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Study on the properties of aluminum plating by electron beam evaporation and magnetron sputtering電子束蒸發與磁控濺射鍍鋁性能分析研究

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Study on the properties of aluminum plating by electron beam evaporation and magnetron sputtering電子束蒸發與磁控濺射鍍鋁性能分析研究

發布日期:2019-01-26 作者:www.tonertimes.com 點擊:

膜厚


嚴格控製發鋁膜的厚度是十分重要的,因為鋁膜的厚度將直接影響Al膜的其它性能,從而影響半導體器件的可靠性。對於高反壓功率管來說,它的工作電壓高,電流大,沒有一定厚度的金屬膜會造成成單位麵積鋁膜上電流密度過高,易燒毀。對於一般的半導體器件,鋁層偏薄,則膜的連續性較差,呈島狀或網狀結構,引起壓焊引線困難,造成不易壓焊或壓焊不牢,從而影響成品率;Al層過厚,引起光刻時圖形看不清,造成腐蝕困難而且易產生邊緣腐蝕和“連條”現象。


采用電子束蒸發,行星機構在沉積薄膜時均勻轉動,各個基片在沉積鋁膜時的幾率均等;行星機構的聚焦點在坩堝蒸發源處,各個基片在一定真空度下沉積速率幾乎相等。采用磁控濺射鍍膜方法,由於沉積電流和靶電壓可以控製,也即是濺射功率可以調節並控製,因此膜厚的可控性和重複性較好,並且可在較大表麵上獲得厚度均勻的膜層。


Al膜厚度的測量可采用金屬膜厚測量儀,它是根據渦流原理設計製造的無損測厚儀。根據工藝參數,91xjcc下载ioses製備了一批試樣,樣品經測試,濺射Al薄膜的平均厚度是1.825μm,電子束蒸發鋁薄膜的平均厚度是1.663μm,均符合半導體器件電極對鋁膜厚的要求(小信號為1.7±0.15μm;大功率為2.5±0.3μm。


為了更進一步地觀測膜厚及表麵形貌,樣品放入環境掃描電子顯微鏡philipsXL30-ESEM中進行觀測,並根據視頻打印機輸出的SEM圖片可以看出,電子束蒸發的膜厚分散度較大,即均勻性較差。


附著力


附著力反映了鋁膜與基片之間的相互作用力,也是保證器件經久耐用的重要因素。濺射原子能量比蒸發原子能量高1-2個數量級。高能量的濺射原子沉積在基片上進行的能量轉換比蒸發原子高得多,產生較高的熱能,部分高能量的濺射原子產生不同程度的注入現象,在基片上形成一層濺射原子與基片原了相互溶合的偽擴散層,而且,在成膜過程中基片始終在等離子區中被清洗和激活,清除了附著力不強的濺射原子,淨化且激活基片表麵,增強了濺射原子與基片的附著力,因而濺射鋁膜與基片的附著力較高。


測定附著力所采用的方法是測量鋁膜從基片上剝離時所需要的力或者能量,91xjcc下载ioses采用剝離水法來測定附著力。


設薄膜單位麵積的附著能為γ,則寬度為b,長度為a的薄膜的總附著能E=abγ(1)


用於剝離該薄膜的力F所作的功Wp=Fa(1-sin(θ))(2)


如果是靜態剝離並忽略薄膜彎曲時所產生的彈性能,則F所作的功近似等於薄膜的總附著能,即Wp=E,於是F=bγ/(1-sin(θ))(3)


(3)式中F隨著θ角的變化而變化,不能真正反映薄膜的附著性能。當所加剝離力與薄膜垂直,即θ=0°時,則式簡化為F=bγ(4)


根據測量所得的F便可計算出附著能γ=F/b。如果要直接計算單位長度的附著力f,根據定義並采用上述方法(θ=0°)進行剝離可得f=γ。可見,附著力的大小和附著能γ的數據相同,由於Al膜的附著能γ較高,所以其附著力較大。實驗測得的數據是:濺射Al膜的平均附著力25N,電子束蒸發Al膜的平均附著力為9.8N。這些數據和理論分析結論一致。


致密性


考慮鋁膜的致密性就相當於考慮鋁膜的晶粒的大小,密度以及能達到均勻化的程度,因為它也直接影響鋁膜的其它性能,進而影響半導體嘩啦的性能。


氣相沉積的多晶鋁膜的晶粒尺寸隨著沉積過程中吸附原子或原子團在基片表麵遷移率的增加而增加。


由此可以看出鋁膜的晶粒尺寸的大小將取決環於基片溫度、沉積速度、氣相原子在平行基片方麵的速度分量、基片表麵光潔度和化學活性等因素。


由於電子束蒸發的基片溫度Ts=120°C,蒸發速率20-25A/s,蒸汽鋁原子的能量為0.1-0.3eV,而濺射的基片溫度Ts=120°C,濺射速率8000A/min(133.3A/s)或10000A/min(166.7A/s),濺射閾為13eV,濺射鋁原子的能量比電子束蒸發的鋁原了能量高1-2個數量級,所以電子束蒸發的鋁原子碰到基片,很快失去能量,且遷移率很小,故原子在表麵上重新排列較困難,即沉積的地方就是定位的地方成原子之間的空隙較大,有麵粗糙度很大;濺射的基片溫度較高,鋁原子能量也較高,在而基片表麵的原子遷移率增大,使得薄膜表麵橫向動能較大,易於連結殂成光滑的表麵,穩定性較高,晶粒較大,原子間距較小,因而形成的薄膜表麵粗糙芳減小。


通過環境掃描電子顯微鏡philipsXL30-ESEM觀測,並分析兩種鋁膜的晶粒大小及表麵形貌的SEM照片也能驗證這一結論,電子束蒸發的平均粒徑為266.8nm,濺射的平均粒徑為1.528μm,雖然電子束蒸發鋁膜的粒徑明顯小於濺射的鋁薄膜,但是電子束蒸發的鋁原子較終不得靠得很近,當中存在很多間隙,而且濺射的鋁原子相互靠得很緊,從側麵觀測,濺射的鋁膜平滑而且色澤光亮,說明濺射鋁膜的致密較好。


濺射的晶粒較大還有一個好處,減小了晶界麵積,從而減少電遷移短路通道的數目,有利於增強Al膜的抗電遷移能力,延長Al膜的平均壽命。但晶粒尺寸不可太大,否則影響鋁膜細線條圖形的光刻質量。同時,濺射的鋁膜晶粒雖大,但可以通過的後麵的熱處理使之細化並使性能更加優越。


電導率


金屬與半導體接觸並非一定能夠形成一個純電阻性接觸。如果接觸電阻太大,即電阻率低,則外加的信號電壓就會有相當大的一部分降落在接觸電阻上,造成不必要的電壓降和功率損耗,所以要想獲得低阻的歐姆接觸,膜層的電阻率應盡量小,電導率應盡量高。


鋁膜的電阻率與鋁材的非常接近。電阻率隨結晶粒徑的減小而增加。由於電子束蒸發鋁膜的結晶粒徑明顯小於濺射,所以濺射的電阻率小於電子束蒸發,其電導率較高。


折射率


折射率一般可以反映薄膜的致密程度,隨致密程度的增加而增加,而91xjcc下载ioses所製備的電極引線鋁膜要求致密性好,這就可能通過測試折射率的大小來定性地判斷鋁膜的致密性。而折射率可以通過反射率間接地換算得到。


金屬膜的特性一般用折射率NM=n-ik來表征,設金屬膜厚度為dM,折射率為NM=n-ik,位相厚度為δM=2πNMdM/λ,若考慮垂直入射,金屬膜與Si基底的組合導納為YM=((ngcos(δM)+iNMsin(δM))/(cos(δM)+isin(δM)ng/NM)=YM(1)+iYM(2)(5)從而整個結構的反射率為RM=|(n0-YM)/(n0+YM)|2={[(n0-YM(1))]2+[YM(1)]2}/{[(n0+YM(1))]2+[YM(1)]2}(6)但其描述和計算過程過於複雜,故可以有下麵的描述和計算代替。


當光束垂直入射到單層薄膜有麵時,反射率RM=(n0n2-NM2)/(n0n2+NM2)(7)


則NM={[(1-RM)/(1+RM)]n0n2}1/2(8)


式中RM-----------反射率


n0------------空氣的反射率


NM---------------Al膜的折射率


n2-----------Si片的折射率(約為3.5)


隻要準確測出垂直入射的反射率RM就可以求出鋁膜的反射率NM。通過日本島津生產的UV3101型分光光度計測得的在不同波長範圍內的反射率可知,在可見光400—760nm範圍內,鋁膜的反射率11#樣品為RM=0.82,8#樣品為RM=0.83,通過(7)、(8)式計算,得出鋁薄膜的反射率NM:8#樣品為NM=0.702,


11#樣品為NM=0.688。雖然,濺射鋁膜的折射率大於電子束蒸發的鋁膜,濺射鋁膜的致密性比電子束蒸發好。


結論


電子束蒸發和磁控濺射製Al膜是半導體器件電極製備生產中常用的兩種方法,通過理論與實驗分析,並對樣品進行了膜厚、附著力、致密性,電導率、折射率等指標的綜合測試,實驗表明:電子束蒸發製得的Al薄膜厚度的可控性和重複性較差及分散度較大鋁薄膜與Si基片的附著力較小;鋁薄膜的晶粒雖小,但很疏鬆,導致其致密性較差;鋁膜的電導率、折射率較塊狀鋁材小得多。而磁控濺射製得的鋁膜的性能指標則比電子束蒸發的指標優越。實踐證明,磁控濺射方法製備的鋁薄膜的綜合性能優於電子束蒸發,所以在生產實踐中絕大多數采用磁控濺射沉積半導體電極材料,這也是半導體行業中薄膜行業的發展方向。


此外,濺射還可以解決電子束蒸發帶來的三個問題:


①台階覆蓋度。一般器件的圖形尺寸為2--3μm或更小,要求在1μm左右高的台階部位盡量能鍍覆膜厚均勻的金屬鍍層。采用電子束蒸發和行星回旋式基片架機構組成的裝置,難以得到十分理想的覆蓋度。


②合金膜的成分控製。隨著圓形的微細化,為確保可靠性並提高成品率采用Al-Si、Al-Cu、Al-Si-Cu等鋁合金膜代替純金屬Al膜。如果采用電子束蒸發來製取合金膜,由於組分蒸氣壓不同會引起分解,很難控製合金膜使其達到所要求的成分。


③裝卡基片複雜,難於實現自動化。在高度複雜的元器件製造工藝中,為提高可靠性和重複性,應盡量減少人工操作,提高自動化操作水閏。采用電子束蒸發,行星回旋式支架上隻能一個一個地裝卡Si片,而且隻能采取單批式蒸鍍。因此,難於實現自動化操作。

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

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.

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.

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.


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