GB2088058A

GB2088058A - Measuring Coating Thickness

Info

Publication number: GB2088058A
Application number: GB8135283A
Authority: GB
Original assignee: Leybold Heraeus GmbH
Current assignee: Balzers und Leybold Deutschland Holding AG
Priority date: 1980-11-24
Filing date: 1981-11-23
Publication date: 1982-06-03
Also published as: DE3145309A1; FR2494840A1; GB2088058B; FR2494840B1; AT382963B; ATA571080A

Abstract

A method for measuring the thickness of thin coatings on a substrate in which an oscillator crystal 1 is coated simultaneously with the substrate to form a compound resonator consisting of an oscillator crystal and foreign layer 3. Resonance oscillations are set up in the compound resonator by means of an oscillator 4 connected to a pair of electrodes 2. The method involves measuring the change in frequency or period of two different quasi-harmonic oscillations of the compound oscillator, from which measurements may be determined the thickness or mass per unit area of the foreign layer, the elastic properties of the layer, which may be calculated from the two measurements being taken into account in assessing the thickness of the layer. <IMAGE>

Description

SPECIFICATION Method for Determining the Thickness of Thin Coatings This invention relates to a method for determining the thickness of thin coatings.

Measuring methods for determining the thickness of thin coatings by means of an oscillator crystal are being used on a constantly increasing scale, particularly in the optical and semi-conductor industries. A feature common to all of the existing methods is that the oscillator crystal is coated simultaneously with the substrate. The term "substrate" will be understood to mean the objects to be coated, e.g.

semi-conductor chips or optical components such as lenses and mirrors. The most usual coating processes are vapour deposition or ionic atomisation (sputtering, reactive sputtering), and chemical vapour deposition (cvd). Since the particular nature of the coating method is basically unimportant to the oscillator crystal iayer-thickness measurement, the general term "coating-applying process" will be used hereinafter. The oscillator crystal coatingthickness measuring process is in practice suitable for all coating materials used in industry.

The coating of the oscillator crystal in the coating-applying process results in a compound resonator consisting of the oscillator crystal and the applied coating. The oscillator crystal, hereinafter also referred to as the crystal (quartz) for the sake of brevity, consists of a quartz monocrystal lamella including the electrodes necessary for setting up the electrical oscillation.

The applied coating will be referred to as the foreign layer.

All of the currently used oscillator crystal coating-thickness measuring methods measure either the frequency f or the changing in frequency A f or the period z or the change in period AT of the basic resonance oscillation of the compound resonator. The geometric thickness of the foreign layer and the increase in the foreign layer can be determined respectively from the measured oscillation quantity for T, and the change therein.

The currently used oscillator crystal coatingthickness measuring methods and the measuring equipment based on these methods can be divided into two kinds. In the methods of the first kind, the elastic properties of the foreign layer, generally deviating from those of the crystal, are taken into account when determing the thickness of the foreign layer or determining its growth from the measured oscillation measuring quantity or from the change therein. Methods of the second kind are based on an approximately proportional relationship between the increase in thickness of the foreign layer and the change in the oscillation measured quantity.

For providing a better understanding of this classification, use can be made of the following formula, which is derived from the literature on the subject and which shows the relationship between the frequencies fn of the resonance oscillations of the compound resonator and the thickness IF of the foreign layer.

in which: pF, pQ designate the density by volume of the foreign layer mass and of the crystal mass respectively.

1, I desi IFT IQ designate the thickness of the foreign layer and of the crystal respectively.

PFIFB PQIQ designate the density per unit of area of the foreign layer and of the crystal mass respectively, fQ designates the basic resonance frequency of the oscillator crystal, and Z,, ZQ designates the characteristic acoustic impedance of the foreign layer and of the crystal respectively.

Instead of the frequencies fn and fQ, the periods

respectively, may be introduced.

The index n gives the consecutive number of the compound resonance frequency in the frequency spectrum of the compound resonance.

Whereas a homogeneous resonator has a harmonic frequency spectrum with equidistantly disposed resonance frequencies (for example f,=3f,, f6=5f1, etc.,) in the case of the compound resonator, for ZF=ZQ, the resonance frequencies do not occur equidistantly in the spectrum. To take this situation into account, the compound resonance frequencies are referred to as quasi-harmonic hereinafter. The equation (1) not only indicates the basic frequency f1 of the compound resonator, but all of the mechanically possible quasi-harmonic resonance frequencies.

However, since in the case of an oscillator crystal only the odd-number harmonic resonance frequencies of the oscillator crystal are set up piezo-electrically and, as a first approximation, this also applies in the case of the quasi-harmonic resonance frequencies of the compound resonator, only the value n=1, 3, 5, 7,... are considered.

When equation (1) is used for determining the thickness IF of the foreign layer, the effects of the elastic properties of the foreign layer on the measured value for the thickness of the coating are taken into account by the parameter ZF.

Coating-thickness measuring equipment of the first kind, which take into account the elastic properties of the foreign layer -- generally deviating from that of the crystal - have therefore hitherto required, in addition to the introduction of the mass density PF' the introduction of a parameter, characterising the elastic properties of the foreign layer, for example ZF or the relationship ZF7ZQ. Since

wherein VF designates the phase velocity of the piezoelectrically generated sound wave in the foreign layer and CF the effective elastic rigidity constant of the foreign layer, it would be possible, however, to take into account the elastic properties of the foreign layer, e.g. by introducing Vp or CF.

Coating-thickness measuring equipment of the second kind disregards the effects of the elastic properties of the foreign layer, differing from those of the crystal, during determination of the thickness of the foreign layer or of the increase in this thickness during a coating-applying process.

These measuring instruments use approximation formulae which can be derived from equation (1) by assuming Pplp Po1w

The application of equation 2 is also known in the literature by the name "period measuring method", and that of equation 3 by the name "frequency measuring method". Whereas when ZZQ, equation 2 corresponds precisely with equation 1, equation 3 is a broader approximation, which provides accurate coating thickness measurements for mass coverings pFiJpl2%. The validity range of equation 2 is dependent upon the magnitude of the deviation from ZF and ZQ; the guide value in the case of the usual foreign layer materials is pFI, /pall 0%.

When use is made of planoconvex crystals and assuming sufficiently accurate knowledge of a parameter characterising the inherent elastic properties of the foreign layer, the validity range of equation 1 could be shown to exist for mass coverages PFIF/PQIQ670%.

To summarise, it can be stated regarding the prior art, that two types of measuring methods are used for determining the thickness of thin layers by means of an oscillator crystal. The first type takes into account the effects of the elastic properties of the foreign layer on the measured value of the thickness of the coating and thus permits the use of the crystal for up to very thick foreign layer coverings. This means that the crystal needs to be changed only after a very large number of coating-applying processes.However, instruments for measuring thickness of coating that use a method of the first kind require the introduction of a parameter characterising the elastic properties of the foreign layer, and at present this usually takes the form of the relationship ZJZ,. The need for introducing this parameter complicated not only the operation of the measuring equipment but also carries with it the problem that the elastic properties, that actually take effect and are inherent in the foreign layer, are not known. In practice, therefore, use is made of the Z values of the corresponding solid materials, i.e. the "bulk values" as they are called; however, these can be applied, very specifically, only to thin coatings.

In the second type of measuring method, the crystal must be changed after a relatively small number of coating-applying processes in order to ensure the accuracy necessary for satisfactory reproducibility of the thicknesses of coating that are produced.

The present invention seeks to avoid the above-enumerated disadvantages of the previously used measuring methods by providing a measuring method for determining the thickness of thin coatings on a substrate by means of an oscillator crystal, which crystal is coated simultaneously with the substrate, so that a compound resonator consisting of an oscillator crystal and a foreign layer is created, wherein the change in the oscillation measured quantity, frequency or period, of said compound resonator is used for determining the thickness or density per unit of area of the foreign layer, in particular, the measured quantities of two different quasiharmonic resonance oscillations of the compound resonator are measured, and are used for determining the thickness or the density per unit of area of the foreign layer, and wherein the elastic properties of the foreign layer, which can be calculated from the two measurements, are taken into account when determining the thickness of the layer.

It will be seen that, in the method of the invention the oscillation measured amounts, frequency or period are measured for each two different quasi-harmonic resonance oscillations of the compound resonator. The measured quantities of each of the two resonance oscillators are used for determining the thickness or the density per unit of area of the foreign layer, and the effective elastic properties of the foreign layer that can be calculated from the two measurements are taken into account. Use is made of the fact that a compound resonator has a non-harmonic spectrum and the measured amounts of two resonance oscillations generally provide two linearly independent relationships for determining the unknown thickness of foreign layer. Thus, the effect of the generally likewise unknown or not precisely known parameter, characterising the resilient properties of the foreign layer, on the measured value of the coating thickness to be determined, can be taken into account by eliminating or calculating this parameter.

The microprocessors normally used in instruments for measuring the thickness of coatings, on the one hand exhibit, during the coating-applying process, a not always adequate computing capacity, while on the other hand, during a single coating-applying operation, the rise in the coating thickness/oscillation measured amount characteristic curve for the usual thickness values of a single coating is constant enough to result in excellent approximation.

Therefore, in accordance with an embodiment of the invention, it may be advantageous if measurement of the oscillation measured quantities of the two resonance oscillations is carried out only prior to commencement of the particular coating-applying process and, from the ascertained thickness of the foreign layer or from a calculated characteristic elastic quantity of the foreign layer, there is calculated the specifically applicable proportionality factor between a change in oscillation measured quantity and the associated change in thickness of the foreign layer, and this proportionality factor is used during the coating-applying process for determining the increase in thickness of the coating, from the measurement of the oscillation amount of only one particular resonance oscillation.This feature also facilitates the adaption of the earlier instruments for measuring the thickness of coatings for use in the method of the invention.

Since the effects of the characteristic sound impedance on the measured value of the thickness of the layer that is to be determined rises with the third power of the increase in thickness of the foreign layer, it is advantageous, as provided for in a further embodiment of the invention, if the measurement of the oscillation measured amounts of both resonance oscillations is begun upon reaching a particular thickness of foreign layer on the crystal, above which the effect of the elastic properties of the foreign layer on the oscillation measured amount becomes significant.

In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawing which is a block diagram of an apparatus for carrying out the method of the invention. In this example, the frequency is selected as the oscillation measured amount. The first and third quasi-harmonic compound resonance frequencies are measured.

However, the third and the fifth, or the first and the fifth could also conceivably be used; the important factor as regards the method of the invention is that two compound resonator frequencies be measured.

Figure 1 illustrates diagrammatically, on the left, the compound resonator consisting of an oscillator crystal 1, electrodes 2 and foreign layer 3. Resonance oscillations are set up in this compound resonator by an oscillator 4. It is advantageous to use what is called an AT-section crystal, which operates as a thickness shear vibrator and has a temperature coefficient which falls with resonance frequency. The frequency range of the oscillator reaches 3 to 5 MHz in the lower position of a switch 5, and 9 to 1 5 MHz in the upper position of the switch. The range of the oscillator is changed over by, for example, a suitable band filter consisting of a damped LC filter or an RC filter, in the feedback branch of the oscillator.Thus, in the lower position of the switch, the compound resonator is actuated on the basic frequency f1, and in the upper position of the switch on the third quasi-harmonic resonance frequency f,. The switch is normally in the lower position, and the counter 6 measures the basic frequency f,. After each completion of the measuring time interval, the contents of the counter are fed into a micro-computer 8 through an input duct 7. Before a coating-applying operation begins, the microcomputer 8 moves the switch 5, which latter may, for example, take the form of a reed relay, into the upper position for the duration of a measuring time interval.The measured frequency is designated f30, and the frequency measured before commencement of the coating-applying process and with the switch in the lower position, is designated frO.

To evaluate the two measured frequencies f10 and f30, these are introduced into equation 1, so that two equations are obtained which are for ZF+ZQ i.e. are generally linearly independent. By division of the two equations a defining equation for ZF/ZQ iS obtained:

The value for ZF/ZQ obtained from this equation can now be introduced into equation 1, so that, during the coating-applying operation, the measurement, each time, of an oscillation measuring amount -- f, in the example suffices for continuously determining IF from equation 1.

The above-described arithmetical operations are carried out by the microcomputer. The only parameter that has to be introduced by way of the microcomputer input keyboard 9 for determining IF is the volume density of the foreign layer mass p. The value IF determined for thickness of layer can be displayed in digital form by the instrument 10 and/or can be used for automatically terminating the coating-applying operation when a prescribed required thickness of coating is reached. The way in which the value IF' determined by the above described method, is used is irrelevant. A further possibility is that of determining the density per unit of area of the foreign layer mass p,l, instead of the thickness IF.

For the purpose of relieving the microcomputer of load during the coating-applying process, the rise P(Zp, faO) of the lVf1 characterising curve at the point frO can also be used for determining IF, instead of equation 1. P(Zp, f O) is obtained by differentiation of equation 1 with respect to f, at the point frO P(Zp, fO) requires to be calculated only prior to commencement of the coatingapplying process; during this process IF is determined from the following simple relationship.

IF P(Zp, fao) . (f. fro) Therein, Alp designates the growth in the thickness of the foreign layer during the coatingapplying process; AIIplrn, wherein IF designates the entire thickness of the foreign layer at a given moment, and IFO designates the thickness of the foreign layer before commencement of each coating-applying process.

It is unimportant whether the oscillation measured amount is measured directly or indirectly in the form of a multiple or a fraction.

Particularly when adapting existing measuring equipment to suit the above-described method, it may be advantageous to insert, between the oscillator and the counter, a divider which divides the quasi-harmonic resonance frequency fn by the number n of the quasi-harmonic resonance frequency. In the example herein described, this would be a 1:3 divider. This would result in the following counter requiring approximately the same capacity for measuring f3/3 as for measuring f,.

In conclusion, special reference is made to the fact that the nature of the relationship between IF and pFI, respectively, and fn or Tn that is used has nothing to do with the nature of the invention.

The equation 1, here quoted by way of example, reproduces the resonance frequency spectrum of the compound resonator when the driving oscillator drives the oscillator crystal in a highly resistive manner. For this limiting case of open electrodes, what is called the parallel resonance spectrum is excited. If the driving oscillator drives the oscillator crystal on a very low resistance basis, the series resistance spectrum, as it is called, that occurs when the electrodes are shortcircuited, is excited. In this case and for the purpose of achieving high measuring accuracy, equation 1 is replaced by an appropriately modified equation which gives the series resonance spectrum. The only decisive factor affecting the method described herein is that use be made of a relationship between IF and p,l,, respectively, and the oscillation measured amount that takes into account, as accurately as possible, the effect of the elastic properties of the foreign layer on the value to be determined for the thickness of the coating.

Claims

1. A measuring method for determining the thickness of thin coatings on a substrate by means of an oscillator crystal, which crystal is coated simultaneously with the substrate, so that a compound resonator consisting of an oscillator crystal and a foreign layer is created, wherein the change in the oscillation measured quantity, frequency or period, of said compound resonator is used for determining the thickness or density per unit of area of the foreign layer, in particular, the measured quantities of two different quasiharmonic resonance oscillations of the compound resonator are measured, and are used for determining the thickness or the density per unit of area of the foreign layer, and wherein the elastic properties of the foreign layer, which can be calculated from the two measurements, are taken into account when determining the thickness of the layer.

2. A measuring method according to claim 1, wherein the measurement of the oscillation measured quantities of the two resonance oscillations is carried out only prior to commencement of the particular coating-applying process and, from the ascertained thickness of the foreign layer or from a calculated characteristic elastic quantity of the foreign layer, there is calculated the specifically applicable proportionality factor between a change in oscillation measured quantity and the associated change in thickness of the foreign layer, and this proportionality factor is used during the coatingapplying process for determining the increase in thickness of the coating, from the measurement of the oscillation amount of only one particular resonance oscillation.

3. A measuring method according to claim 1 ol- claim 2, wherein the measurement of the oscillation measured amounts of the two resonance oscillations is begun only upon reaching a particular thickness of foreign layer on the crystal, beyond which the effect of the elastic properties of the foreign layer on an oscillation measured amount becomes significant.

4. A measuring method for determining the thickness of thin coatings substantially as hereinbefore described.