JPH07111286A - Evaluating method for surface conditions of thin film - Google Patents

Abstract

PURPOSE:To obtain quickly, simply and quantitatively the information about the top surface of a thin film by bringing an A electrolytic solution into contact with the thin film covering the surface of a substrate, applying a voltage between a substrate electrode and a reference electrode immersed in the electrolytic solution and measuring an output voltage between the source and drain electrodes. CONSTITUTION:An ISFET 10 with an insulating thin film 5 to be measured, which was formed on the surface of a silicon substrate 1, is immersed in an electrolytic solution 7 in a solution tank 6. In this electrolytic solution 7, a glass electrode 8 using AgCl as an electrolytic liquid 81 is immersed as a reference electrode; and a DC voltage from a power source 9 is applied between the glass electrode 8 and a substrate electrode provided at the ISFET 10. Then, a phase boundary potential is generated in response to the flow of electrons at the phase boundary between the electrolytic solution 7 and the thin film 5. Therefore, the cleanliness level of the thin film such as th level of surface conditions related to the absorption site density of surface can be quantitatively evaluated from output voltage between a drain terminal D and a source terminal S of the ISFET 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the surface condition of a thin film used in the manufacture of semiconductor devices, for example, using a thin film formed on a semiconductor substrate as a functional material.

[0002]

2. Description of the Related Art The surface cleanliness of a thin film such as an oxide film or a nitride film formed on the surface of a semiconductor substrate or the wettability of a liquid to the surface cleanliness of subsequent semiconductor devices or the characteristics of finished devices. Have a great influence on. Therefore, if information on the surface state of such a thin film can be obtained and evaluated at an arbitrary stage, it is extremely advantageous for improving the characteristics of the semiconductor device or for production control.

Conventional methods for evaluating the surface condition of thin films have been:
Ion micro analyzer (IMA), photoelectron spectrometer
(ESCA), Slow Ion Scattering Spectrometer (IS
S), scanning Auger electron spectroscopy (SAM), low-speed electron diffraction (LEED), X-ray absorption broad continuous fine structure (EX)
Instrument analysis such as AFS) and Rutherford backscattering device (RBS) is known as an effective means. Table 1 shows the information obtained by these instrumental analyses, ⊚ is very effective, ∘ is effective, Δ is conditionally effective, and × is ineffective.

[0004]

[Table 1] It is well known that very useful information can be obtained by combining these instruments, and it is expected that such analysis methods will become more sophisticated.

[0005]

Each of the evaluation means shown in Table 1 is generally disadvantageous in that the equipment is expensive, the evaluation takes time, and it is relatively difficult to obtain information on the outermost surface of the thin film. Have. In order to obtain information on the outermost surface of a thin film, constraints such as rapid evaluation and the need for extremely low energy analytical means are essential. The wettability evaluation and the like have been utilized as one means for obtaining information on the outermost surface, but there is a problem as a quantitative evaluation method.

An object of the present invention is to solve the above-mentioned problems and to provide a method for evaluating the surface state of a thin film, by which information on the outermost surface of the thin film can be obtained quickly, simply and quantitatively.

[0007]

In order to achieve the above object, the method for evaluating the surface state of a thin film according to the present invention is such that the source and drain regions of the second conductivity type are selectively formed with a space between each other. The surface of the first conductivity type layer is covered with a silicon oxide film, and a source electrode and a drain electrode that respectively contact the source and drain regions at the opening of the silicon oxide film and a substrate electrode that contacts the first conductivity type layer are provided. The surface of the silicon substrate is coated with a thin film to be evaluated, an electrolyte solution is brought into contact with the thin film, and a source is applied when a voltage is applied between the reference electrode and the substrate electrode immersed in the electrolytic solution,
The output voltage between the drain electrodes is measured, and the measured value is used. It is effective that the reference electrode is a glass electrode. It is also effective to calculate the output voltage per pH from the output voltage between the source and drain electrodes measured using electrolyte solutions having different pH values and use that value.

[0008]

[Function] As one means for quickly, easily and quantitatively obtaining information on the outermost surface in order to evaluate the surface state of a thin film, attention is paid to the energy change at the solid / liquid interface as seen in the evaluation of wettability. The evaluation method can be considered. In particular, when considering the evaluation of thin films formed in semiconductor processes, it is more in-situ that the evaluation can be realized in semiconductor processes.
It is targeted, quick and convenient. The present invention focuses on the operating principle of an ISFET (Ion Sensitive FET) as a means for that purpose, and utilizes it. The operating principle of the ISFET is as follows. FIG. 1 is a cross-sectional view showing a base structure of an ISFET. A P-type source region 21 and a drain region 22 are selectively formed on the surface of an N-type silicon substrate 1, and the source and drain regions 21 and 22 have a surface. The source electrode 41 and the drain electrode 42 are in contact with each other through the contact holes formed in the upper silicon oxide film 3. Further, an insulator thin film 5 to be measured is formed on this. This ISFET10
Is immersed in the electrolyte solution 7 in the liquid tank 6 as shown in FIG. In this electrolyte solution 7, a glass electrode 8 using AgCl as an electrolyte solution 81 is immersed as a reference electrode, and the glass electrode 8 and IS
Between the substrate electrode (not shown) provided on the FET 10,
A DC voltage is applied by the power supply 9. By immersing the ISFET 10 having the structure shown in FIG. 1 in the electrolyte solution 7, an interface potential is generated according to the degree of electron transfer at the interface between the electrolyte solution 7 and the thin film 5. The relationship between the output voltage between the source terminal S and the drain terminal D of the ISFET 10 and the surface state of the thin film 5 is described by, for example, WM Sin et al.
ED, Vol ED-26 (1979) p.1805 or Nakajima et al., As already described as "Site Binding Model", as described in Chemical Society of Japan (1980) No.10, p.1499. This model is an intermediate case between two extreme cases: (1) complete transfer of electrons and (2) no transfer of electrons at the interface between the electrolyte solution and the thin film. , So to speak, it is a model that handles the real system. The case of (1) is an ideal system according to the Nernst equation, and the'Site Binding Model 'corresponds to the case where the density of adsorption sites of ions and atoms on the surface of the thin film in contact with the electrolyte solution is high, and the output per pH is 2.3 kT / q (=
59 mV). That is, the density of adsorption sites on the thin film surface can be estimated by evaluating the output per pH. As shown in Table 2, the results of evaluating the output per pH of various gate surface materials of ISFET have been reported. [Matsuo: Applied Physics, Volume 49 (198
0) See p586, etc.).

[0009]

[Table 2] From this result, it is possible to estimate what kind of film is formed on the surface of the thin film. FIG. 3 shows the relationship between ΔpH and flat band voltage shift for different adsorption site densities under a certain condition, with the adsorption site density N being a paramate (see the report by W. M. Sin et al. Above).
. Since the structure of the ISFET shown in FIG. 1 is a structure that can be arbitrarily set during the manufacturing process of a semiconductor device such as an IC, if the thin film 5 to be measured in the surface state is formed as the thin film 5, the semiconductor device manufacturing process can be performed. A function as a process monitor can be added.

[0010]

[Example] A thermal nitride film was selected as a thin film for surface evaluation, and FIG.
In the ISFET having the above structure, a gate oxide film 3 having a thickness of 50 nm was formed, and a thermal nitride film having a gate length of 30 μm and a gate width of 270 μm was formed with a thickness of 50 nm as a gate formed of the thin film 5 to be measured. Source electrode 41 and drain electrode 42
Is made of Pt and these electrodes and Al shown in FIG.
The lead wires 11, 12, and 13 are made of electrolyte solution 7 at the time of measurement.
It was protected with a vinyl chloride resin so that it would not come into contact with. HCl or HNO ₃ is used as the acid, caustic soda is used as the alkali, and the mixture is appropriately mixed with water to obtain an acidic solution having a pH of 4.1 and a pH of 6.8.
A neutral solution and an alkaline solution having a pH of 9.2 are prepared as electrolyte solutions 7, respectively, and pH sensitivity to pH change from acidic to neutral, neutral to alkaline, alkaline to acidic,
That is, the value obtained by dividing the difference between the output voltages obtained from the S and D terminals by the pH difference was obtained. Table 3 shows these sensitivities before boiling and 1
The results of evaluation after boiling twice, boiling twice, and boiling three times are shown. The boiling was always carried out using freshly prepared pure water at 100 ° C. for 1 hour.

[0011]

[Table 3] As is clear from Table 3, the sensitivity increased as the boiling was repeated. This means that the adsorption site density on the surface of the thermal nitride film 5 has increased, that is, the cleanliness of the surface of the thermal nitride film 5 has increased. This is consistent with the result of the decrease in the amount of carbon or the like adhering to the result of instrumental analysis such as ESCA.

[0012]

According to the present invention, the ISFET based on the Site Binding Model at the interface between the thin film surface and the electrolyte solution.
By using output, it is not possible to perform analytical evaluation such as element identification and quantification like instrumental analysis, but to some extent quantitatively evaluate the level of surface condition related to surface adsorption site density such as cleanliness level of thin film surface. it can. Furthermore, this evaluation method does not require a sample for further evaluation, and it can be built at the same time as a semiconductor device as a monitor at any place in the wafer, and its area can be reduced to about one cell of the semiconductor device. -It also has the effect of being able to perform in-situ evaluation.

[Brief description of drawings]

FIG. 1 is a sectional view of an ISFET used according to the present invention.

FIG. 2 is a sectional view showing a measurement system of ISFET.

FIG. 3 is a diagram showing the relationship between the difference in pH value using the adsorption site density as a parameter and the flat band voltage shift.

[Explanation of symbols]

1 N Silicon substrate 21 P ⁺ source region 22 P ⁺ drain region 3 Gate oxide film 41 Source electrode 42 Drain electrode 5 Measured thin film 7 Electrolyte solution 8 Glass electrode 10 ISFET

Claims (3)

[Claims] 1. A surface of a first conductivity type layer in which source and drain regions of the second conductivity type are selectively formed with an interval between each other is covered with a silicon oxide film, and the source is formed in the opening of the silicon oxide film. A surface of a silicon substrate having a source electrode and a drain electrode in contact with the drain region and a substrate electrode in contact with the first conductivity type layer is covered with a thin film to be evaluated, and the thin film is contacted with an electrolyte solution to form an electrolyte. A method for evaluating a surface state of a thin film, which comprises measuring an output voltage between a source electrode and a drain electrode when a voltage is applied between a reference electrode immersed in a solution and a substrate electrode, and using the measured value. 2. The method for evaluating the surface condition of a thin film according to claim 1, wherein the reference electrode is a glass electrode. 3. The output voltage per pH is calculated from the output voltage between the source and drain electrodes measured using electrolyte solutions having different pH values, and the value is used.
A method for evaluating the surface condition of the thin film described.