JPS6166104A - Method for measuring thickness of thin metal film - Google Patents

Abstract

PURPOSE:To measure the thickness of very thin films highly accurately, by providing two current coils inducing eddy currents so as to face the upper and lower surfaces of the thin metal films. CONSTITUTION:The oscillating coil of a Copitts-type oscillator is divided into two parts L1 and L2, which are both eddy-current inducing coils. Thin films to be measured are provided between the coil L1 and the coil L2, and the measurement is carried out. Namely, three thin films to be measured 31, whose thicknesses t=t1, t2 and t3 are accurately measured, are prepared. A distance lbetween the two coils L1 and L2 is fixed at a constant value. The thin film 31 and an insulating substrate 30 are held between the measuring coils. A distance (d) between the coil L1 and the surface of the thin films 31 are variously changed, and the oscillating amplitude of the oscillator is measured. The value of (d) is made to be the value in the vicinity of l/2. Thus the measurement with few errors can be carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring the thickness of a metal thin film used in the manufacturing process of semiconductor devices, printed wiring boards, etc.

(Prior art and its problems) One method of measuring the thickness of a metal thin film is to bring a coil to which diagonal gold is applied close to the thin film to be measured to generate eddy currents in the thin film. There is a method of measuring the film thickness by utilizing the fact that eddy current loss is proportional to the film thickness of the thin film and this reduces the Q of the coil.

Conventional measurement methods using this method are as follows.

For example, as shown in Fig. 4, a coil L, a capacitor C,
, C, , the coil L of the oscillator O8C consisting of the amplifier T
e. Wrap it in a small size as indicated by L in FIG.
'i dl, measure the oscillation amplitude (output) A volt of the oscillator O8C, and from this value A, calculate the film thickness t of the thin film using the oscillation width vs. film thickness curve JDt in Figure 6. It is something to know. However, the "oscillation width vs. film thickness curve" in Figure 6
For D□, the film thicknesses of various thin films having the same material and film structure as the thin film to be measured are accurately measured using a separate precise measurement method, and the thicknesses of these films are measured at a distance d: = d. It had been drawn and prepared in advance by carefully measuring the oscillation amplitude.

This conventional measurement method has the following drawbacks. That is, coil L
Distance d from force ζdl to the surface of the film to be measured 31 d2
．． When changing to d3, the "oscillation amplitude vs. film thickness curve" changes from curve DI (d=dl) to D2 (d=dz
), D3 (d=d,), therefore, during measurement, the measurement error will increase unless the distance d is exactly matched to d.

For example, in a commercially available measuring device that employs this conventional measuring method, in order to measure a film thickness of approximately 1 μm with an error of ±0.01 μm, the distance d must be within the range of dl ±5 μm. This often means that it cannot be measured. This is because in many cases, ±5 μm already exceeds the range of curves or irregularities in the L portion of the coil at 3° of the substrate. That is, when attempting to measure an extremely thin film having a thickness below a certain level, the conventional measuring method shown in FIGS. 4 and 5 cannot be used.

OBJECTS OF THE INVENTION The present invention aims to overcome this drawback of the prior art methods.

(Structure of the Invention) The present invention sandwiches a metal thin film to be measured and installs two coils facing each other on the front and back sides of the thin film to induce an eddy current in the thin film, and uses the energy loss caused by the eddy current to The above objective was achieved by measuring the film thickness of the film.

(Embodiment) FIG. 1 shows an oscillator according to an embodiment of the present invention, in which the Colpitts-type oscillator oscillation coil L shown in FIG. 4 is divided into two, TJI and L.
2, these are both eddy current induction coils, and a thin film to be measured is placed between the coil L1 and the coil L2, and the measurement is performed.

Figure 2 shows the measurement situation.

In these figures 1 and 2, the film thickness ""tl + '2 g '3
Prepare three thin films 31 to be measured that have been accurately measured, and place them between the measuring coils with the distance e between two (7): 'I/' Ll and L2 fixed at a constant value, as shown in Figure 2. Thin film 31. Sandwiching the insulating substrate 30, the coil L! The oscillation width of the oscillator shown in FIG. 1 was actually measured by varying the distance d between the oscillator and the surface of the thin film 31, and a "curve of oscillation width versus distance d" was drawn using t as a parameter. T+ (t=tt) in Figure 3
, T2 (t=t2) and Ta (t=ts) curves.

FIG. 3 shows a curve T1° (t=b) obtained by measuring the same sample using the conventional measurement method shown in FIGS. 4 and 5.

T2'' (t=12) and Tso (t=ts) are also shown with dotted lines.

The curves T+, T2, and Ta each show a minimum value at dokey e/, and are curved as quadratic curves. Therefore, by setting the distance d close to %, measurement with less error is possible.

To give one example, insulating coated copper wire is attached to a core with a diameter of 65 mm.
Two 80μH coils are wrapped around each other, Ll,
used as L2 and using a frequency of 200 KHz,
When trying to measure the thickness of a 1 μm thin film with an error of 0.01 μm, the error allowed for the distance d was ±5QIMn.

With a 160 μH coil using the conventional measurement method described above,
This can be said to be a significant improvement compared to the tolerance error of d which was ±5 μm.

According to experiments, it was possible to measure the film thickness with the same good results as L1 and L2 even if the wires were connected in opposite directions.

In addition, when using this measuring device, the unevenness of the surface of the film to be measured 31, the curvature of the insulating substrate 30, etc. is generally less than 10/jT1 within the range of the coil size. was found to have sufficient practicality.

FIG. 7 shows the measurement results of another example.

The object to be measured is an aluminum thin film of ~2 μm deposited on a silicon single crystal substrate with a thickness of 500 μm, and two 68 μH coils are set as Ll and L2, and the distance between them is 6 = 3.
．． 5 m11 were fixed facing each other, and the insertion place of the substrate to be measured was fixed in the center thereof, and repeated measurements were performed to obtain [oscillator output vs. film thickness curve] B. Even when the measurements were repeated, the results were always within the range of the thickness of curve B.

When similar measurements were made in the conventional manner using a coil with L=150 μH, a curved band B° was obtained. When the measurement is repeated, the measurement result floats within this curve band B°, indicating a large error.

Since the method of the present invention measures the energy loss of eddy currents generated in a metal thin film, the measurement can be performed using a resonant circuit as shown in FIG. 8 without using an oscillator.

In Figure 8, the output of crystal oscillator X08C is output to amplifier AM.
It becomes a constant value through P+ and is applied to the resonant circuit of the coil LI+L2 and the capacitor C, and the terminal voltage of the resonant circuit is read by the meter M through the buffer AMP2. Board to be measured 30. Similarly to the above, the thin film 31 is inserted between the coils L and L2 and measured as shown in the figure.

In addition, although we have shown methods for measuring eddy current energy loss using changes in voltage, measurements can also be made using changes in phase. It has many implementations.

Note that the material of the substrate 30 on which the thin metal film 31 to be measured is placed does not necessarily have to be an insulator.

As long as there is a difference in electrical conductivity between the thin film 31 and the substrate 30, it is possible in principle to measure the thickness of the thin film using the method of the present invention. However, the greater the difference in electrical conductivity, the higher the accuracy of measurement, which is advantageous.

(Effects of the Invention) The present invention is as described above, and the thickness of an extremely thin metal film can be measured with high accuracy, and the apparatus can be constructed at low cost.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a measurement oscillator according to an embodiment of the present invention. FIG. 2 is a diagram showing the measurement situation. Figure 3 is a graph of the measurement results. FIG. 4 is a circuit diagram of a conventional measurement oscillator. FIG. 5 is a diagram showing the measurement situation. Figure 6 is a graph of the measurement results. FIG. 7 is a graph comparing the measurement results of another embodiment of the present invention with the measurement results of the conventional method. FIG. 8 is a measurement circuit diagram of another embodiment of the present invention.

Claims (1)

[Claims] In a metal thin film thickness measurement method in which an eddy current is passed through a metal thin film to be measured and the magnitude of energy loss caused by the eddy current is converted into the thickness of the metal thin film, two current coils that induce the eddy current are A method for measuring the thickness of a metal thin film, characterized in that the metal thin film is placed facing each other on the front and back sides thereof.