Electron Beam Evaporation and Magnetron Sputtering Aluminizing Performance Analysis

In order to obtain a good performance semiconductor Al film, we have prepared a series of superior Al films by optimizing the process parameters. Through theoretical calculations and performance tests, the characteristics of aluminum films prepared by electron beam evaporation and magnetron sputtering were analyzed and compared.

Film thickness

It is very important to strictly control the thickness of the aluminum film, because the thickness of the aluminum film will directly affect other properties of the Al film, thereby affecting the reliability of the semiconductor device. For a high back-pressure power tube, its working voltage is high and the current is large. A metal film without a certain thickness will cause the current density on the aluminum film per unit area to be too high, and it is easy to burn. For general semiconductor devices, if the aluminum layer is thin, the continuity of the film is poor, and the island-like or net-like structure causes difficulty in pressure-welding the leads, resulting in difficulty in pressure-welding or pressure-welding, thereby affecting the yield; Al layer Too thick, the pattern cannot be seen clearly when photolithography is caused, causing corrosion difficulties and easily causing edge corrosion and "ribbing".

With electron beam evaporation, the planetary mechanism rotates uniformly when the film is deposited, and the probability of deposition of the aluminum film is equal for each substrate; the focus point of the planetary mechanism is at the crucible evaporation source, and the deposition rate of each substrate is almost equal under a certain degree of vacuum. Magnetron sputtering coating method, because the deposition current and the target voltage can be controlled, that is, the sputtering power can be adjusted and controlled, so the controllability and repeatability of the film thickness is better, and the thickness can be obtained on a larger surface A uniform film layer.

Al film thickness can be measured using a metal film thickness meter, which is a non-destructive thickness gauge designed and manufactured based on the eddy current principle. According to the process parameters, we prepared a batch of samples. After testing the samples, the average thickness of sputtered Al film is 1.825μm, and the average thickness of electron beam evaporated aluminum film is 1.663μm, which is in accordance with the requirements of semiconductor device electrode thickness for aluminum film. (The small signal is 1.7±0.15 μm; the high power is 2.5±0.3 μm.

In order to further observe the film thickness and surface topography, the sample was placed in an environmental scanning electron microscope philips XL30-ESEM for observation. According to the SEM image output by the video printer, it can be seen that the electron beam evaporation has a relatively large film thickness dispersion, ie, Poor uniformity.

Adhesion

Adhesion reflects the interaction between the aluminum film and the substrate and is an important factor in ensuring the durability of the device. Sputtering atom energy is 1-2 orders of magnitude higher than evaporated atom energy. The deposition of high-energy sputtered atoms on the substrate performs much higher energy conversion than the evaporation of atoms, resulting in higher thermal energy. Part of the high-energy sputtering atoms generate different degrees of implantation, forming a layer of sputtered atoms on the substrate. A dummy diffusion layer that is intrinsically compatible with the substrate, and the substrate is always cleaned and activated in the plasma during the film formation process, eliminating sputter atoms with weak adhesion, purifying and activating the surface of the substrate, The adhesion between the sputtered atoms and the substrate is enhanced, and the adhesion of the sputtered aluminum film to the substrate is high.

The method used to measure the adhesion is to measure the force or energy required to peel the aluminum film from the substrate. We used the stripping water method to determine the adhesion.

Assuming that the adhesion energy per unit area of ​​the film is γ, then the total adhesion energy of the film with width b and length a is E=abγ(1)

Work Wp=Fa(1-sin(θ))(2) for the force F for peeling the film

If it is a static strip and ignores the elastic energy generated when the film is bent, the work done by F is approximately equal to the total adhesion energy of the film, that is, Wp=E, so F=bγ/(1-sin(θ))(3)

(3) In the formula, F varies with the angle of θ, and it cannot truly reflect the adhesion performance of the film. When the applied peel force is perpendicular to the film, ie θ = 0°, the formula is simplified as F = bγ(4)

From the measured F, the adhesion energy γ = F/b can be calculated. If the adhesion force f per unit length is to be directly calculated, peeling according to the definition and using the above method (θ=0°) can yield f=γ. It can be seen that the adhesion force is the same as the adhesion energy γ. Since the adhesion energy of the Al film is high, its adhesion is large. The experimentally measured data is: the average adhesion of the sputtered Al film is 25N, and the average adhesion of the electron beam evaporated Al film is 9.8N. These data are consistent with the theoretical analysis.

Denseness

Considering the denseness of the aluminum film is equivalent to considering the size, density, and degree of uniformity of the grain of the aluminum film, since it also directly affects the other properties of the aluminum film, and thus affects the performance of the semiconductor film.

The grain size of the vapor-deposited polycrystalline aluminum film increases as the mobility of adsorbed atoms or atomic groups on the surface of the substrate increases during deposition.

It can be seen that the size of the grain size of the aluminum film will depend on such factors as the substrate temperature, the deposition rate, the velocity component of the gas phase atoms in the parallel substrate, the substrate surface finish, and the chemical activity.

Since the temperature of the substrate evaporated by the electron beam is Ts=120° C., the evaporation rate is 20-25 A/s, the energy of the vapor aluminum atom is 0.1-0.3 eV, and the sputtering substrate temperature Ts=120° C., the sputtering rate is 8000 A. /min (133.3 A/s) or 10000 A/min (166.7 A/s), the sputtering threshold is 13 eV, and the energy of the sputtered aluminum atom is 1-2 orders of magnitude higher than that of the electron beam evaporated aluminum, so the electron beam The evaporated aluminum atoms hit the substrate and lose their energy very quickly. The migration rate is very small, so it is difficult for the atoms to rearrange on the surface. That is, where the deposition is located, the gap between the atoms is larger and the surface is rough. Very large; sputtering substrate temperature is high, aluminum atoms are also high energy, and the atomic mobility on the substrate surface increases, making the film surface lateral kinetic energy is relatively large, easy to link into a smooth surface, stability Higher, larger grains, and smaller atomic distances, resulting in a thinner surface roughness of the film.

The SEM photos of the grain size and surface morphology of the two aluminum films were also observed by an environmental scanning electron microscope philipsXL30-ESEM, and the results were confirmed by the SEM photograph of the electron beam evaporation. The average particle size of the electron beam evaporation was 266.8 nm, and the average grain size of the sputtering was measured. The diameter is 1.528 μm. Although the particle size of the electron beam evaporated aluminum film is significantly smaller than that of the sputtered aluminum film, the aluminum atoms evaporated by the electron beam must not be brought close to each other. There are many gaps in the beam, and the sputtered aluminum atoms are mutually dependent. Very tight, from the side view, sputtered aluminum film is smooth and bright color, indicating that the sputtering aluminum film is better.

The large size of the sputtered grains also has the advantage of reducing the grain boundary area, thereby reducing the number of electromigration short-circuiting channels, which is beneficial to enhancing the electromigration resistance of the Al film and prolonging the average life of the Al film. However, the grain size must not be too large, otherwise the lithographic quality of the aluminum film fine line pattern will be affected. At the same time, although the sputtered aluminum film grains are large, they can be refined by the subsequent heat treatment and are superior in performance.

Conductivity

Metal-semiconductor contact does not necessarily result in a purely resistive contact. If the contact resistance is too large, ie, the resistivity is low, a significant portion of the applied signal voltage will fall on the contact resistance, causing unnecessary voltage drop and power loss, so to obtain a low-resistance ohmic contact, the film The resistivity of the layer should be as small as possible and the conductivity should be as high as possible.

The resistivity of the aluminum film is very close to that of the aluminum material. The resistivity increases as the crystal grain size decreases. Since the grain size of the aluminum film evaporated by the electron beam is significantly smaller than that of the sputtering, the resistivity of the sputtering is smaller than the electron beam evaporation, and its electrical conductivity is high.

Refractive index

The refractive index can generally reflect the degree of denseness of the film and increase with the increase of the density. However, the aluminum film of the electrode lead prepared by us requires good compactness, which may qualitatively determine the compactness of the aluminum film by measuring the size of the refractive index. . The refractive index can be indirectly converted by the reflectivity.

Metal film properties are generally characterized by the refractive index NM = n-ik, set the metal film thickness dM, refractive index NM = n-ik, phase thickness δM = 2πNMdM / λ, if you consider normal incidence, metal film and Si The combined admittance of the substrate is YM=((ngcos(δM)+iNMsin(δM))/(cos(δM)+isin(δM)ng/NM)=YM(1)+iYM(2)(5) The reflectance of the structure is RM=|(n0-YM)/(n0+YM)|2={[(n0-YM(1))]2+[YM(1)]2}/{[(n0+YM) (1)) 2+[YM(1)]2}(6) However, the description and calculation process is too complicated, so it can be replaced by the following description and calculation.

When the beam is perpendicularly incident on a single-layer film, the reflectance RM = (n0n2-NM2)/(n0n2+NM2) (7)

Then NM={[(1-RM)/(1+RM)]n0n2}1/2(8)

RM----------- reflectivity

N0------------Air reflectivity

NM---------------Refractive Index of Al Film

The refractive index of n2-----------Si sheet (about 3.5)

The reflectance NM of the aluminum film can be found by accurately measuring the reflectance RM at normal incidence. The reflectance in different wavelength ranges measured by the UV3101 spectrophotometer manufactured by Shimadzu, Japan shows that in the visible light range of 400-760 nm, the reflectivity of the aluminum film 11 # sample is RM = 0.82, 8 # sample is RM = 0.83, obtained by formula (7), (8) calculation, the reflectance of aluminum film NM: 8 # sample is NM = 0.702,

The 11# sample is NM=0.688. Although the refractive index of the sputtered aluminum film is larger than that of the electron beam evaporated aluminum film, the sputtered aluminum film is denser than the electron beam evaporation.

in conclusion

Electron beam evaporation and magnetron sputtering to form Al film are two methods commonly used in electrode preparation of semiconductor devices. Through theoretical and experimental analysis, the film thickness, adhesion, compactness, conductivity, refractive index, etc. are performed on the sample. The comprehensive test of the index shows that the controllability and reproducibility of Al film thickness produced by electron beam evaporation are poor and the dispersion degree is larger. The adhesion of aluminum film and Si substrate is smaller; the grain of aluminum film is small , but it is loose, leading to its poor compaction; aluminum film conductivity, refractive index is much smaller than the bulk aluminum. The performance index of the aluminum film obtained by magnetron sputtering is superior to the electron beam evaporation index. Practice has proved that the overall performance of the aluminum thin film prepared by magnetron sputtering method is superior to electron beam evaporation. Therefore, most of the production practice uses magnetron sputtering to deposit semiconductor electrode materials, which is also the development direction of the thin film industry in the semiconductor industry.

In addition, sputtering can also solve three problems caused by electron beam evaporation:

1 step coverage. In general, the device has a pattern size of 2 - 3 μm or less, and it is required that a metal film with a uniform film thickness be coated as much as possible at a step height of about 1 μm. With the device consisting of electron beam evaporation and planetary orbital substrate carriers, it is difficult to obtain a very satisfactory coverage.

2 alloy film composition control. With the miniaturization of the circle, Al-Si, Al-Cu, Al-Si-Cu, etc. aluminum alloy films are used instead of the pure metal Al film in order to ensure the reliability and improve the yield. If electron beam evaporation is used to prepare the alloy film, it will be difficult to control the alloy film so as to achieve the required composition because the vapor pressure of the components will cause decomposition.

3 The card substrate is complex and difficult to automate. In highly complex component manufacturing processes, to improve reliability and repeatability, manual operations should be minimized and automated operations should be enhanced. With electron beam evaporation, Si wafers can only be mounted one by one on planetary orbital supports, and only single-batch evaporation can be used. Therefore, it is difficult to achieve automation.