A method for measuring thin film work function

Abstract

A method for measuring the work function of a thin film. The steps are: A. Calibrate the surface potential measured by a Kelvin probe force microscope using a standard sample; B. Process an isolation groove on the film to be measured to separate the area to be measured from the surrounding film area; C. Thin the isolated area to be measured to eliminate the effects of surface oxidation and adsorption on the measurement; D. Use a Kelvin probe force microscope to measure the potential of the film area to be measured relative to the conductive needle tip; E. Calculate the work function of the film based on the relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be measured, i.e., the work function of the film to be measured is equal to the known work function of the standard sample plus the difference in relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be measured. The present invention eliminates the effects of metal or alloy surface oxidation and adsorption effects to obtain a true, high-precision work function value; and can provide nanometer-level spatial resolution and positioning capabilities, making it suitable for in-situ measurement of thin films.

Description

Technical Field

[0001] The present invention relates to a method for detecting the work function of a metal thin film material, in particular to a method for measuring the work function of a thin film. Background Art

[0002] Metal films have a wide range of applications. In semiconductor devices, they are used as electrode materials, gate materials, and electrical interconnect materials for contact with semiconductors, making them an indispensable component of modern electronic and optoelectronic devices. Semiconductor devices generally have specific requirements for the work function of metal films. For example, when in contact with semiconductor materials, the work function of metals and alloys is the main factor in modulating the electrical properties of metal-semiconductor contacts. When used as an interconnect layer, accurately measuring its work function can help optimize the preparation process of the interconnect layer, thereby improving the performance and reliability of the circuit. When used as a gate, accurately measuring its work function can help optimize the preparation process of the gate oxide and improve the performance and reliability of the transistor. Therefore, accurately measuring the work function of metal films plays an important role in the design, preparation, and performance evaluation of semiconductor devices.

[0003] Currently, methods for measuring the work function of metal materials include photoelectron spectroscopy (PES), thermionic emission (TE), field emission (FE), and contact potential difference (CPD). These methods each have their own unique characteristics, but also certain limitations. First, the measurement results are limited by practical factors, resulting in low accuracy. Most work function measurements rely on the surface state of the material. Adsorption and oxidation on the metal surface can significantly interfere with the electron work function, or work function. Conventional surface cleaning and deoxidation processes can significantly affect the electrical properties of metal films with thicknesses of only tens or even a few nanometers. Currently, the metal work functions reported in materials handbooks and literature are primarily measured using these methods, with numerical accuracies typically ranging from 50 to 100 millielectronvolts (meV). However, modern low-threshold, low-power semiconductor device fabrication has achieved control over the work function of metal layers with accuracies of 10 millielectronvolts or even lower. Second, the measurement targets are limited. Most methods are effective for measuring the work function of elemental metals, but are less applicable to alloys, especially thin-film alloys, or the analysis of the measurement results can easily deviate from the true value. The metal gate films of current advanced CMOS devices are already made of compound metals such as TiN and AlTi. To determine the work function of these alloy films, the microelectronics industry typically uses capacitance-voltage (CV) and flat-band voltage methods. However, these methods require the preparation of a complete device structure for testing, resulting in lengthy processes, low efficiency, and difficulty in timely evaluation and feedback. Furthermore, measurement results are susceptible to factors such as capacitance attenuation and crosstalk, making it difficult to ensure repeatability and accuracy. With the continued reduction of CMOS dimensions and the demands for lower gate voltage and reduced leakage, the thickness of TiN, AlTi, and other thin films in transistors has been reduced to the nanometer scale. The atomic structure and electronic state near the film surface are significantly affected by the substrate and adjacent layers, causing the work function of the alloy material to vary with film thickness. Therefore, accurately measuring the work function of alloy films of varying thicknesses has become a critical requirement for the research and application of semiconductor electronic devices and materials science. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for measuring the work function of a thin film, which mainly solves the problems existing in the above-mentioned prior art. The method of the present invention provides a new solution for work function measurement for key metal and alloy thin film process control in microelectronics and other fields. The main feature is to eliminate the influence of metal or alloy surface oxidation and adsorption effects to obtain a true, high-precision work function value; at the same time, the method can provide nanometer-level spatial resolution and positioning capabilities, and is suitable for in-situ measurement of thin films.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0006] A method for measuring the work function of a thin film, characterized in that it comprises the following steps:

[0007] A. Calibrate the surface potential using a Kelvin probe force microscope (i.e., KPFM) using a standard specimen.

[0008] B. Processing an isolation groove on the film to be tested to separate the area to be tested from the surrounding film area;

[0009] C. Thin the isolated area to be measured to eliminate the effects of surface oxidation and adsorption on the measurement;

[0010] D. Use Kelvin probe force microscopy to measure the electric potential of the film area to be tested relative to the conductive needle tip;

[0011] E. Calculate the work function of the film based on the relative potentials measured on the standard sample and the film to be tested by Kelvin probe force microscopy (i.e., using Kelvin probe force microscopy, KPFM). That is, the work function of the film to be tested is equal to the known work function of the standard sample plus the difference in relative potentials measured on the standard sample and the film to be tested by Kelvin probe force microscopy.

[0012] The method for measuring the work function of a thin film is characterized in that: in step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope, wherein the standard sample is a thin film material with a known work function, a smooth surface and not easily oxidized.

[0013] The method for measuring the work function of a thin film is characterized in that the thin film material with a known work function is a newly dissociated highly oriented pyrolytic graphite (HOPG) or a freshly deposited precious metal film (such as gold or platinum).

[0014] The method for measuring the work function of a thin film is characterized by using a standard sample in step A to calibrate the surface potential measured by a Kelvin probe force microscope. When measuring the surface potential of the standard sample using the Kelvin probe force microscope, the relative height, drive voltage, drive phase, and other parameters of the probe are adjusted to stabilize the measurement, and the measured relative surface potential is equal to the work function difference between the standard sample and the tip material. Surface potential measurement requires the use of a fresh conductive probe, such as a highly doped silicon probe, or a probe coated with a metal such as gold or platinum.

[0015] The method for measuring the work function of a thin film is characterized in that: in step B, an isolation groove is processed on the thin film to be measured to separate the area to be measured from the surrounding thin film area, which means that the thin film material is locally removed until the substrate, and the isolation groove forms a complete closed loop, thereby preventing the measurement of the surface potential of the thin film in the isolated area in step C from being affected by the surrounding area.

[0016] The method for measuring the work function of a thin film is characterized in that the isolation groove is formed by scanning and carving the surface of the thin film using an atomic force microscope probe with a hard coating attached thereto.

[0017] The method for measuring the work function of a thin film is characterized in that: in step C, the thin film in the isolated area to be measured is thinned, and an atomic force microscope probe with a hard coating is used, a contact mode is applied, and the probe pressure is controlled to scan to achieve the purpose of removing the surface oxide layer and adsorbents.

[0018] Wherein: the isolation and thinning treatment can be performed by atomic force microscopy. The specific method and steps are: using a probe with a harder hardness higher than that of the film material to be tested, applying appropriate pressure to the probe in contact mode to scan the film to be tested, which can gradually remove the surface oxide layer and film material. The area to be tested can be isolated with greater pressure, and the periphery of the area to be tested is repeatedly scanned to gradually thin the metal or alloy material. To ensure that the peripheral film has no electrical effect on the area to be tested, the depth of the thinned isolation groove needs to reach the substrate layer, and a closed-loop isolation groove needs to be formed around the area to be tested. Removal of the surface oxide or adsorption layer of the area to be tested should be done with a smaller pressure scan, and the thickness removed by scanning is slightly greater than the thickness of the oxide layer on the metal or alloy surface, usually 1 to 2 nanometers.

[0019] In step E, when measuring the relative surface potential of the film's test area, the tip type and conditions used are consistent with those used for measuring the standard sample. The work function of the film to be tested is equal to the known work function of the standard sample plus the difference in relative potentials measured on the standard sample and the test sample using Kelvin probe force microscopy.

[0020] Through the above technical solution, the present invention has the following advantages:

[0021] 1. The method of the present invention offers high measurement reliability and repeatability. Using tip scanning, thin films can be thinned with sub-nanometer precision, eliminating the effects of surface adsorption and oxidation on potential and work function measurements. Furthermore, localized deep thinning isolates the electrical connections of the surrounding film from affecting work function measurements in the fresh region.

[0022] 2. The measurement accuracy of the method of the present invention is high. By calibrating the potential measurement conditions and parameters of the scanning probe method using standard samples, the error of the work function can be less than 30 millielectron volts.

[0023] 3. The method of the present invention has spatial resolution capability and is suitable for measuring thin films in micro-nano regions, and is particularly suitable for alloy materials that are difficult to accurately measure using other methods.

[0024] 4. The method of the present invention can be used for online measurement of thin films. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of the TiN thin film for measuring the work function in an embodiment of the present invention.

[0026] Figure 2 It is a schematic diagram of the method of the present invention for measuring the surface potential on a Kelvin probe force microscope using a standard sample of gold (Au) film or highly oriented pyrolytic graphite (HOPG) with a known work function for calibration.

[0027] Figure 3 FIG. 1 is a schematic diagram of partially isolating and thinning the TiN film to form a region to be tested in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] The present invention discloses a method for measuring the work function of a thin film, which comprises the following steps:

[0029] A. Use standard samples to calibrate the surface potential measured by Kelvin probe force microscope;

[0030] B. Processing an isolation groove on the film to be tested to separate the area to be tested from the surrounding film area;

[0031] C. Thin the isolated area to be measured to eliminate the effects of surface oxidation and adsorption on the measurement;

[0032] D. Use Kelvin probe force microscopy to measure the electric potential of the film area to be tested relative to the conductive needle tip;

[0033] E. Calculate the work function of the film based on the relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be tested. That is, the work function of the film to be tested is equal to the known work function of the standard sample plus the difference between the relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be tested.

[0034] In step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope, wherein the standard sample is a thin film material with a known work function, a flat surface and is not easily oxidized.

[0035] The thin film material with known work function is newly dissociated highly oriented pyrolytic graphite or freshly deposited noble metal film.

[0036] In step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope. When the Kelvin probe force microscope measures the surface potential of the standard sample, the measurement is stabilized by adjusting the relative height, driving voltage, driving phase and other condition parameters of the probe, and the measured relative surface potential value is equal to the difference in work function between the standard sample and the needle tip material.

[0037] In step B, an isolation groove is processed on the film to be tested to separate the area to be tested from the surrounding film area, which means that the film material is locally removed until the substrate, and the isolation groove forms a complete closed loop, thereby preventing the measurement of the surface potential of the film in the isolated area in step C from being affected by the surrounding area.

[0038] The isolation groove is formed by scanning and carving on the surface of the film using an atomic force microscope probe with a hard coating attached thereto.

[0039] In step C, the thin film of the isolated area to be tested is thinned, and an atomic force microscope probe with a hard coating is used in contact mode to control the probe pressure for scanning, thereby removing the surface oxide layer and adsorbents.

[0040] Example

[0041] The following uses a TiN metal film grown on a SiO2 / Si substrate as an example of the method of the present invention to further describe the method of the present invention. Figure 1 As shown, the thickness of the TiN film is 1 to tens of nanometers.

[0042] The measurement of thin film surface potential and atomic-level thinning and isolation are all performed using a Multimode Nanoscope IV scanning probe microscope produced by Veeco, USA. The operating modes are Kelvin probe force microscopy (KPFM, also known as surface potential imaging) and contact atomic force microscopy mode. The potential measurement and atomic force thinning tips are selected from heavily doped silicon probes and diamond-coated probes, respectively.

[0043] First, the Kelvin probe force microscope (KPFM) is calibrated. The purpose is to use a standard sample with a known work function to calibrate the conditions and parameters for measuring the surface potential using the Kelvin probe force microscopy method. The standard sample is a fresh gold film or highly oriented pyrolytic graphite (HOPG) whose surface is not easily oxidized. The Kelvin probe force microscope is affected by several factors when measuring the surface potential of the material and calculating the work function, including the relative height of the needle tip, the driving voltage of the probe, the phase, the position of the microscope spot on the probe cantilever, etc. By adjusting the measurement conditions and parameters, the potential measured by the needle tip on the standard sample can be made equal to the difference between the work functions of the two, that is, V CPD-标样 =(Φ 针尖 -Φ 标样 ) / e. Figure 2 It shows that with the adjustment of measurement parameters, the value of the work function gradually approaches the true value of the standard sample (gold or highly oriented pyrolytic graphite).

[0044] Then, a diamond-coated probe is used as a nano-scalpel to perform local isolation and surface thinning on the test film sample in contact atomic force microscopy mode. Specifically, the sample to be tested is first adhered and fixed to the sample base plate, with the TiN metal layer facing upward. A rectangular area of ​​micrometers square is selected on the surface of the film as an isolation area. Then, a diamond-coated probe is used to scan along the edge of the pre-designed rectangle, gradually removing the TiN material until the oxide layer (SiO2) or the substrate Si is formed. An isolation groove is formed, which isolates the film within the rectangular range from the surrounding unthinned TiN film, obtaining an isolation area for measuring the work function, as shown in FIG. Figure 3 shown.

[0045] The next step is to thin the isolated area, that is, to remove the oxide layer on the TiN surface. Using a diamond-coated probe, the isolated area is scanned in contact atomic force microscopy mode. By controlling the force between the probe tip and the sample, the surface material within this range is scraped off. For TiN thin films, a fresh TiN surface can be obtained by removing a thickness of about 1 to 1.5 nanometers using the probe tip. The thinned thickness can be obtained by simultaneously scanning the isolated and thinned area and the surrounding unthinned area and measuring the height difference between the two. In previous measurements or studies, only a local area of ​​the metal surface is processed to obtain a local fresh surface. However, the charge effect formed by the oxidation of the surface or interface of the surrounding thin film material still affects the work function of the metal material in the thinned area. Specifically in this embodiment, the area where the oxide layer is thinned and removed only occupies a very small part of the TiN film. If it is not isolated, electrons can still move between the thinned area and the surrounding area, thereby affecting the relative surface potential and work function of the measured area.

[0046] Finally, the potential of the isolated and thinned TiN film relative to the conductive needle tip is measured to infer the work function. Specifically, in the Kelvin probe force microscopy mode, a heavily N-type doped silicon probe is used to measure the surface potential V of the film to be tested. CPD-薄膜 The surface potential is the difference between the work function of the TiN film and the electron work function of the silicon tip, that is, V CPD-薄膜 =(Φ 针尖 -Φ 薄膜 ) / e. In this embodiment, the surface potential of a 4 nm thick TiN metal film grown on a SiO2 / Si substrate was measured using a silicon conductive probe to be 0.55 V. The electron work function of heavily N-type doped silicon is known to be Φ 针尖 is 4.05eV, so the work function of the 4 nm thick TiN film is 4.60eV. Alternatively, the work function of the TiN film can be obtained by comparing the relative potential difference measured on the standard sample (highly oriented pyrolytic graphite) and the TiN film using the Kelvin probe force method, that is, the work function Φ of the TiN film.薄膜 =e(V CPD-薄膜 -V CPD-标样 )+Φ 标样 get.

[0047] The method of the present invention is aimed at the requirement of accurately controlling the electron work function of metal and alloy films in the field of semiconductor device manufacturing. It combines the nanoscale precision processing and local potential measurement capabilities of scanning probe microscopy technology to achieve a method for quantitative analysis of the work function of metal or alloy films.

[0048] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for measuring the work function of a thin film, characterized in that: It includes the following steps: A. Use standard samples to calibrate the surface potential measured by Kelvin probe force microscope; B. Processing an isolation groove on the film to be tested to separate the area to be tested from the surrounding film area; C. Thin the isolated area to be measured to eliminate the effects of surface oxidation and adsorption on the measurement; D. Use Kelvin probe force microscopy to measure the electric potential of the film area to be tested relative to the conductive needle tip; E. Calculate the work function of the film based on the relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be tested. That is, the work function of the film to be tested is equal to the known work function of the standard sample plus the difference between the relative potentials measured by the Kelvin probe force microscope on the standard sample and the film to be tested.

2. The method for measuring the work function of a thin film according to claim 1, wherein: In step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope, wherein the standard sample is a thin film material with a known work function, a flat surface and is not easily oxidized.

3. The method for measuring the work function of a thin film according to claim 2, wherein: The thin film material with known work function is newly dissociated highly oriented pyrolytic graphite or freshly deposited noble metal film.

4. The method for measuring the work function of a thin film according to claim 1, wherein: In step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope. When the Kelvin probe force microscope measures the surface potential of the standard sample, the measurement is stabilized by adjusting the relative height, driving voltage, driving phase and other condition parameters of the probe, and the measured relative surface potential value is equal to the difference in work function between the standard sample and the needle tip material.

5. The method for measuring the work function of a thin film according to claim 1, wherein: In step B, an isolation groove is processed on the film to be tested to separate the area to be tested from the surrounding film area. This means that the film material is locally removed until it reaches the substrate, and the isolation groove forms a complete closed loop to prevent the measurement of the surface potential of the film in the isolated area in step C from being affected by the surrounding area.

6. The method for measuring the work function of a thin film according to claim 5, wherein: The isolation groove is formed by scanning and carving on the surface of the film using an atomic force microscope probe with a hard coating attached thereto.

7. The method for measuring the work function of a thin film according to claim 1, wherein: In step C, the thin film in the isolated test area is thinned, and an atomic force microscope probe with a hard coating is used in contact mode to control the probe pressure for scanning to remove the surface oxide layer and adsorbents.

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