JPH10106984A - Polishing method and controller in polishing equipment for semiconductor wafer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a controller in a polishing equipment for semiconductor wafers in which a highly accurate polishing is made possible, reflecting such polishing conditions as the updated situation of the surface of a polishing nonwoven fabric and the updated components and temperature of a polishing liquid on the polishing. SOLUTION: This equipment has a polishing-time calculating unit 3, a polishing-time distributing unit 4, and a polishing controlling portion 5. When polishing semiconductor wafers initially, inputting a desired value MT of the film thickness of the wafer and an initial polishing time T0 from an inputting device 2, the first semiconductor wafer is polished only during the time T0 in the polishing portion of the polishing equipment. Thereafter, from the time TO, a measurement value M1 of the pre-polishing film thickness and a measurement value MF1 of the post-polishing film thickness, an optimum polishing time T1 which is applied to each of the second wafer to the wafer with a predetermined sheet number is calculated to polish each semiconductor wafer during the foregoing optimum time T1 . Thereafter, whenever the polishing is completed, the optimum polishing time which is applied to each of the wafer with a specific sheet number to the wafer with a new predetermined sheet number is calculated in succession to polish each semiconductor wafer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor polishing apparatus (polishing apparatus) for polishing a semiconductor wafer.
The present invention relates to a polishing method and a polishing control device in a semiconductor wafer polishing apparatus for polishing a semiconductor with an optimum polishing time to accurately maintain a film thickness to be polished at a predetermined value.

[0002]

2. Description of the Related Art In a polishing apparatus (polishing apparatus) for polishing (polishing) the surface of a semiconductor wafer to a mirror surface, the polishing of the wafer film to a predetermined value is performed by controlling the polishing time. That is, if the polishing amount is constant under polishing conditions such as the polishing speed and the surface pressure, the polishing amount is proportional to the polishing time. Therefore, by controlling the polishing time, the film thickness can be maintained at a predetermined value.

Conventionally, in order to obtain this polishing time, an operator first measures the film thickness of one semiconductor wafer before polishing (in a normal polishing apparatus, the thickness is measured outside the apparatus), and then, the polishing is performed. The semiconductor wafer is polished by a polishing apparatus for a predetermined time, and after polishing, the film thickness of the semiconductor wafer is measured (in some cases, it is measured inside a polishing apparatus or outside the apparatus). The optimum polishing time is calculated from the film thickness, the target film pressure, and the polishing time, and with this optimum polishing time,
The subsequent polishing of the semiconductor wafer is performed.

[0004]

However, the method of setting the polishing time as described above is a difficult operation by hand, and at the same time, the polishing time is set by one operation, so that the polishing is continued. There is a problem that the polishing conditions change over time, and the polishing amount differs even with the same polishing time. For example, the surface of the polishing non-woven fabric is worn out during long polishing, changes over time, and a difference in the polishing amount occurs even with the same polishing time due to a component of the supplied polishing liquid or a change in temperature. There is a problem.

Further, as disclosed in, for example, Japanese Patent Publication No. 7-100297, the polishing amount and the polishing time in a polishing cycle of a polished semiconductor wafer are sequentially accumulated, and an average polishing rate is calculated based on these accumulated values. In some cases, the optimum polishing time is calculated based on the average speed and the polishing margin of the semiconductor wafer in the next polishing cycle, and the polishing in the next polishing cycle is performed based on the optimum polishing time. However, the method of calculating the optimum polishing time based on the average polishing speed of the accumulated value of the polishing time as described above cannot always set the optimum polishing time.

[0006] One semiconductor wafer is required for each lot.
In a polishing machine that continuously grinds each sheet,
The condition of the surface of the nonwoven fabric for polishing at the end of the previous semiconductor wafer, the composition and temperature of the polishing solution have a great effect on the current and next semiconductor wear polishing, and are not necessarily the average of the accumulated values of past polishing times. The polishing rate does not significantly affect the current or next polishing. Therefore, there is a problem that high-precision film thickness polishing cannot be performed.

The present invention has been made in view of the above points, and eliminates the above-mentioned problems, and reflects the latest polishing conditions such as the state of the surface of the nonwoven fabric for polishing and the components and temperature of the polishing solution in polishing.
An object of the present invention is to provide a polishing method and a polishing control device in a semiconductor wafer polishing apparatus capable of performing highly accurate polishing.

[0008]

According to a first aspect of the present invention, there is provided a polishing apparatus in which a plurality of semiconductor wafers each comprising a large number of lots are loaded in a polishing apparatus for each lot, and the semiconductor wafers are loaded one by one into a target film. A polishing method in a semiconductor wafer polishing apparatus for polishing to a thickness value, comprising: a polishing time after polishing the semiconductor wafer, a target film thickness value, a measured film thickness value before polishing the semiconductor wafer, and a measured film thickness value after polishing. From the above, the optimum polishing time is calculated for each polishing of the semiconductor wafer, and the optimum polishing time calculated for each polishing is applied to the polishing of the next and subsequent semiconductor wafers, and is continued until the total number of the semiconductor wafers of one lot is completed. It is characterized by the following.

According to a second aspect of the present invention, there is provided a polishing apparatus in which a plurality of semiconductor wafers of one lot are loaded into a polishing apparatus for each lot, and the semiconductor wafers are continuously polished one by one. A control device, comprising: input means for inputting various data; film thickness measuring means for measuring a film thickness before and after polishing of a semiconductor wafer; a polishing time calculating means; and a polishing section control means. At the time of initial polishing, a target film thickness value is inputted from the input means, a predetermined initial polishing time required for polishing to the target film thickness value is inputted, and the polishing section is controlled by the polishing section control means so that 1 is obtained. After the first semiconductor wafer is polished for the initial polishing time, the polishing time calculating means calculates the initial polishing time and the measured film thickness before polishing and the measured film thickness after polishing measured by the film thickness measuring means. The predetermined number of times thereafter The optimal polishing time is calculated in the following, the next and subsequent predetermined number of semiconductor wafers are polished by the optimal polishing time, and each time the polishing is completed, the optimal polishing time and the film thickness measuring means are measured by the polishing time calculating means. From the measured film thickness value before polishing and the measured film thickness value after polishing, an optimum polishing time for a predetermined number of semiconductor wafers thereafter is calculated, and the predetermined number of semiconductor wafers are polished with the optimum polishing time. And
Thereafter, each time polishing is sequentially completed, the optimum polishing time for the subsequent predetermined number of semiconductor wafers is calculated, and the predetermined number of semiconductor wafers are polished with the optimum polishing time.

According to the first and second aspects of the present invention, the optimum polishing time is calculated each time after polishing as described above, and the subsequent polishing of the semiconductor wafer is performed with the calculated optimum polishing time. For example, even if the polishing conditions such as the condition of the surface of the non-woven fabric for polishing and the composition and temperature of the polishing liquid change, the changed latest polishing conditions can be reflected in the subsequent polishing of the semiconductor wafer. The thickness can be polished to a highly accurate thickness.

According to a third aspect of the present invention, in the polishing control device of the semiconductor wafer polishing apparatus according to the second aspect, the film thickness measuring means measures the film thickness at a plurality of points on the semiconductor wafer, and averages the measured values. Is the measured film thickness.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a polishing control device of a semiconductor wafer polishing apparatus according to the present invention. As shown in the figure, the present polishing control device includes a film thickness measuring unit 1, an input device 2, a polishing time calculation unit 3, a polishing time distribution unit 4, and a polishing unit control unit 5.

The film thickness measuring section 1 has a pre-polishing measuring function 1a for measuring the film thickness of the semiconductor wafer before polishing and a post-polishing measuring function 1b for measuring the film thickness of the semiconductor wafer after polishing. measuring film thickness value M _I and the measured thickness value M _F after polishing
Is input to the polishing time calculation unit 3. As will be described in detail later, the polishing time calculation unit 3 calculates the optimum polishing times T _{1 to} T _N for the next and subsequent semiconductor wafers each time polishing is completed, and outputs the calculated polishing times to the polishing time distribution unit 4. The measurement of the film thickness at the film thickness measuring unit 1 measures the thickness of the plurality of points of the surface of the semiconductor wafer, to obtain a measurement thickness value M _I and M _F from the average value. As a result, a highly accurate measured film thickness value can be obtained.

The polishing section control section 5 includes a polishing time storage section 5a for storing an optimum polishing time for each semiconductor wafer to be polished, and the polishing time distribution unit 4 stores the optimum polishing time calculated by the polishing time calculation unit 3. T _{1 to} T _N are respectively set to the second and third polishing time storage units 5a,
..,..., N−1 and N. The polishing section control section 5 controls the polishing section (polishing section) of the polishing apparatus with the optimum polishing times T _{1 to} T _N stored in the polishing time storage section 5a to polish each semiconductor wafer, as will be described in detail later.

FIG. 2 is a diagram showing a schematic configuration of a polishing apparatus using the above-mentioned polishing control device. The polishing apparatus includes a loading / unloading unit 21 for carrying in and carrying storage cases 25 and 25 'storing a large number of semiconductor wafers;
Transport device 22, polishing (polishing) unit 23, cleaning unit 2
4 and a film thickness measuring unit 1 of the polishing control device shown in FIG. 1 are arranged in a predetermined arrangement.

The transfer device 22 is composed of a robot or the like, and takes out semiconductor wafers one by one from the storage case 25 carried into the load / unload unit 21 and sends the semiconductor wafers to the film thickness measuring unit 1. The film thickness measuring section 1 first measures the film thickness before polishing.
Subsequently, the transfer device 22 transfers the semiconductor wafer, for which the film thickness measurement before polishing has been completed, to the polishing unit 23. In the polishing section 33, for example, the semiconductor wafer is brought into contact with the top ring at a predetermined pressure against a nonwoven fabric surface for polishing attached to the upper surface of the turntable, and the semiconductor wafer is moved for a predetermined time by the mutual movement due to the rotation of the top ring and the rotation of the turntable. Grind. The polishing time is determined by the optimum polishing times T _{1 to} T _N stored in the respective storage areas of the polishing time storage unit 5a of the polishing unit control unit 5 as described above.

The polished semiconductor wafer is sent to the cleaning unit 24 by the transfer unit 22, washed and dried by the cleaning unit 24, and then sent to the film thickness measurement unit 1 by the transfer unit 22. In step 1, the thickness of the polished film is measured. After the film thickness is measured, the polished semiconductor wafer is transferred to the load / unload unit 21 and is separately disposed in the load / unload unit 21 for storing the polished semiconductor wafer. Is stored in. The semiconductor wafers of one lot stored in the storage case 25 as described above are taken out one by one by the transfer device 22, and the operations of polishing, washing / drying, and measuring the film thickness after polishing are repeated. When the above-mentioned operation for all the semiconductor wafers in the storage case has been finished, ie, when the polishing of one lot of semiconductor wafers has been finished, the polishing operation for the next lot is started.

FIG. 3 and FIG. 4 are diagrams showing the flow of the polishing processing operation of the above-mentioned polishing control device. The polishing processing operation will be described with reference to FIG. First, in FIG. 3, one lot (for example, 2 lots) requiring polishing in the load / unload unit 21 of the polishing apparatus is used.
(5 sheets) A storage case 25 storing semiconductor wafers is set. Then, enter the target thickness value M _T and the initial polishing from the input device 2 (polishing) time T _0. Is the target thickness value M _T is transferred to the polishing time calculation unit 3, an initial polishing time T ₀ is transferred to the polishing unit control unit 5 and the polishing time calculation unit 3, the polishing unit control unit 5 in the polishing time storage unit 5a It is stored in the first storage area.

The initial polishing time T ₀ is the thickness is calculated the target thickness value empirically from the pre-polishing film thickness and the target film thickness value at the time required to polish to a M _T. As described above, the transfer device 22 takes out the _first semiconductor wafer W ₁ from the storage case 25 and transfers it to the film thickness measuring unit 1.
At this time, the second semiconductor wafer W ₂ enters a standby state in the storage case 25. The film thickness measuring unit 1 is provided with the semiconductor wafer W
The film thickness before polishing of ₁ was measured, and transfers the measured film thickness M _I in the polishing time calculating unit 3.

Subsequently, the transfer device 22 is a semiconductor wafer W
₁ is transferred to the polishing section 23. The polishing section 23 polishes the semiconductor wafer W ₁ for an initial polishing time T _{0 under} the control of the polishing section control section 5. When the polishing is completed, the transfer device 22 transfers the semiconductor wafer W ₁ to the cleaning unit 24, and the semiconductor wafer W ₁ cleaned and dried in the cleaning unit 24 is transferred again to the film thickness measuring unit 1 by the transfer device 22. It is, where the measured thickness after the polishing of the semiconductor wafer W ₁ is the measured thickness value M _F1 is transferred to the polishing time calculating unit 3. After the completion of the film thickness measurement, the semiconductor wafer W ₁ is transferred to the load / unload unit 21 by the transfer device 22.
And is stored in a storage case 25 'for storing the semiconductor wafer after polishing.

Subsequently, in the polishing time calculation unit 3, the initial polishing time T ₀ , the semiconductor wafer W ₁
Measurements thickness value before polishing of M _I, measured thickness value M _F1 and the target thickness value M _T (optimal polishing time to the target thickness value M _T) optimum polishing time from the polished T ₁ { T ₁
= F (M _T , M _I , M _F1 , T ₀ )}. The optimum polishing time T ₁ is transferred to the polishing time distribution unit 4 and stored in the second and third storage areas of the polishing time storage unit 5 a of the polishing unit control unit 5.

After confirming that the optimum polishing time T ₁ has been calculated, the operator cancels the waiting and releases the waiting time as shown in FIG.
As shown in (2), the polishing operation for the second and subsequent semiconductor wafers is started. The transfer device 22 is a second semiconductor wafer W
₂ is taken out of the storage case 25 and transferred to the polishing section 23. The control of the polishing unit 23 in the polishing unit control unit 5 is stored in the second sheet storage area of the polishing time storing unit 5a polishing the optimum polishing time T ₁ by the second sheet of the semiconductor wafer W _2.

The transfer device 22 transfers the polished semiconductor wafer W ₂ to the cleaning unit 24, and the washed and dried semiconductor wafer W ₂ is transferred to the film thickness measuring unit 1 again by the transfer device 22. The film thickness measuring unit 1 is a semiconductor wafer W ₂
The film thickness after the polishing were measured, and transfers the measured film thickness M _F2 in polishing time calculating unit 3. The semiconductor wafer W ₂ that the film thickness after polishing is measured is transferred to the loading and unloading unit 21 by the transfer device 22, it is 25 'housed in the housing case for housing the semiconductor wafer after polishing.

The polishing time calculation unit 3 calculates the target film thickness M _T , the measured film thickness M _I before polishing of the _second semiconductor wafer W ₂ , the measured film thickness M _F2 after polishing, the optimum polishing time. Optimal polishing time T ₄ ｛T ₄ = f for polishing the _fourth semiconductor wafer W ₄ from T ₁ (M _T , M _I , M _F2 ,
T ₁ )｝ is calculated, and this optimum polishing time T ₄ is transferred to the polishing time distribution unit 4 and stored in the fourth storage area of the polishing time storage unit 5 a of the polishing unit control unit 5.

A polishing operation for the third semiconductor wafer W ₃ is started in parallel with the polishing operation for the _second semiconductor wafer W ₂ . The transfer device 22 takes out the third semiconductor wafer W ₃ from the storage case 25 and performs the polishing section 23.
Transferred to the control of the polishing section 23 in the polishing unit control unit 5, to polish the 3rd stored in the storage area optimal polishing time T ₁ in 3rd semiconductor wafer W ₃ of the polishing time storage unit 5a .

The polished semiconductor wafer W ₃ is transferred to the cleaning unit 24 by the transfer unit 22, and the washed and dried semiconductor wafer W ₃ is transferred to the film thickness measuring unit 1, and polished by the film thickness measuring unit 1. Is measured, and the measured film thickness M
_F3 is transferred to the polishing time calculation unit 3. Semiconductor wafer W ₃ that the film thickness after polishing is measured is transferred to the loading and unloading unit 21 by the transfer device 22, it is housed in the housing case 25 'which houses the semiconductor wafer after polishing.

The polishing time calculation unit 3 calculates the target film thickness M _T , the measured film thickness M _I before polishing of the _third semiconductor wafer W ₃ , the measured film thickness M _F3 after polishing, the optimum polishing time. optimum polishing time _{T 5 (T 5 = f (} M T, M I, M F3 from T ₁ to polish the semiconductor wafer W ₅ of 5 th,
T ₃ )) is calculated, and this optimum polishing time T ₅ is transferred to the polishing time distribution unit 4 and stored in the fifth storage area of the polishing time storage unit 5 a of the polishing unit control unit 5.

The polishing operation of the semiconductor wafer W ₄ of the fourth sheet is transferred to the polishing section 23 takes out the semiconductor wafer W ₄ of the fourth sheet by the transfer device 22, the control of the polishing unit control unit 5 in the polishing section 23 to polish the 4 th semiconductor wafer W ₄ at 4th stored in the storage area optimal polishing time T ₄ of the polishing time storage unit 5a.

The polished semiconductor wafer W ₄ is cleaned and dried in the cleaning unit 24, further transferred to the film thickness measuring unit 1, where the polished film thickness is measured by the film thickness measuring unit 1. the value M _F4 is transferred to the polishing time calculation unit 3, the semiconductor wafer W ₄ is accommodated in the storage case 25 'which houses the semiconductor wafer after polishing at the load-unload 21.

The polishing time calculation unit 3 calculates the sixth semiconductor wafer from the target film thickness M _T , the measured film thickness M _I before polishing, the measured film thickness M _F4 after polishing, and the optimum polishing time T _4. optimum polishing time T ₆ to polish the wafer W ₆ {T ₆
= F (M _T , M _I , M _F4 , T ₄ )}, and transfers this optimum polishing time T ₆ to the polishing time distribution unit 4 to store the optimum polishing time T ₆ in the polishing time storage unit 5 a of the polishing unit control unit 5. The image is stored in the sixth storage area.

Similarly, the polishing operation of the _fifth semiconductor wafer W ₅ is performed by the polishing time storage unit 5 of the polishing unit control unit 5.
The optimal polishing time T ₅ stored in the fifth memory area of a
, And after the polishing is completed, the optimal polishing time T ₇ for the seventh sheet
The polishing of the _sixth semiconductor wafer W ₆ is polished with the optimum polishing time T ₆ stored in the sixth storage area of the polishing time storage unit 5 a of the polishing unit control unit 5 and the polishing is completed. Thereafter, the optimum polishing time T8 for the _eighth wafer is calculated.

After the polishing of the _K-th semiconductor wafer W _K is completed, the measured film thickness M _I before polishing, the measured film thickness M _FK after polishing, and the K + 2 semiconductor wafer from the optimum polishing time T _K. Optimal polishing time T _{K + 2} for polishing wafer W _{K + 2} (T
_{K + 2} = f (M _T , M _I , M _FK , T _K )) is calculated and stored in the (K + 2) th storage area of the polishing time storage unit 5 a of the polishing unit control unit 5.

In the above embodiment, the film thickness before polishing is measured only on the _first semiconductor wafer W1, and the measured film thickness value is used. May be measured, and the measured film thickness values M _I1 , M _{I2 to} M _IN may be obtained each time.

In the above embodiment, the optimum polishing time T ₀ , the measured film thickness M _I before polishing, the measured film thickness M _F1 after polishing, and the optimum film thickness M _T obtained from the target film thickness MT are calculated. Polishing time T ₁ {T ₁ = f (M _T , M _I , M _F1 , T ₀ )}
Is used for polishing the second and third semiconductor wafers,
While seeking by calculating it from the optimum polishing time of the semiconductor wafer to hit the second sheet to the polishing each termination below, but the invention is not limited thereto, for example, the second sheet the optimum polishing time T _1, 3 sheet , used to polish 4 th semiconductor wafer, then the semiconductor wafer to hit the third sheet may be determined by calculating the optimal polishing time and the optimum polishing time T ₁ to the second sheet to the polishing each ends below It may be used for polishing a semiconductor wafer, and the optimum polishing time of the next semiconductor wafer may be calculated and obtained every time the polishing is completed.

[0035] In short, the first sheet of the semiconductor wafer after polishing in the initial polishing time T _0, from the measured thickness value M _F1 after polishing and measurement thickness value M _I before initial polishing time T ₀ and polishing Calculate the optimal polishing time from the next time to the predetermined number of wafers, polish the semiconductor wafers up to the predetermined number of wafers with the optimal polishing time, and thereafter, each time polishing is completed, the optimal polishing time of the subsequent predetermined number of semiconductor wafers May be calculated and the predetermined number of semiconductor wafers may be polished at the optimum polishing time.

[0036]

As described above, according to the first and second aspects of the present invention, the polishing time after polishing the semiconductor wafer, the target film thickness value, the measured film thickness before polishing the semiconductor wafer. The optimum polishing time is calculated for each polishing of the semiconductor wafer from the values and the measured film thickness values after polishing, and the optimum polishing time calculated for each polishing is applied to the subsequent polishing of the semiconductor wafer. An excellent effect is obtained in that the thickness of the semiconductor wafer can be polished with high precision, which is reflected in polishing.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a polishing control device of a semiconductor wafer polishing apparatus according to the present invention.

FIG. 2 is a diagram showing a schematic configuration of a polishing apparatus using the polishing control device of the present invention.

FIG. 3 is a diagram showing a flow of a polishing processing operation of the polishing control device of the present invention.

FIG. 4 is a diagram showing a flow of a polishing processing operation of the polishing control device of the present invention.

[Explanation of symbols]

 Reference Signs List 1 film thickness measurement unit 2 input device 3 polishing time calculation unit 4 polishing time distribution unit 5 polishing unit control unit 21 load / unload unit 22 transfer device 23 polishing unit 24 cleaning unit 25 storage case 25 'storage case

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Atsushi Shigeta Inventor 72, Horikawa-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture Inside of Toshiba Kawasaki Works Co., Ltd. Address Toshiba Corporation Yokkaichi Plant

Claims (3)

[Claims] 1. A polishing apparatus is loaded with a lot of semiconductor wafers, one lot of which is a lot, and the semiconductor wafer is loaded with one lot.
A polishing method in a semiconductor wafer polishing apparatus for polishing a wafer to a target film thickness value one by one, wherein a polishing time in the polishing after polishing the semiconductor wafer, a target film thickness value, a measured film thickness value before polishing the semiconductor wafer, and a post-polishing time. The optimum polishing time is calculated for each polishing of the semiconductor wafer from the measured film thickness value, and the optimum polishing time calculated for each polishing is applied to the polishing of the next and subsequent semiconductor wafers. A polishing method for a semiconductor wafer polishing apparatus, wherein the polishing is continued until the polishing is completed. 2. A polishing apparatus is loaded with a lot of semiconductor wafers, one lot of which is a lot, and the semiconductor wafers are loaded in one lot.
What is claimed is: 1. A polishing control device for a semiconductor wafer polishing apparatus for polishing a wafer to a target film thickness value, comprising: an input unit for inputting various data; and a film thickness for measuring a film thickness before polishing and a film thickness after polishing of the semiconductor wafer. A measuring unit, a polishing time calculating unit, and a polishing unit control unit, and a predetermined initial polishing required for inputting a target film thickness value from the input unit and polishing to the target film thickness value during initial polishing. After inputting the time, the polishing section is controlled by the polishing section control means and the first semiconductor wafer is polished with the initial polishing time, and then the initial polishing time and the film thickness measuring means are calculated by the polishing time calculation means. From the measured film thickness value before polishing and the measured film thickness value after polishing, the optimal polishing time for the next and subsequent predetermined number of wafers is calculated, and the polishing of the semiconductor wafers for the next and subsequent predetermined number of wafers is performed. In the laboratory After each polishing, a predetermined number of semiconductor wafers from the optimum polishing time, the measured film thickness value before polishing, and the measured film thickness value after polishing measured by the film thickness measuring means by the polishing time calculation means. The optimum polishing time is calculated, and the predetermined number of semiconductor wafers are polished with the optimum polishing time. Thereafter, each time the polishing is sequentially completed, the optimum polishing time of the predetermined number of semiconductor wafers is calculated, and the optimum polishing time is calculated. A polishing controller for polishing a predetermined number of semiconductor wafers. 3. The apparatus for polishing a semiconductor wafer according to claim 2, wherein said film thickness measuring means measures film thicknesses at a plurality of points on the semiconductor wafer and sets an average value as a measured film thickness.