Under high vacuum, the filament of the electron gun is heated to emit thermally emitted electrons, which are then accelerated by the anode to acquire substantial kinetic energy and bombard the evaporation material. This kinetic energy is converted into heat, causing the evaporation material to be heated and vaporized, thereby achieving electron-beam evaporation deposition. The electron-beam evaporation source consists of a thermionic cathode that emits electrons, an electron-accelerating electrode, and the coating material serving as the anode. The energy of the electron beam can be highly concentrated, enabling localized high temperatures on the coating material and promoting its evaporation. By adjusting the power of the electron beam, the evaporation rate of the coating material can be conveniently controlled, which is particularly advantageous for high-melting-point metals and high-purity metallic and compound materials.
Characteristics of Electron-Beam Evaporation Coating
Electron-beam evaporation coating systems are widely used in industrial thin-film manufacturing. Their unique characteristics make them highly effective in the production of thin films, as the films are primarily formed through heating by the electron beam within the system.
1. Advantages of Electron-Beam Heating Evaporation Coating
① Compared with conventional resistance heating, electron-beam heating in coating machines generates a much higher flux density, which is advantageous for the evaporation of high-melting-point materials and can also increase the evaporation rate to a certain extent.
② During operation, the evaporation coating machine places the source material to be evaporated into a water-cooled copper crucible, thereby preventing contamination and enabling the production of high-purity thin films.
③ Electron-beam evaporation imparts relatively high kinetic energy to the deposited particles, which is conducive to achieving high film precision and strong adhesion.
2. Disadvantages of Electron-Beam Heating Evaporation Coating
① Electron beam The overall structure of an evaporation coating machine is relatively complex, and its price is comparatively higher than that of other coating equipment.
② During operation, if the vapor density near the evaporation source is relatively high, interactions between the electron beam and vapor particles can occur, thereby affecting the electron flux—leading to flux loss or beam deflection. Moreover, such conditions may also induce excitation and ionization of both the vapor and residual gases, which in turn can compromise the quality of the entire thin film.
Electron beam Applications of Evaporation—Electron-Beam Evaporation Deposition of TiO 2 Thin film
Electron-beam evaporation deposition of TiO 2 Thin films are deposited using the INTEGRITY-39 fully automated optical coating system, which employs ion-beam-assisted electron-beam evaporation as illustrated in the figure below.
1. Cooling water inlet; 2. Cooling water outlet; 3. Crucible; 4. Beam focusing coil; 5. Electron beam emitter; 6. Heating lamp; 7. Substrate holder; 8. Motor; 9. Monitoring wafer; 10. Ion source
The electron gun for coating the sample operates at a voltage of 10 kV and a current of 200 A, with a deposition temperature in the vacuum chamber of 145–155°C. The target material is black, granular Ti with a purity of 99.99%. 2 O 3 , using a CC-105 cold-cathode ion beam for assisted deposition, with the vacuum chamber filled with 99.99% pure O during deposition. 2 As the reaction gas, the O₂ reaction gas is simultaneously controlled using an MKS Model 1179A mass flow meter. 2 of the flow, with a base made of 25 mm-diameter circular K9 glass.
Prior to coating, the substrate is first cleaned with a glass cleaning solution, rinsed with deionized water, and dried with nitrogen purging. It is then ultrasonically cleaned for 15 minutes each in acetone (99.9% purity) and anhydrous ethanol, followed by drying with a dedicated wiping paper before being loaded into the vacuum chamber.
During the coating process, a mechanical pump and a diffusion pump are used to evacuate the chamber to a vacuum level of 6.5×10 -
4 At the designated time, the automatic deposition program is initiated. Once the substrate is heated to the deposition temperature of 150°C, the ion source begins bombarding the substrate at an energy of 60–90 eV for a duration of 10 minutes. Subsequently, the electron gun is automatically activated to heat and evaporate the film material, initiating thin-film deposition at a rate of 0.38–0.42 nm/s. When the film thickness reaches the design value of 440 nm, the program automatically shuts off the electron gun, thereby completing the deposition process.
After coating, the vacuum chamber is allowed to cool naturally to room temperature before the sample is removed. The sample’s TiO₂ is then characterized using a Lambda 900 spectrophotometer (with a measurement range of 175–3,300 nm). 2 Spectral testing was conducted, and TiO was calculated using the Macleod software envelope method. 2 The actual thickness, extinction coefficient, and refractive index of the thin film.
Different flow rates of high-purity oxygen were introduced into the vacuum chamber to investigate the effects of varying vacuum levels on TiO. 2 the effects on film-forming quality, refractive index, and absorption coefficient.
TiO deposition by ion-source-assisted evaporation under high vacuum 2 During thin-film deposition, the vacuum level varies with the oxygen flow rate as shown in the following table.
As the oxygen molecules introduced into the vacuum chamber are ionized into oxygen ions, they fully react with Ti. 2 O 3 Vapor molecules react, resulting in Ti 2 O 3 The lost oxygen is replenished through decomposition, thereby forming a TiO film. 2 The composition is relatively pure, but if the oxygen supply is insufficient or Ti 2 O 3 With O 2 If the reaction is insufficient, a highly absorbent titanium suboxide film, Ti, will form. n O 2
n-1 (n=1, 2, ……, 10). As the oxygen flow rate increases, TiO 2 During the evaporation and ascent of vapor molecules, the probability of collisions with oxygen molecules increases, leading to energy loss and resulting in the deposition of TiO on the substrate surface. 2 Reduced kinetic energy adversely affects the adhesion and density of the deposited film.
For optical thin films, ion-beam-assisted deposition can increase the kinetic energy of film-forming molecules on the substrate surface, which not only significantly affects the refractive index of the film but also enhances its density and moisture resistance, while markedly improving its adhesion to the substrate.
II. Coating Results and Analysis
The spectra of four samples were measured using a Lambda 900 spectrophotometer, and the resulting spectra are shown in the figure below. As can be seen from the figure, Sample No. 1 exhibits TiO 2 The thin film exhibits significant absorption, with a maximum transmittance of 81%. Sample No. 2 shows a marked improvement in transmittance compared with Sample No. 1, achieving a maximum transmittance of 90%. Sample No. 3 attains a maximum transmittance of 92%, which is essentially identical to the transmittance of the substrate, indicating virtually no absorption. Sample No. 4 also achieves a maximum transmittance of 92%; however, the difference between the peak and valley transmittances decreases, and the refractive index of the material is reduced.
2. Calculation of TiO based on the envelope method 2 Refractive Index and Extinction Coefficient of Thin Films
For optical coating systems designed to achieve high transmittance in the visible region, the absorption coefficient of the coating materials must not be excessively high; otherwise, the final transmittance of the thin-film product will be compromised, leading to degraded optical performance. At the same time, the refractive index should not be too low, as this can adversely affect parameters such as the bandwidth of the cutoff region in the coating system design.
The Bloch–Polar method was proposed by Manifacier in 1976 and involves inverting the optical thin-film parameters of a coating layer by analyzing the extrema of its transmittance (or reflectance) at locations where the optical thickness of the layer is an integer multiple of λ/4. In practical measurements, one first sequentially connects the transmittance maxima T lambda
/2 Point and Local Minimum T lambda
/Four Point forms T maximum two envelope curves, (λ) and Tmin(λ); then, by selecting points on these envelope curves, the extremum of the transmittance T at any wavelength can be obtained. lambda
/2 and T lambda
/Four ;Finally, the extinction coefficient and refractive index of the film layer are calculated using the extrema of transmittance, and the film thickness is determined based on the calculated refractive index and the wavelengths corresponding to the extrema.
The advantages of this method are its simplicity, the ability to simultaneously determine the refractive index, extinction coefficient, and thickness of a thin film, and the fact that the measurement does not require physical contact with the sample, thereby minimizing sample damage. It is an ideal approach for comparing results obtained from different measurement instruments and, when properly applied, can serve as a reliable means of determining all optical constants of a thin film.
The envelope method is used to calculate the linear refractive index n and thickness L of the thin film at wavelength λ.
Among them:
In the formula: n 0 and n 1 These are the refractive indices of air and the substrate, respectively; Tmax and Tmin denote the maximum and minimum transmittance at wavelength λ; λ1 and λ2, along with n(λ1) and n(λ2), correspond to the wavelengths and refractive indices at two adjacent peaks or valleys on the transmittance curve. Using Macleod software, the envelope method is employed to calculate TiO. 2 Refractive index and extinction coefficient of the thin film.
As shown in Figure (a), the spectral range spans the ultraviolet–visible–near-infrared region, and the refractive indices of all four samples decrease. Sample No. 1 exhibits a refractive index ranging from 2.50 to 2.15 across the 400–1,000 nm wavelength band. Samples No. 2 and No. 3 have slightly higher refractive indices, whereas Sample No. 4 shows a slightly lower refractive index at the same wavelengths, ranging from 2.45 to 2.15.
As shown in Figure (b) above, Sample No. 1 exhibits obvious absorption, with the extinction coefficient increasing across the UV–vis spectral range, indicating the formation of low-valence titanium oxides due to insufficient oxygen supply. In contrast, Sample No. 2 shows a significant increase in maximum transmittance, with the extinction coefficient remaining approximately at 2.5 × 10 -3 The following still affect the final spectral transmittance; the extinction coefficients of spectra for Sample No. 3 and Sample No. 4 are at 10. -4 The order of magnitude has a negligible effect on the final spectral transmittance.
For samples No. 1 and No. 2, the extinction coefficient increases with increasing wavelength, whereas for the other two samples the extinction coefficient remains essentially constant across the wavelength range. Under these vacuum conditions, the transmittance of samples No. 1 and No. 2 decreases as the wavelength increases, indicating that the oxygen content is too low at this point, leading to the formation of metallic titanium; notably, the extinction coefficient of titanium itself increases with increasing wavelength.
By comparing four types of samples, it was found that as the oxygen flow rate increases, the vacuum level during deposition decreases, and TiO 2 The refractive index of the thin film first increases and then decreases. An increase in oxygen supply leads to a higher concentration of oxygen ions ionized by the ion source, which in turn raises the ion flux bombarding the film, resulting in a denser film layer and thus a higher refractive index. However, when the oxygen flow is further increased and the deposition chamber vacuum is relatively low, excess oxygen molecules and TiO 2 The collision of thin-film molecules reduces TiO. 2 the kinetic energy of the thin-film molecules, thereby enabling the deposition of TiO 2 The reduced molecular migration rate in the thin film leads to decreased film density, which in turn further lowers the refractive index of the film.
If the oxygen supply is insufficient—i.e., when the deposition vacuum level is high—the limited number of oxygen molecules will be unable to replenish Ti. 2 O 3 The oxygen content in the film material during evaporation and decomposition with oxygen loss results in the deposited thin film containing low-valent titanium oxides, which adversely affects the optical properties of the film. Therefore, selecting an appropriate oxygen flow rate is crucial for TiO 2 The deposition of optical thin films is crucial.
3. TiO 2 Dispersion of Thin-Film Refractive Index
For visible and near-infrared optical thin-film materials, the dispersion relationship conforms to the Cauchy equation n(λ) = A. n + B n /lambda 2 + C n /lambda 4 , A n , B n , C n For fitting parameters.
As shown in the figure below, for TiO 2 The dispersion relation of the refractive index of the thin film in the 400–1,400 nm spectral range, as fitted by Origin data-analysis software, almost completely overlaps with the curve obtained using the envelope method, with a squared correlation coefficient of 0.99946. The expression is n(λ) = 2.17 + 6.12 × 10 4 /lambda 2 +2.98×10 8 /lambda 4 Since the dispersion behavior is in excellent agreement with that predicted by the Cauchy equation, the refractive index obtained via envelope-method calculation can be used as a material parameter, and subsequent software-based multilayer design yields results that closely match the target specifications.
Oxygen flow rate on TiO 2 The optical properties of the thin film are significantly affected. By controlling the oxygen flow rate, the vacuum level within the vacuum chamber can be adjusted, TiO 2 The peak spectral transmittance of the thin film increases as the vacuum level decreases, while the refractive index and extinction coefficient first increase and then decrease with decreasing vacuum; when the vacuum level is 2.0×10 -
3 At time Pa, the prepared TiO 2 The thin film exhibits high transmittance in the visible spectral region, with a maximum transmittance of 92%, a refractive index ranging from 2.50 to 2.20, and an extinction coefficient of 10. -4 The following.
The squared correlation coefficient between the fitted curve and the results calculated using the envelope method is 0.99946, and the Cauchy dispersion equation for the refractive index is n(λ) = 2.17 + 6.12 × 10 4 /lambda 2 + 2.98×10 8 /lambda 4。
Composition and Structure of Electron-Beam Evaporation Coating Equipment
An electron-beam evaporation coating system primarily consists of a control system, a vacuum piping system, and an electron-gun system.
1. Vacuum Piping System in Electron Beam Evaporation Coating Equipment
The vacuum system comprises two pumps: a foreline mechanical pump and a high-vacuum cryogenic pump. The primary function of the cryogenic pump is to achieve vacuum by exploiting the adsorption properties of cold surfaces at extremely low temperatures. During the startup sequence of the high-vacuum cryogenic pump, the foreline mechanical pump first evacuates the pipeline to a specified vacuum level. Once the pipeline reaches this target vacuum, the transfer valve opens, allowing the foreline mechanical pump to continue operating and maintaining both the pipeline and the cryogenic pump under vacuum conditions. Subsequently, the foreline valve repeats this cycle.
2. Electron-beam Heating Evaporation Source
Electron beam The evaporation source primarily consists of an electron gun and a crucible, and may optionally be equipped with a set of auxiliary machinery for supplying raw materials to the coating system. In most cases, the control unit for generating and modulating the electron beam is integrated with the crucible design within the coating machine. The following are several typical structural configurations of electron-beam-heated evaporation sources:
① An evaporator heated by a linear cathode and electrostatic focusing.
② Evaporator employing a ring-shaped cathode and electrostatic focusing for heating.
③ Evaporators that employ axial guns and electrostatic far-field focusing for heating.
④ An evaporator that employs axial gun heating, magnetic focusing, and 90-degree magnetic deflection.
⑤E-type electron gun evaporator: The electron trajectories emitted by the E-type electron-beam evaporation source resemble the shape of the letter “e,” hence the name “E-type gun.”
Currently, this type of evaporation source is the most widely used in vacuum vapor deposition processes.
Maintenance Techniques for Electron-Beam Evaporation Coating Machines
During operation, electron-beam evaporation coating systems may experience malfunctions of varying severity due to the inherent characteristics of the equipment; the most common failures are typically caused by circuit shorting and residues from the coating materials.
1. There is a burnt smell inside the vacuum chamber.
During operation, if a burnt odor emanates from the vacuum chamber, the first step is to determine whether any components have been damaged or burned. If, after thorough inspection, no abnormalities or damage are found, the maintenance personnel must conduct a systematic, step-by-step troubleshooting of the equipment. During the repair process, the chamber should be tested using a method of elimination. The electron-beam evaporation source is the primary structural component of the entire system and, therefore, the most critical area to inspect when diagnosing problems.
2. The electron gun is not functioning properly.
Due to Electron beam It relies on the electron gun; if the electron gun fails to operate properly, the entire device will be affected. The first step is to verify that the high-voltage cable connections are secure. If they are, the next step is to check whether the surrounding environment is excessively humid, as a damp environment can lead to short circuits and increase the likelihood of equipment malfunctions.
3. The electron beam current cannot be properly loaded.
This type of fault may be caused by carbon deposits or short circuits inside the electrode or the voltage regulator. The equipment may operate normally at low currents, but once the current is increased to a certain level, breakdown occurs, triggering the circuit breaker to trip. Such issues are sometimes difficult to detect visually; it is therefore advisable to use a megohmmeter to perform an insulation resistance test to ground.
