AU2018697A

AU2018697A - Determining characteristic parameters by polarised light

Info

Publication number: AU2018697A
Application number: AU20186/97A
Authority: AU
Inventors: Jacek Michal Misiura
Original assignee: MANAKATO Pty Ltd
Current assignee: LEOTEK Pty Ltd
Priority date: 1996-03-19
Filing date: 1997-03-19
Publication date: 1997-10-10

Description

DETERMINING CHARACTERISTIC PARAMETERS BY POLARISED LIGHT

FIELD OF THE INVENTION

THIS INVENTION relates to a method and apparatus for in-situ determination

of one or more characteristic parameters of a material. In particular but not

limited thereto it relates to a modified single beam ellipsometry technique for

in-situ, real time determination of the one or more characteristic parameters..

BACKGROUND TO THE INVENTION

Ellipsometry is well-known as an optical technique for monitoring

events at the interface between two media. In a general scheme of ellipsometry, a beam of polarised light is directed onto a changing surface. The beam interacts with the surface which results in a change in the

polarisation state of the light. Measurements of the initial and final

polarisation states are analysed to determine parameters describing the interaction.

In typical experimental set-up found in the prior art, a beam from a

suitable light source (usually a laser) is passed through a polaπser to produce

light of a known polarisation. This light interacts with the optical system

(surface) under study and its polarisation is modified. The modified state of

polarisation is obtained by a polarisation analyser followed by a photodetector

The polarisation analyser is commonly a rotating polaπser and the

photodetector is commonly a photomultiplier.

Reflection ellipsometry is used for the study of surfaces and thin films.

The technique can be used to determine the parameters of surface growth (eg. Oxidation, deposition, adsorption, diffusion, etc) or surface removal (eg.

Etching, desorption, sputtering, diffusion, etc).

Reference is made to "Ellipsometry and Polarised Light" by R.M.A.

Azzam and N.M. Bashara published by North Holland, Amsterdam, 1977,

which describes ellipsometry. Determination of parameters requires analysis of

the basic ellipsometry equation:

tanψe^iΔ=-£

where Ψ and Δ are the ellipsometric parameters given by:

and

Δ =δ p -δ ^vs

where R is Fresnel reflection coefficient

E is electric vector

δ is phase shift

p, s are parallel and perpendicular components respectively

In essence, ellipsometry involves the measurement of tan Ψ, the change

in the amplitude ratio upon reflection, and Δ, the change in phase upon reflection. These parameters are functions of the refractive index of the

surface, the refractive index of the substrate, the wavelength of light used, the

angle of incidence, temperature and the film thickness.

In order to determine physical properties of a material from the optical

measurements a mathematical model must be used based upon the above

equations. One such mode has been described by D.E. Aspnes and A. A. Studna in "Applied Optics", 14, 1, (1975) 220 and Y. Hayashi in Japanese

Journal Applied Physics, 29, 11 (1990) 2514 and defines:

Ψ=-arccos(-α) 2 ^v '

and

The values a and b are determinable experimentally from the photodetector signals as:

and

where l₀ is the average reflected intensity over one full rotation of the

analysing polariser and \_κ is the measured intensity when the analysing

polariser is at angle A_κ.

Each ellipsometric measurements of polarisation state change yields one

value for Ψ and one value for Δ. Thus, with the best prior art techniques only two surface properties can be determined providing values for other

parameters are known or assumptions are made.

Prior art methods have sought to overcome this limitation by taking

multiple measurements under a variety of conditions. One such technique is

described in United States Patent No. 5166752 which describes a technique

for determining Ψ and Δ at a variety of angles of incidence of the laser beam.

In the citation the variety of angles is achieved by directing parallel light

through one or more lenses to focus the light onto the surface.

Another approach is to provide multiple ellipsometers with identical set-

ups but different angles of incidence.

The known prior art techniques are not able to effectively monitor

surface parameters in real time. Furthermore, there is no ellipsometric based

technique that can measure etch rate/deposition rate in real time. Although

current methods can measure important surface parameters they require

accurate angle of incidence control and careful mechanical and optical

alignment. There is a need for an apparatus and method that can determine and/or

monitor surface parameters in materials processing in-situ and preferably in

real time.

OBIECT OF THE INVENTION

It is an object of the present invention to provide a method and

apparatus for determining and/or monitoring one or more characteristic

parameters of a material in-situ and preferably in real time during processing.

It is a further object to overcome one or more of the limitations evident

in the prior art relating to determining and/or monitoring of surface properties

by ellipsometry.

Other objects will be evident from the following description.

DISCLOSURE OF THE INVENTION

In one aspect therefor, the present invention resides in a method of in-

situ determining and/or monitoring one or more characteristic parameters of a material during materials processing including the steps of:

directing light of known polarisation at a material;

analysing light reflected from the material to determine changes in

polarisation state;

monitoring the changes in polarisation state over time to obtain a

periodicity of the changes in the polarisation state; and

calculating one or more characteristic parameters of the material from

the obtained periodicity.

In another aspect therefor, the present invention resides in a

method of in-situ determining and/or monitoring a rate of change of thickness of a material in a surface etching or deposition process including the steps of:

directing light of know polarisation at a known material;

analysing light reflected form the material to determine changes in

polarisation state;

monitoring the changes in polarisation state over time to obtain a

periodicity of the changes in polarisation state; and

calculating the rate by dividing a characteristic thickness of the material

derived from the obtained periodicity by time required for etching or

depositing the characteristic thickness.

The step of analysing light reflected from the material may further include the step of directing the reflected light through a rotating analyser or

polariser, detecting the light with a photodetector and processing signals from

the photodetector in a processing means.

The step of processing signals from the photodetector in a processing

means may suitably be performed in a computer using ellipsometric equations.

The calculated one or more characteristic parameters may be dependent

upon one or more known parameters. In one application, if the material is

known its characteristic thickness can be determined, and an etch or

deposition rate can also be calculated. In another application, if the material is

not known but the thickness of the material deposited or removed is known,

the material can be identified. The surface temperature can also be calculated

in one preferred form of the invention.

The method may further include the step of for example performing a

Fourier transform on the signals form the photodetector to identity multiple signals of different periodicity.

The obtained periodicity may suitably be calculated by curve fitting

techniques.

The step of directing light of known polarisation at the material may

include directing multiple beams of light wherein each beam may be at a

different angle, a different wavelength or both. The changes in polarisation

state may then be monitored for different angles and wavelength.

In a further aspect therefor, the present invention resides in an apparatus

for in-situ determining and/or monitoring one or more characteristic parameters

of a material during materials processing comprising:

A source of light of known polarisation;

means for directing the light at a material;

means for analysing light reflected from the material to determine

changes in polarisation state; means for monitoring the changes in polarisation state over time to

obtain a periodicity of the changes in the polarisation state; and

processing means for calculating one or more characteristic parameters

o the material from the obtained periodicity.

BRIEF DETAILS OF THE DRAWINGS

To assist in understanding the invention preferred embodiments will

now be described with reference to the following figures in which:

Figure 1 is schematic of an apparatus for determining and/or monitoring

characteristic parameters of a wafer in plasma etching ;

Figure 2 shows a periodic nature of polarisation state with surface layer thickness;

Figure 3 shows determination of real time etch rate according to the

invention;

Figures 4, 5 and 6 show determination of etch endpoint detection

according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Figure 1 , there is shown an apparatus for determining and

monitoring a surface of a material during plasma etching. In this embodiment

the material is a polysilicon wafer 6. The apparatus comprises a source of

coherent light 1 which in this case is a laser. In the embodiment of Figure 1

the source 1 is Helium Neon laser, model LGR 7631 A from Siemens. The

laser has an associated power supply 2.

The state of polarisation of the incident beam 3 is determined by fixed

polariser 5. The incident beam 3 may be linearly polarised, elliptically

polarised or circularly polarised. Whatever the polarisation is must be fixed

for the embodiment of the invention shown in Figure 1. Conceivably, the

fixed polariser 5 may be incorporated in the laser 1 so that a separate element

is not required.

The incident beam impinges upon the semiconductor wafer 6 at an

angle ψ and is reflected toward rotating polariser 7. In prior art ellipsometric

methods a knowledge of the angle ψ is critical. As will become evident

below, knowledge of this angle is not critical in the method of the invention.

The rotating polariser 7 rotates at a known frequency determined by the

modular power supply 8. In the embodiment described the polariser has two speeds, fast (3 Hz) and slow (1 .5 Hz). Although a rotating polariser is

preferred any element that modulates the polarisation of the reflected beam

can be used.

Although the preferred embodiment is described in terms of a fixed

polariser 5 and rotating polariser 7, the converse can also be used. That is, the

polariser adjacent the source may rotate and the polariser adjacent the detector

may be fixed. This arrangement may have advantage in a multiple beam

application.

A laser line interference filter 9 filters certain optical noise from the

reflected beam 10. A detector 1 1 produces an analog signal 12 proportional to the intensity of light incident on the detector 1 1 . The detector 1 1 is

energised by power supply 13. In the embodiment of Figure 1 the detector 1 1 is a Hammamatsu photomultiplier and the power supply 1 3 is a high voltage

power supply. The signal 12 is converted from analog to digital in a PCL718 A/D

converter 14. The digital signal 1 5 is processed in a computer 16.

The apparatus can be used in various applications. In Figure 1 the

apparatus is shown applied to a plasma etcher comprising a chamber 17

having an upper electrode 18 and lower electrode 19, upon which the wafer 6

is mounted. Input optical window 20 and exit optical window 21 are

mounted in the chamber wall.

In one example the wafer 6 has a polysilicon layer on top of a Si0₂

layer. Polysilicon has a refractive index N₂ of 3.6 and Si0₂ has a refractive

index N₃ of 1 .457. The laser is adjusted for a wavelength λ of 632.8 nm and an incident angle of 70° at the surface of the polysilicon layer. Phases shift ό

at the layers and Fresnel coefficients are obtained from the following equation:

2TT δ =(— )N_iCOSθ

where N is refractive index

d is thickness

This equation is well known and is described in Azzam and Bashara referred

to on page 2.

The inventor has found that the polarisation state of the reflected laser light various periodically with the change in thickness of the surface layer of

the wafer 6. Figure 2 shows the periodic nature of Ψ and Δ for the

polysilicon wafer 6. As can be clearly seen these parameters vary periodically

with film thickness. Exemplary etched thickness G of the polysilicon layer and

corresponding polarisation state values of Δ and Ψ are:

G = 0 Angstrom Δ = 180° Ψ = 14.59°

G - 300 Angstroms Δ - 26.59° Ψ = 27.23°

G = 450 Angstroms Δ = 20.68° Ψ - 29.24°

The apparatus described above is used to monitor the polarisation state

of the reflected beam as a function of time during the etch process. Using the

equations described earlier on page 4 the signals from the photomultiplier 1 1

are converted to polarisation state and plotted against time as shown in Figures

4 to 6. The resultant plot is periodic with the period equals to the time it takes

to deposit (etch away) a characteristic quantity of the material. For polysilicon this characteristic quantity is 90 nm. Thus the etch rate (conversely the growth

rate in a deposition process) is directly determinable as:

T E =- ^r P

where E_r is the etch rate, T_c is the characteristic thickness and P is the time, in

seconds, required for a characteristic thickness of the material to be deposited

or removed.

Characteristic thickness of other materials can be determined

theoretically or experimentally. Once the characteristic thickness is

determined the etch rate is directly obtainable in-situ and in real time from the

polarisation state periodicity.

The characteristic thickness is a function of the wavelength as well as

the material. At shorter wavelengths the function of the wavelength is less for

the same material. The characteristic thickness of a number of materials at a

wavelength of 632.8 nm (HeNe laser) are listed in the table below. The

wavelength dependence of the characteristic thickness can be exploited in

complex systems as an additional degree of discrimination.

Material Tc(nm)

polysilicon 90

silicon nitride 129.1

Si0₂ 140 It will be appreciated that it is not necessary to measure an entire period

before the etch rate can be determined. Curve modelling techniques can be used

in the signal processing to predict the periodicity after only a few date points are

obtained. The confidence level of the prediction will increase as the quality of

data increases.

The etch rate can be displayed as a plot of etch rate versus plasma

processing time. Such a plot in a differential mode is shown in Figure 3. Figure

3 shows etch rate versus plasma processing time plots for polysilicon and SIMOX.

The plasma chamber pressure was 200mT, the gas flow was SF₅ at 20 seem and

He at 10 seem, the power density in the plasma was 0.57 W/cm² RF. The measurements were taken in-situ and in real time using the apparatus of Figure

1 . A knowledge of the refractive indices of the materials is not required.

It is also possible to determine differential etch rate in alloy materials or

identify alloy composition from the obtained etch rates of materials. The

periodicity of the changing polarisation state of an alloy will be the superposition

of the periodicities of the individual components. The periodicities can be

separated using Fourier transform techniques and analysed as above.

Other parameters can be determined from the experimental data. If the

quantity or thickness of material removed or deposited is known the material can

be identified by counting the number of periods. For example, if a plot or

polarisation state against time shows 5 periods, and the total material removed is

measured as 450 nm the material must be polysilicon (450 nm divided by 5

equals 90 nm which is the characteristic thickness of polysilicon).

End point can be determined by monitoring the differential change in

polarisation state. Figure 4 shows a plot of ΘΔ

where v₅ is polarisation state

t is time The end point is clearly evident.

End point can also be determined by directly monitoring the polarisation

state with time. Figure 5 shows a plot of polarisation state ( in this case Ψ is

plotted) against time. The periodic nature of the polarisation state is clearly seen and the end point is easily identified.

Figure 6 shows a plot of polarisation state Δ against time. Again, the

periodic nature of the polarisation is clearly seen and the end point is easily

identified.

The method can also be applied to the measurement of the surface

temperature during etching or deposition if other parameters are known. This is

possible because the characteristic thickness is a function of refractive index which is temperature dependent.

The description of the preferred embodiments has generally been in terms

of determining and monitoring characteristic parameters of materials during

plasma etching. It will be appreciated by those skilled in the art of ellipsometer

that the technique described herein is not limited to any one situation but can be

applied to any surface modification process.

Although the apparatus of Figure 1 shows a single beam ellipsometer, the

method and apparatus can be extended to multiple beam systems. This may be useful if monitoring of a large wafer is to occur at a number of points across the

surface. In this application the rotating polariser would most conveniently be

located adjacent the light source, as previously mentioned. In a multiple beam

apparatus each light source may be incident at the material at a different angle

and may be at a different wavelength. Multiple beams facilitates the application

of the method and apparatus to complex systems.

The invention conceives that the technique can be applied to at least the following situations:

* etch rate control

* deposition rate control

* chemical composition determination

* contamination determination

* multi-wavelength analysis

* surface temperature determination

* surface homogeneity determination

* layer thickness measurement

Whilst the above has been given by way of illustrative example of the

present invention, many variations and modifications thereto will be apparent to

those skilled in the art without departing from the broad ambit and scope of the

invention as herein set forth.

Claims

1 . A method of in-situ determining and/or monitoring one or more

characteristic parameters of a material during materials processing including the

steps of:

directing light of known polarisation at a material;

analysing light reflected from the material to determine changes in

polarisation state;

monitoring the changes in polarisation state over time to obtain a periodicity of the changes in the polarisation state; and

calculating one or more characteristic parameters of the material from the

obtained periodicity.

2. The method according to claim 1 wherein the light in the directing step is one or more beams of coherent light.

3. The method according to claim 2 wherein each beam is at a different angle

of incidence and/or at a different wavelength.

4. The method according to any one of claims 1 to 3 wherein the light is laser.

5. The method according to any one of claims 1 to 4 wherein the light is

linearly polarised, elliptically polarised or circularly polarised.

6. The method according to any one of claims 1 to 5 wherein the polarisation

in the directing step or the analysing step is fixed or modulated.

7. The method according to claim 6 wherein the directing step of the

analysing step includes using a fixed polarisation means for determining a

polarisation state of the light.

8. The method according to claim 6 wherein the directing or the analysing

step includes using a modulation means for modulating the light.

9. The method according to claim 8 wherein the modulation means is a

modulation element, a rotary analyser element, a rotary polariser element or a

rotary compensator element.

10. The method according to any one of claims 1 to 9 wherein the material is a solid, fluent or gaseous body.

1 1 . The method according to any one of claims 1 to 10 wherein the material

is formed of a single or plurality of substances.

12. The method according to claim 1 1 wherein the plurality of substances are arranged in layers or in a composite form.

13. The method according to claims 1 1 or 12 wherein the substances include

polysilicon, silicon nitride, silicon oxide and SIMOX.

14. The method according to any one of claims 1 to 13 wherein the material

is a semiconductor wafer having a polysilicon or SIMOX layer.

1 5. The method according to any of claims 8 to 14 wherein the analysing step

includes at least one of the following further steps:

directing light reflected from the material through the modulating means,

filtering the reflecting light with a filtering means for eliminating or

reducing optical noise,

detecting the reflected light with a light detection means,

converting analogue signals from the detection means to digital signals

with an analogue to a digital converting means, and

processing signals from the detection means or the converting means.

16. The method according to claim 1 5 wherein a laser line interference filter

is used in the filtering step and a processing means including a computer is used

in the processing step.

1 7. The method according to claim 1 5 wherein in the processing means

ellipsometric equations are used for processing the signals from the detection

means or the converting means.

18. The method according to any one of claims 1 to 1 7 wherein the

characteristic parameters include material thickness in total or in each layer, material thickness deposited or removed, a substance or substances from which

the material is formed, and temperature at the material.

19. A method of in-situ determining and/or monitoring a rate of change of

thickness of a material in a surface etching or deposition process including the steps according to any one of claims 1 to 18 and the further step of:

calculating the rate by dividing a characteristic thickness of the material

derived from the obtained periodicity by time required for etching or depositing

the characteristic thickness.

20. An apparatus for in-situ determining and/or monitoring one or more

characteristic parameters of a material during materials processing the apparatus

comprising.

a source of light of known polarisation;

means for directing the light at a material;

means for analysing light reflected from the material;

means for monitoring changes in polarisation state overtime to obtain a

periodicity of the changes in the polarisation state; and processing means for calculating one or more characteristic parameters of

the material from the obtained periodicity.

21 . The apparatus according to claim 20 wherein the light is coherent and

having one or more beams.

22. The apparatus according to claim 21 wherein each beam is at a different

angle of incidence and/or at a different wavelength.

23. The apparatus according to any one of claims 20 to 22 wherein the light

is laser.

24. The apparatus according to any one of claims 20 to 23 wherein the light

is linearly polarised, elliptically polarised or circularly polarised.

25. The apparatus according to any one of claims 20 to 24 wherein the

apparatus further comprising a fixed polariser means and a modulation means

arranged respectively in the directing means and the analysing means or vice

versa.

26 The apparatus according to claim 25 wherein the modulation means is a

modulation element, a rotary analyser element, a rotary polariser element or a

rotary compensator element.

27. The apparatus according to any one of claims 20 to 26 wherein the

monitoring means having a filter means for eliminating or reducing optical noise

in the reflected light, and a photo detector for detecting the reflected light.

28. The apparatus according to claim 27 wherein the photo detector is in the

form of a photomultiplier.

29. The apparatus according to any one of claims 20 to 28 wherein the

processing means is in the form of a computer.

30. The apparatus according to any one of claims 20 to 28 wherein the

apparatus further comprising a plasma etching chamber having an input window

through which the light is directed and an exit window for the reflected light, and

an upper electrode and a lower electrode arranged in the chamber, in use the

material is positioned on the lower electrode.