Method for measuring work function of thin film

Abstract

The invention discloses a method for measuring a film work function. The method comprises the following steps: A, calibrating the surface potential measured by the Kelvin probe force microscope by using a standard sample; b, processing an isolation groove in the to-be-detected film, and separating a to-be-detected area from a peripheral film area; c, thinning the isolated to-be-measured area, and eliminating the influence of surface oxidation and adsorption on measurement; d, measuring the potential of the to-be-measured area of the film relative to the conductive tip by using a Kelvin probe force microscope; and E, calculating the work function of the film according to the relative potentials measured on the standard sample and the film to be measured by the Kelvin probe force microscope, namely, the work function of the film to be measured is equal to the difference between the known work function of the standard sample and the relative potentials measured on the standard sample and the film to be measured by the Kelvin probe force microscope. According to the invention, the influence of metal or alloy surface oxidation and adsorption effect is eliminated, and a real and high-precision work function value is obtained; the device can provide nano-scale spatial resolution and positioning capability, and is suitable for in-situ measurement of a thin film.

Description

Technical Field

[0001] The invention relates to a method for detecting the work function of a metal film material, in particular to a method for measuring the work function of a 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 that contact semiconductors, and are therefore an indispensable component of modern electronic and optoelectronic devices. Semiconductor devices usually 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, accurate measurement of its work function can help optimize the preparation process of the interconnect layer, thereby improving the performance and reliability of the circuit. When a metal film is used as a gate, accurate measurement of its work function can help optimize the preparation process of the gate oxide and improve the performance and reliability of the transistor. Therefore, accurate measurement of the work function of metal films plays an important role in the design, preparation, and performance evaluation of semiconductor devices.

[0003] At present, the methods for measuring the work function of metal materials include photoelectron spectroscopy (PES), thermal electron emission (TE), field emission (FE) and contact potential difference (CPD). These methods have their own characteristics and limitations. First, the measurement effect is limited by practical factors and the accuracy is not high. Most work function measurements rely on the surface state of the material. The adsorption and oxidation of the metal surface will significantly interfere with the work function of the electron, that is, the work function. The conventional surface cleaning and deoxidation process has a significant impact on the electrical properties of metal films with a thickness of only tens or even a few nanometers. At present, the metal work functions reported in material manuals and literature are basically measured by the above methods, and the numerical accuracy is usually 50 to 100 millielectron volts (meV), while the control accuracy of the work function of the metal layer in the preparation of modern low-threshold, low-power semiconductor devices has reached 10 millielectron volts or even lower. Secondly, the object of measurement is limited. Most methods are more effective in measuring the work function of single metal materials, but they are not easy to apply to alloy materials, especially the work function measurement of thin film alloys, or the analysis of the measurement results is easy to deviate from the true value. The metal gate films of current advanced process CMOS devices have been made of compound metals such as TiN and AlTi. In order to determine the work function of these alloy films, the microelectronics industry usually uses capacitance-voltage (CV) and flat-band voltage methods. However, these methods require the preparation of a complete device structure for testing, which has a long process, low efficiency, and is difficult to evaluate and feedback in a timely manner. At the same time, the measurement results are also easily affected by factors such as capacitance attenuation and crosstalk, making it difficult to ensure repeatability and accuracy. With the further reduction of CMOS size and the requirements of reducing gate voltage and leakage, the thickness of TiN, AlTi and other films in transistors has been reduced to the nanometer scale. The atomic structure and electronic state near the surface of the film are greatly affected by the substrate and adjacent layers. The work function of the alloy material changes with the thickness of the film. Therefore, accurate measurement of the work function of alloy films of different thicknesses has become an important 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, mainly to solve 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 and high-precision work function value; at the same time, the method can provide nanometer-level spatial resolution and positioning capability, 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. Use standard samples to calibrate the surface potential measured by Kelvin probe force microscope (i.e., Kelvin probe force microscopy method, KPFM);

[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 tested to eliminate the influence of surface oxidation and adsorption on the measurement;

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

[0011] E. Based on the relative potentials measured on the standard sample and the film to be tested by Kelvin probe force microscopy (i.e., using the Kelvin probe force microscopy method, KPFM), the work function of the film to be tested is calculated, 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 sample to be tested.

[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 is 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 in that: in step A, a standard sample is used to calibrate the surface potential measured by the Kelvin probe force microscope, wherein when the Kelvin probe force microscope measures the surface potential of the standard sample, the relative height, driving voltage, driving phase and other condition parameters of the probe are adjusted to stabilize the measurement, and the measured relative surface potential value is equal to the work function difference between the standard sample and the needle tip material. The surface potential measurement requires the use of a fresh conductive probe, such as a highly doped silicon probe, or a probe with a metal coating 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 avoiding 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 attached with a hard coating.

[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 tested is thinned, an atomic force microscope probe with a hard coating is used, a contact mode is applied, and the probe pressure is controlled to scan, so as to achieve the purpose of removing the surface oxide layer and adsorbents.

[0018] Wherein: the isolation and thinning treatment can be performed by an atomic force microscope. The specific method and steps are: use a probe with a harderness higher than the film material to be tested, apply appropriate pressure to the probe in contact mode to scan the film to be tested, and gradually remove the surface oxide layer and film material. A larger pressure can be used to isolate the area to be tested, and repeatedly scan around the area to be tested to gradually thin the metal or alloy material. To ensure the electrical effect of the surrounding film 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 scanned with a smaller pressure, and the thickness removed by scanning is slightly greater than the oxide layer thickness on the metal or alloy surface, usually 1 to 2 nanometers.

[0019] In step E, when measuring the relative surface potential of the film to be tested, the needle tip type and condition parameters used are consistent with those 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 potential measured on the standard sample and the sample to be tested by the Kelvin probe force microscopy method.

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

[0021] 1. The measurement reliability and repeatability of the method of the present invention are high. Using needle tip scanning, the film can be thinned with sub-nanometer precision, thereby removing the influence of surface adsorption and oxidation on the potential and work function measurement; at the same time, local deep thinning will isolate the influence of the electrical connection of the film in the surrounding area on the work function measurement of the fresh area.

[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 areas, 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 calibrating the surface potential by measuring the surface potential on a Kelvin probe force microscope using a standard sample of a known work function, a gold (Au) film or a highly oriented pyrolytic graphite (HOPG).

[0027] Figure 3 It 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 Kelvin probe force microscope to measure surface potential;

[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 tested to eliminate the influence of surface oxidation and adsorption on the measurement;

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

[0033] E. Based on the relative potential measured by the Kelvin probe force microscope on the standard sample and the film to be tested, the work function of the film to be tested is calculated, 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 sample to be tested.

[0034] In the 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 adopts 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 partially removed until the substrate, and the isolation groove forms a complete closed loop to avoid the measurement of the surface potential of the film in the isolated area in step C 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 attached with a hard coating.

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

[0040] Example

[0041] The following uses a TiN metal film grown on a SiO2 / Si substrate as an embodiment 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 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 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 of the Kelvin probe force microscopy method for measuring surface potential. 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 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, the diamond-coated probe is used as a nano-scalpel to perform local isolation and surface thinning on the test film sample in the contact atomic force microscope mode. Specifically, the sample to be tested is first adhered and fixed on the sample base plate, with the TiN metal layer facing up. A rectangular area of ​​micrometers square is selected on the surface of the film as an isolation area, and then the diamond-coated probe is used to scan along the edge of the pre-designed rectangle to gradually remove the TiN material until the oxide layer (SiO2) or the substrate Si forms an isolation groove band, so that the film within the rectangular range is isolated from the surrounding unthinned TiN film, and an isolation area for measuring the work function is obtained, such as Figure 3 shown.

[0045] The next step is to thin the isolated area, that is, to remove the oxide layer on the surface of the TiN. Use a diamond-coated probe to scan the isolated area in contact atomic force microscopy mode, and by controlling the force between the needle tip and the sample, scrape off the surface material within the range. For the TiN film, a fresh TiN surface can be obtained by removing a thickness of about 1 to 1.5 nanometers by scanning with the needle tip. The thinned thickness can be obtained by simultaneously scanning the isolated 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 was processed to obtain a local fresh surface. However, the charge effect formed by the oxidation of the surface or interface of the surrounding film material will still affect the work function of the metal material in the thinned area. Specifically in this embodiment, the area where the oxide layer is removed by thinning 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 work function of the electrons at 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-nanometer 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 by the Kelvin probe force method, that is,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 electronic 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 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 specification 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 Kelvin probe force microscope to measure surface potential; 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 tested to eliminate the influence of surface oxidation and adsorption on the measurement; D. Use Kelvin probe force microscopy to measure the potential of the film test area relative to the conductive needle tip; E. Based on the relative potential measured by the Kelvin probe force microscope on the standard sample and the film to be tested, the work function of the film to be tested is calculated, 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 sample to be tested.

2. A method for measuring a thin film work function according to claim 1, characterized in that: In the 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. A method for measuring the work function of a thin film according to claim 2, characterized in that: The thin film material with known work function adopts newly dissociated highly oriented pyrolytic graphite or freshly deposited noble metal film.

4. A method for measuring a thin film work function according to claim 1, characterized in that: 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, characterized in that: 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 partially removed until the substrate, and the isolation groove forms a complete closed loop to avoid the measurement of the surface potential of the film in the isolated area in step C being affected by the surrounding area.

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

7. A method for measuring a thin film work function according to claim 1, characterized in that: In step C, the 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, so as to remove the surface oxide layer and adsorbents.

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