents a low coherence interferometer with reference plane in non-collinear configuration including ray paths

Figure 6 depicts an interferogram of light reflected from reflective and nontransparent

sample

Figure 7 is an example of an interferogram (expanded scale) Figure 8 depicts a temperature drift of the measured distance using system shown in

Figure 1.

Figure 9 depicts an interferogram of light reflected from reflective and nontransparent

sample suing probe with reference plane

DESCRIPTION OF THE INVENTION

In Figure 1 light emitted by a low coherence source 501 is split by means of a

beam-splitter 503 into two beams the first beam called later reference beam propagates in the reference arm of the interferometer 508, the second beam called signal beam propagates in the signal arm 505. The polarization of the reference beam is controlled by means of polarization controller 509, and is collimated by means of lens 510 on reflective element 511. Reflective element 511 resides on delay stage such that the length of the

optical path of the reference beam is controlled by means of optical delay stage 511. The reference beam is reflected from the reflective element, passes again through polarization controller 509 is partially transmitted by beam-splitter 503 and directed to detector 502. The signal beam 505 is collimated by lens 506 and impinges sample 507. The reflected portion of the signal beam is directed by means of beam splitter 503 towards detector 502.

In Figure 2 a low coherence interferometer is shown with a reference plane in configuration not requiring the use of a cube beam-splitter. In this embodiment of the invention a low coherence interferometer controller 5 is connected by means of electric cables 6 to a computer 7. Single mode fiber-optic cable 4 guides light from a controller 5 to a beam shaping assembly 3. The beam-shaping assembly is equipped with the optical flat 2 mounted to this assembly by means of a mount 20. Beam shaping assembly 3 is mounted on a wafer chuck frame 10. Measured wafer 1 is also placed on the wafer chuck frame 10.

In Figure 3 a low coherence interferometer is shown with a reference plane in a configuration not requiring the use of a beam-splitter including ray paths. Light 101

produced in optical controller 5 is transmitted through an optical cable 4 and emanating

from the beam shaping assembly 3 is impinging the first surface of an optical flat 2, is partially transmitted through the second surface of optical flat 2. A reflected portion of the light 103 from the second surface is collected by the beam shaping assembly 3. The transmitted portion of the beam 101 is reflected from the wafer 1 and forms a reflected beam 102. The reflected beam 102 passes through an optical flat 2 and is collected by a beam shaping assembly 3. The beam shaping assembly relays the beams 102 and 103 through an optical cable 4 to a controller box 5. Controller 5 is measuring the incoming radiation and transmits a measured signal using electrical cables 6 to a computer 7. A computer 7 analyses the measured signal and displays the results of measurements.

In Figure 4 a low coherence interferometer is shown with a reference plane in configuration using a cube beam-splitter. In this embodiment of the invention low coherence interferometer controller 5 is connected by means of electric cables 6 to a computer 7. Single mode fiber-optic cable guides light from a controller 5 to a beam- shaping assembly 3. In Figure 5 the beam-shaping assembly is equipped with a cube beam splitter 201 mounted to this assembly by means of mount 20. Beam shaping assembly 3 is mounted on wafer chuck frame 10. Measured wafer 1 is also placed on the wafer chuck frame 10.

In Figure 5 ray traces are shown in the interferometer described in Figure 3. Light 9 emanating from the beam shaping assembly 3 propagates through the beam splitter cube 201. A portion of the radiation is split into a separate beam 91 and directed towards a mirror 200. A mirror 200 reflects this portion of radiation toward the beam splitter 201 by forming an optical beam 92. Optical beam-splitter 201 is directing a portion of the beam 93 towards a beam-shaping assembly 3. Other portions of the beam 9 leave the beam splitter cube 201 and form a beam 93. This beam 93 is reflected from the wafer and forms another beam 94. Just like in Figure 2, the beam shaping assembly relays the beams 94 and 92 through an optical cable 4 to a controller box 5 The controller 5 is measuring the incoming radiation and transmits a measured signal using electrical cables 6 to a computer 7 The computer 7 analyses the measured signal and displays the results of measurements In Figure 6 an example is shown of the interferogram of light reflected from the

surface of reflective (and nontransparent) When optical paths of the signal and reference beams are approximately equal strong interference feature is observed This feature is referred sometimes in Fourier transform interferometry as "center burst"

In Figure 7 details of the center burst oscillations are revealed, which are spaced by approximately half of the wavelength of incident radiation λ /2 as shown in Figure 7 representing expanded interferogram

In Figure 8 the result of the distance ranging measurement using the system in

Figure 1 is shown The result reveals large thermal drift of the system

In Figure 9 an interferogram of light is shown reflected from a reflective and nontransparent sample using probe with reference plane as described in Figure 4 The interferogram reveals two features The left feature corresponds to reflection from a reference plane, while the right feature represents reflection from the reference plane

In Figure 10 the result of the range measurement are shown using the system shown in Figure 1 The result reveals that thermal drift of the system has been reduced

by about factor of 100

In order to eliminate the influence of the thermal drift of the length of the optical path in fiber, the optical head is redesigned in such way as to introduce the additional reference plane residing in the signal arm of the interferometer as discussed in Figures 2, 3, 4, and 5. The interferometer in this configuration is measuring the interference features resulting from the reflection from the reference plane and reflections from the surface of the sample. A typical interferogram for such a measurement (in this particular case we used configuration shown in Figure 5) is shown in Figure 9. The interferogram reveals two features, one corresponding to reflection from the reference plane and a second feature corresponding to the reflection from the sample surface. The absolute position of each of these two features is subject to thermal drift due to changes of the refractive index in the optical fibers. The difference between the positions of these two features does not depend on drift of the optical path in fibers; both features suffer the same drift. The measurements using this configuration demonstrated that thermal drift

was reduced to below 0.6 μm in 10 minutes interval.

Claims

What is claimed

1 An apparatus for distance ranging of the surface of materials by means of an optical low coherence Michelson interferometer, in which a signal arm of the interferometer comprises a single mode fiber-optic guide connected to a beam shaping assembly which comprises an optical partially reflective optical flat of an optical probe constituting a reference plane which directs a portion of the signal beam directly to the interferometer

2 An apparatus described in Claim 1 in which the reflective element is a semitransparent mirror

3 An apparatus described in Claim 1 in which the reflective element is a slab of uncoated transparent material

4 An apparatus for distance ranging of the surface of materials by means of an optical low coherence Michelson interferometer, in which a signal arm of the interferometer comprises a single mode fiber-optic guide connected to a beam shaping assembly which comprises an optical beam splitter directing a portion of an optical beam towards a mirror constituting a reference plane which is essentially perpendicular to a portion of the optical beam and directing it towards the beam splitter and back to the interferometer

5 An apparatus as described in Claim 4 in which the beam-splitter is a cube beam¬

splitter

6 An apparatus as described in Claim 4 in which the beam-splitter is a flat plate beam-splitter

7. An apparatus as described in Claim 4 in which the beam-splitter is a 2x2 fiber optic coupler.

8. An apparatus as described in Claim 1 further comprising two sensors positioned such that light emanating from the first sensor propagates along the same line as light emanating from the second sensor, in a direction towards the first sensor while light emanating from the second sensor is propagating towards the first sensor, while both sensors are separated by a certain distance allowing insertion of slab of measured materials which are positioned in such way that surfaces of the slab are approximately perpendicular to the direction of propagation of light emanating from the sensors.

9. An apparatus as described in Claim 4 further comprising two sensors positioned such that light emanating from the first sensor propagates along the same line as light emanating from the second sensor, in the direction towards the second sensor while light emanating from the second sensor is propagating towards the first sensor, while both sensors are separated by a certain distance allowing insertion of a slab of measured materials which is positioned in such way that surfaces of the slab are approximately perpendicular to the direction of propagation of light emanating from the sensors.

10. A method of measurement of the distance between a partially reflective optical flat of an optical probe, called a reference plane, and a measured surface comprising of the following steps: measurement of the interferogram resulting from the interference of light

propagating in signal arm of an interferometer and reflected by a reference plane residing in the signal arm with the light propagating in the delay arm of the interferometer, calculating the position of the reference plane using the interferogram, measurement of the interferogram resulting from the interference of light propagating in the signal arm of an interferometer and reflected by the measured surface, calculating the position of the measured surface using the interferogram, and subtracting the calculated position of the surface from position of the reference plane in order to obtain the relative position of the measured surface with respect to the reference plane.

11. A method as described in Claim 10 further comprising the following steps: measurement of the interferogram resulting from the interference of light propagating in a signal arm of interferometer and reflected by a reference plane residing in the signal arm with the light propagating in the delay arm of the interferometer, calculating the position of the reference plane using the interferogram, measurement of the interferogram resulting from the interference of light propagating in signal arm of interferometer and reflected by the measured

surface, calculating the position of the measured surface using the interferogram, and subtracting the calculated position of the surface from position of the

reference plane in order to obtain a relative position of the measured surface

with respect to the reference plane.

12. A method as described in Claim 10 further comprising the steps of: measurement of the optical path between first surface of a transparent material and reference plane of the optical probe, measurement of the optical path between second surface of the transparent material and the reference plane, calculating optical path of light traveling between first and second surface of

the transparent material, and calculating thickness of optical material using formula: t = LI n where L is optical path of light traveling between first and second surface of the transparent material, and n is a group refractive index of the transparent material.

13. A method as described in Claim 12 further compromising the steps of: positioning material of known thickness T between two optical probes such that first optical probe is positioned in close proximity of the first surface, and the second probe is positioned in proximity of the second surface of the

material of unknown thickness,

measuring of the distance d_x between reference plane of the first probe and

first surface of the material of known thickness,

measuring of the distance d₂ between reference plane of the second probe and

second surface of the material of known thickness, calculating distance D between the reference plane of the first probe and

reference plane of the second probe using formula: D = d_λ + d₂ + T , positioning material of measured unknown thickness t_x between said optical

probes such that first optical probe is positioned in close proximity of the first surface, and the second probe is positioned in proximity of the second surface of the material of unknown thickness comprising of the following steps,

measuring of the distance d_xx between reference plane of the first probe and

first surface of the material of known thickness,

measuring of the distance d_2X between reference plane of the second probe

and second surface of the material of known thickness, and

calculating measured thickness t_x using following formula:

t_x - D — t_λX — I_2x .