Study on the properties of aluminum plating by electron beam evaporation and magnetron sputtering电子束蒸发与磁控溅射镀铝性能分析研究

膜厚

严格控制发铝膜的厚度是十分重要的，因为铝膜的厚度将直接影响Al膜的其它性能，从而影响半导体器件的可靠性。对于高反压功率管来说，它的工作电压高，电流大，没有一定厚度的金属膜会造成成单位面积铝膜上电流密度过高，易烧毁。对于一般的半导体器件，铝层偏薄，则膜的连续性较差，呈岛状或网状结构，引起压焊引线困难，造成不易压焊或压焊不牢，从而影响成品率;Al层过厚，引起光刻时图形看不清，造成腐蚀困难而且易产生边缘腐蚀和“连条”现象。

采用电子束蒸发，行星机构在沉积薄膜时均匀转动，各个基片在沉积铝膜时的几率均等;行星机构的聚焦点在坩埚蒸发源处，各个基片在一定真空度下沉积速率几乎相等。采用磁控溅射镀膜方法，由于沉积电流和靶电压可以控制，也即是溅射功率可以调节并控制，因此膜厚的可控性和重复性较好，并且可在较大表面上获得厚度均匀的膜层。

Al膜厚度的测量可采用金属膜厚测量仪，它是根据涡流原理设计制造的无损测厚仪。根据工艺参数，我们制备了一批试样，样品经测试，溅射Al薄膜的平均厚度是1.825μm，电子束蒸发铝薄膜的平均厚度是1.663μm,均符合半导体器件电极对铝膜厚的要求(小信号为1.7±0.15μm;大功率为2.5±0.3μm。

为了更进一步地观测膜厚及表面形貌，样品放入环境扫描电子显微镜philipsXL30-ESEM中进行观测，并根据视频打印机输出的SEM图片可以看出，电子束蒸发的膜厚分散度较大，即均匀性较差。

附着力

附着力反映了铝膜与基片之间的相互作用力，也是保证器件经久耐用的重要因素。溅射原子能量比蒸发原子能量高1-2个数量级。高能量的溅射原子沉积在基片上进行的能量转换比蒸发原子高得多，产生较高的热能，部分高能量的溅射原子产生不同程度的注入现象，在基片上形成一层溅射原子与基片原了相互溶合的伪扩散层，而且，在成膜过程中基片始终在等离子区中被清洗和激活，清除了附着力不强的溅射原子，净化且激活基片表面，增强了溅射原子与基片的附着力，因而溅射铝膜与基片的附着力较高。

测定附着力所采用的方法是测量铝膜从基片上剥离时所需要的力或者能量，我们采用剥离水法来测定附着力。

设薄膜单位面积的附着能为γ，则宽度为b,长度为a的薄膜的总附着能E=abγ(1)

用于剥离该薄膜的力F所作的功Wp=Fa(1-sin(θ))(2)

如果是静态剥离并忽略薄膜弯曲时所产生的弹性能，则F所作的功近似等于薄膜的总附着能，即Wp=E,于是F=bγ/(1-sin(θ))(3)

(3)式中F随着θ角的变化而变化，不能真正反映薄膜的附着性能。当所加剥离力与薄膜垂直，即θ=0°时，则式简化为F=bγ(4)

根据测量所得的F便可计算出附着能γ=F/b。如果要直接计算单位长度的附着力f，根据定义并采用上述方法(θ=0°)进行剥离可得f=γ。可见，附着力的大小和附着能γ的数据相同，由于Al膜的附着能γ较高，所以其附着力较大。实验测得的数据是：溅射Al膜的平均附着力25N，电子束蒸发Al膜的平均附着力为9.8N。这些数据和理论分析结论一致。

致密性

考虑铝膜的致密性就相当于考虑铝膜的晶粒的大小，密度以及能达到均匀化的程度，因为它也直接影响铝膜的其它性能，进而影响半导体哗啦的性能。

气相沉积的多晶铝膜的晶粒尺寸随着沉积过程中吸附原子或原子团在基片表面迁移率的增加而增加。

由此可以看出铝膜的晶粒尺寸的大小将取决环于基片温度、沉积速度、气相原子在平行基片方面的速度分量、基片表面光洁度和化学活性等因素。

由于电子束蒸发的基片温度Ts=120°C，蒸发速率20-25A/s，蒸汽铝原子的能量为0.1-0.3eV，而溅射的基片温度Ts=120°C，溅射速率8000A/min(133.3A/s)或10000A/min(166.7A/s)，溅射阈为13eV，溅射铝原子的能量比电子束蒸发的铝原了能量高1-2个数量级，所以电子束蒸发的铝原子碰到基片，很快失去能量，且迁移率很小，故原子在表面上重新排列较困难，即沉积的地方就是定位的地方成原子之间的空隙较大，有面粗糙度很大;溅射的基片温度较高，铝原子能量也较高，在而基片表面的原子迁移率增大，使得薄膜表面横向动能较大，易于连结殂成光滑的表面，稳定性较高，晶粒较大，原子间距较小，因而形成的薄膜表面粗糙芳减小。

通过环境扫描电子显微镜philipsXL30-ESEM观测，并分析两种铝膜的晶粒大小及表面形貌的SEM照片也能验证这一结论，电子束蒸发的平均粒径为266.8nm，溅射的平均粒径为1.528μm，虽然电子束蒸发铝膜的粒径明显小于溅射的铝薄膜，但是电子束蒸发的铝原子较终不得靠得很近，当中存在很多间隙，而且溅射的铝原子相互靠得很紧，从侧面观测，溅射的铝膜平滑而且色泽光亮，说明溅射铝膜的致密较好。

溅射的晶粒较大还有一个好处，减小了晶界面积，从而减少电迁移短路通道的数目，有利于增强Al膜的抗电迁移能力，延长Al膜的平均寿命。但晶粒尺寸不可太大，否则影响铝膜细线条图形的光刻质量。同时，溅射的铝膜晶粒虽大，但可以通过的后面的热处理使之细化并使性能更加优越。

电导率

金属与半导体接触并非一定能够形成一个纯电阻性接触。如果接触电阻太大，即电阻率低，则外加的信号电压就会有相当大的一部分降落在接触电阻上，造成不必要的电压降和功率损耗，所以要想获得低阻的欧姆接触，膜层的电阻率应尽量小，电导率应尽量高。

铝膜的电阻率与铝材的非常接近。电阻率随结晶粒径的减小而增加。由于电子束蒸发铝膜的结晶粒径明显小于溅射，所以溅射的电阻率小于电子束蒸发，其电导率较高。

折射率

折射率一般可以反映薄膜的致密程度，随致密程度的增加而增加，而我们所制备的电极引线铝膜要求致密性好，这就可能通过测试折射率的大小来定性地判断铝膜的致密性。而折射率可以通过反射率间接地换算得到。

金属膜的特性一般用折射率NM=n-ik来表征，设金属膜厚度为dM,折射率为NM=n-ik,位相厚度为δM=2πNMdM/λ,若考虑垂直入射，金属膜与Si基底的组合导纳为YM=((ngcos(δM)+iNMsin(δM))/(cos(δM)+isin(δM)ng/NM)=YM(1)+iYM(2)(5)从而整个结构的反射率为RM=|(n0-YM)/(n0+YM)|2={[(n0-YM(1))]2+[YM(1)]2}/{[(n0+YM(1))]2+[YM(1)]2}(6)但其描述和计算过程过于复杂，故可以有下面的描述和计算代替。

当光束垂直入射到单层薄膜有面时，反射率RM=(n0n2-NM2)/(n0n2+NM2)(7)

则NM={[(1-RM)/(1+RM)]n0n2}1/2(8)

式中RM-----------反射率

n0------------空气的反射率

NM---------------Al膜的折射率

n2-----------Si片的折射率(约为3.5)

只要准确测出垂直入射的反射率RM就可以求出铝膜的反射率NM。通过日本岛津生产的UV3101型分光光度计测得的在不同波长范围内的反射率可知，在可见光400—760nm范围内，铝膜的反射率11#样品为RM=0.82，8#样品为RM=0.83，通过(7)、(8)式计算，得出铝薄膜的反射率NM：8#样品为NM=0.702,

11#样品为NM=0.688。虽然，溅射铝膜的折射率大于电子束蒸发的铝膜，溅射铝膜的致密性比电子束蒸发好。

结论

电子束蒸发和磁控溅射制Al膜是半导体器件电极制备生产中常用的两种方法，通过理论与实验分析，并对样品进行了膜厚、附着力、致密性，电导率、折射率等指标的综合测试，实验表明：电子束蒸发制得的Al薄膜厚度的可控性和重复性较差及分散度较大铝薄膜与Si基片的附着力较小;铝薄膜的晶粒虽小，但很疏松，导致其致密性较差;铝膜的电导率、折射率较块状铝材小得多。而磁控溅射制得的铝膜的性能指标则比电子束蒸发的指标优越。实践证明，磁控溅射方法制备的铝薄膜的综合性能优于电子束蒸发，所以在生产实践中绝大多数采用磁控溅射沉积半导体电极材料，这也是半导体行业中薄膜行业的发展方向。

此外，溅射还可以解决电子束蒸发带来的三个问题：

①台阶覆盖度。一般器件的图形尺寸为2--3μm或更小，要求在1μm左右高的台阶部位尽量能镀覆膜厚均匀的金属镀层。采用电子束蒸发和行星回旋式基片架机构组成的装置，难以得到十分理想的覆盖度。

②合金膜的成分控制。随着圆形的微细化，为确保可靠性并提高成品率采用Al-Si、Al-Cu、Al-Si-Cu等铝合金膜代替纯金属Al膜。如果采用电子束蒸发来制取合金膜，由于组分蒸气压不同会引起分解，很难控制合金膜使其达到所要求的成分。

③装卡基片复杂，难于实现自动化。在高度复杂的元器件制造工艺中，为提高可靠性和重复性，应尽量减少人工操作，提高自动化操作水闰。采用电子束蒸发，行星回旋式支架上只能一个一个地装卡Si片，而且只能采取单批式蒸镀。因此，难于实现自动化操作。

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 the other properties of the Al film, thus affecting the reliability of semiconductor devices. For the high back pressure power tube, its working voltage is high and its current is large. Without a certain thickness of metal film, the current density on the unit area aluminum film will 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 structure is island or network, which makes it difficult to crimp the welding leads, and it is not easy to crimp or the crimp welding is not firm, thus affecting the yield; if the aluminum layer is too thick, it will cause the pattern not to be seen clearly during photolithography, which will cause the corrosion difficulty and easy to produce edge corrosion and "strip" phenomenon.

   With electron beam evaporation, the planetary mechanism rotates evenly when depositing the film, and the probability of each substrate is equal when depositing the aluminum film; the focus point of the planetary mechanism is at the evaporation source of the crucible, and the deposition rate of each substrate is almost equal under a certain vacuum degree. With the method of magnetron sputtering, the deposition current and target voltage can be controlled, that is, the sputtering power can be adjusted and controlled, so the film thickness is controllable and repeatable, and the film with uniform thickness can be obtained on a large surface.

   The metal thickness gauge can be used to measure the thickness of Al film. It is a non-destructive thickness gauge designed and manufactured according to the eddy current principle. According to the process parameters, we have prepared a batch of samples. After testing, the average thickness of sputtering Al film is 1.825 μ m, the average thickness of electron beam evaporation aluminum film is 1.663 μ m, which all meet the requirements of semiconductor device electrode for aluminum film thickness (small signal is 1.7 ± 0.15 μ m, high power is 2.5 ± 0.3 μ m).

  In order to further observe the film thickness and surface morphology, the samples were put into the environmental scanning electron microscope Philips xl30-esem for observation. According to the SEM pictures output by the video printer, it can be seen that the film thickness of electron beam evaporation has a large dispersion, that is, the uniformity is poor.

adhesion

  The adhesion reflects the interaction between the aluminum film and the substrate, and is also an important factor to ensure the durability of the device. The energy of sputtering atom is 1-2 orders of magnitude higher than that of evaporation atom. The energy conversion of high-energy sputtered atoms deposited on the substrate is much higher than that of the evaporated atoms, resulting in higher heat energy. Some of the high-energy sputtered atoms produce injection phenomenon in varying degrees, forming a layer of pseudo diffusion layer on the substrate, in which sputtered atoms and the substrate are mutually dissolved. Moreover, during the film-forming process, the substrate is always cleaned and activated in the plasma area, and removed The sputtered atoms with weak adhesion can purify and activate the substrate surface and enhance the adhesion between sputtered atoms and substrate, so the adhesion between sputtered aluminum film and substrate is higher.

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

If 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 = ab γ (1)

The work of force F used to peel the film WP = fa (1-sin (θ)) (2)

If it is static peeling and neglecting the elastic energy produced 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 = B γ / (1-sin (θ)) (3).

(3) In the formula, f changes with the change of θ angle, which can not really reflect the adhesion performance of the film. When the applied peeling force is perpendicular to the film, i.e. θ = 0 °, the formula is simplified as F = B γ (4).

According to the measured F, the adhesion energy γ = f / B can be calculated. If the adhesion force F per unit length is to be calculated directly, f = γ can be obtained by peeling according to the definition and using the above method (θ = 0 °). It can be seen that the size of adhesion is the same as the data of adhesion energy, because the adhesion energy of Al film is higher, its adhesion is larger. The results show that the average adhesion of sputtering Al film is 25N, and that of electron beam evaporation Al film is 9.8N. These data are consistent with the conclusion of theoretical analysis.

Compactness

Considering the density of aluminum film is equivalent to considering the grain size, density and the degree of homogenization of aluminum film, because it also directly affects other properties of aluminum film, and then affects the performance of semiconductor crash.

The grain size of the polycrystalline aluminum film increases with the increase of the mobility of the adsorbed atoms or clusters on the substrate surface.

It can be seen that the grain size of the aluminum film depends on the temperature of the substrate, the deposition rate, the velocity component of the vapor atoms in the parallel substrate, the surface finish and chemical activity of the substrate.

As the substrate temperature TS = 120 ° C, the evaporation rate is 20-25a / s, and the energy of the vapor aluminum atom is 0.1-0.3ev, while the substrate temperature TS = 120 ° C, the sputtering rate is 8000A / min (133.3a / s) or 10000a / min (166.7a / s), and the sputtering threshold is 13ev, the energy of the sputtering aluminum atom is 1-2 orders of magnitude higher than that of the original aluminum It is very difficult for atoms to rearrange on the surface because of the low mobility and the rapid loss of energy; The sputtered substrate has higher temperature and higher aluminum atomic energy, while the atomic mobility on the substrate surface increases, which makes the film surface have larger lateral kinetic energy, easy to connect to a smooth surface, high stability, larger grains, and smaller atomic spacing, so the rough surface aromatics formed on the film surface decrease.

The results show that the average particle size of electron beam evaporation is 266.8nm, and the average particle size of sputtering is 1.528 μ M. although the particle size of electron beam evaporation is obviously smaller than that of sputtering, the aluminum atom of electron beam evaporation is not very reliable Recently, there are many gaps, and the sputtered aluminum atoms are close to each other. From the side view, the sputtered aluminum film is smooth and bright, which shows that the sputtered aluminum film is dense.

The larger grain size of the sputtering also has the advantage of reducing the grain boundary area, thus reducing the number of short-circuit channels for electromigration, which is conducive to enhancing the anti electromigration ability of the Al film and prolonging the average life of the Al film. However, the grain size should not be too large, otherwise it will affect the lithographic quality of the thin line pattern of the aluminum film. At the same time, although the grain size of the sputtered aluminum film is large, it can be refined and its performance is superior by the subsequent heat treatment.

conductivity

The contact between metal and semiconductor does not necessarily form a pure resistive contact. If the contact resistance is too large, i.e. the resistivity is low, a considerable part of the external signal voltage will fall on the contact resistance, causing unnecessary voltage drop and power loss. Therefore, in order to obtain ohmic contact with low resistance, the resistivity of the film should be as small as possible, and the conductivity should be as high as possible.

The resistivity of aluminum film is very close to that of aluminum. The resistivity increases with the decrease of crystal size. Because the crystal size of the film is smaller than that of sputtering, the resistivity of sputtering is smaller than that of electron beam evaporation, and its conductivity is higher.

Refractive index

Generally, the refractive index can reflect the compactness of the film, which increases with the increase of the compactness. However, the electrode lead aluminum film we prepared requires good compactness, so it is possible to judge the compactness of the aluminum film qualitatively by measuring the refractive index. And the refractive index can be obtained indirectly by reflectivity.

The characteristics of the metal film are generally characterized by the refractive index nm = n-ik. The thickness of the metal film is DM, the refractive index nm = n-ik, and the phase thickness δ M = 2 π nmdm / λ, (6) but its description and calculation process are too complex, so it can be replaced by the following description and calculation.

The reflectivity RM = (n0n2-nm2) / (n0n2 + Nm2) (7)

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

Where RM ----------- reflectivity

N0 ------------ reflectivity of air

Nm ------------ refractive index of Al film

Refractive index of n 2-Si (about 3.5)

As long as the reflectivity rm of vertical incidence is measured accurately, the reflectivity nm of aluminum film can be calculated. According to the reflectance measured by uv3101 spectrophotometer produced by Shimadzu in different wavelength range, in the visible light range of 400-760nm, the reflectance of 11 × samples is RM = 0.82, and that of 8 × samples is RM = 0.83. The reflectance of 8 × samples is nm = 0.702,

11. The sample is nm = 0.688. Although the refractive index of sputtered aluminum film is higher than that of electron beam evaporation, the density of sputtered aluminum film is better than that of electron beam evaporation.

Conclusion

Electron beam evaporation and magnetron sputtering are two commonly used methods in the production of semiconductor device electrode. Through theoretical and experimental analysis, the thickness, adhesion, compactness, conductivity, refractive index and other indicators of the sample were tested. The results show that the thickness of the Al film produced by electron beam evaporation is less controllable and less repetitive, and the dispersion of the aluminum film and Si film is larger The adhesion of the substrate is small; the grain size of the aluminum film is small, but it is very loose, resulting in poor compactness; the conductivity and refractive index of the aluminum film are much smaller than that of the bulk aluminum. The performance index of the aluminum film prepared by magnetron sputtering is superior to that of the electron beam evaporation. It has been proved that the comprehensive properties of the aluminum films prepared by magnetron sputtering are better than that by electron beam evaporation, so the vast majority of the semiconductor electrode materials are deposited by magnetron sputtering in the production practice, which is also the development direction of the film industry in the semiconductor industry.

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

① Step coverage. Generally, the figure size of the device is 2-3 μ m or smaller, and it is required that the metal coating with uniform thickness can be coated at the step with a height of about 1 μ M. It is difficult to obtain ideal coverage by using the device composed of electron beam evaporation and planetary cyclotron substrate frame mechanism.

② Composition control of alloy film. In order to ensure the reliability and improve the yield, Al Si, Al Cu, Al Si Cu and other aluminum alloy films are used instead of pure metal Al films. If electron beam evaporation is used to make alloy film, it will cause decomposition due to different vapor pressure of components, so it is difficult to control the alloy film to reach the required composition.

③ It is difficult to realize the automation because of the complexity of mounting the substrate. In order to improve the reliability and repeatability of highly complex component manufacturing process, it is necessary to reduce manual operation and improve the water leap of automatic operation. With the electron beam evaporation, the Si plate can only be installed on the cyclotron support one by one, and only single batch evaporation can be used. Therefore, it is difficult to realize automatic operation.