Electrical Reliability of Integrating Low-K Dielectrics with Cu Metallization and Passivation Study of Cu Films
|Keywords:||低介電常數介電層;銅金屬化製程;銅抗氧化;Low-K Dielectrics;Cu Metallization;Cu Passivation|
|Abstract:||本論文探討以旋覆式和化學氣相沈積兩種方式成長之低介電常數介電層與銅金屬化製程整合的電性可靠度及銅膜之抗氧化處理；前者係先研究這些低介電常數介電層的物理特性，以及經過高溫退火處理後的熱穩定性，再利用金屬絕緣層半導體(MIS)結構及單層銅鑲嵌式結構(single level Cu damascene)來分析他們與銅製程整合所衍生的電性可靠度問題；後者係以離子佈植方式將BF2+和鎂原子摻雜於銅膜之表面層，藉此探討這兩種雜質對銅膜抗氧化能力提升的效果及其失效機制。
在旋覆式沈積之低介電常數介電層的研究方面，吾人選擇三種有機aromatic polyether類的高分子聚合物作為評估對象，分別是Schumacher的PAE-2、AlliedSignal的FLARE™和Dow Chemical的 SiLK™。在PAE-2方面，吾人發現Cu/PAE-2/Si電容結構經過30分鐘200℃的熱退火處理後，銅金屬即已擴散進入PAE-2中，並造成Cu/PAE-2/Si電容結構漏電流的增加與崩潰電壓的劣化。然而，以厚度25 nm的濺鍍鉭膜(Ta)和反應式濺鍍氮化鉭膜(TaN)所製成的Cu/(Ta, TaN)/PAE-2/Si電容結構的高溫熱穩定性分別可達400及450℃。由於濺鍍鉭膜與PAE-2間的附著力較差，無法抵擋PAE-2在高溫outgassing所產生的外向應力，致使濺鍍鉭膜的抗銅擴散能力不如反應式濺鍍氮化鉭膜。
在FLARE™和SiLK™方面，吾人發現這兩種介電層經過8小時400℃的熱退火處理後，均展現極佳的高溫熱穩定性。再者，以加溫偏壓法(BTS)施加於Cu/USG/(FLARE™, SiLK™)/SiO2/Si電容結構之測試結果顯示，這兩種介電層本身皆具備優越的抗銅擴散能力，但也顯示這兩種介電層材料內部極易受外加電場的影響，而產生介電層極化(dielectric polarization)的電性不穩定現象。
吾人亦使用單層銅鑲嵌式測試結構來探討以單甲基甲烷摻雜二氧化矽作為金屬間絕緣層(IMD)的銅導線之間所呈現的漏電流機制。從實驗結果的分析，吾人發現銅導線間的漏電流大部分是由流經單甲基甲烷摻雜二氧化矽的漏電流所構成，而其緣由則可能為與介電層內的雜質殘留或鍵結缺陷有關的Frenkel-Poole機制。此外，測試結構中的蝕刻終止層(etching stop layer)之厚度與品質，以及銅膜之是否覆蓋鈍化保護層(passivation layer)，都對銅導線間的漏電流具有重大影響。
This thesis studies the thermal stabilities and physical characteristics for a number of low dielectric constant (low-k) aromatic polyether polymers and F- as well as C-doped oxides, which are deposited using spin-on (SO) and chemically vapor deposited (CVD) techniques, respectively. In addition, electrical reliabilities with respect to the integration of these low-k dielectrics with Cu metallization are evaluated using planar MIS capacitors and/or single level Cu damascene structures. Moreover, the passivation of Cu films against oxidation in the structure of Cu/SiO2/Si with BF2+ and Mg doping by the ion implantation technique are also investigated. Electrical measurements and various material analyses are used to determine the optimal implantation condition with respect to the effectiveness in improving the oxidation-resistant capability of Cu films. To start with, the electrical reliability with regard to the integration of thin sputtered Ta and reactively sputtered TaN barriers with Cu and an early version of organic spin-on aromatic polyether polymers, Schumacher’s PAE-2 (k=2.8), is investigated. It is found that Cu readily penetrates into PAE-2 and degrades its dielectric strength in the MIS capacitors of Cu/PAE-2/Si structure at temperatures as low as 200℃. Thin Ta and TaN films of 25 nm thickness sandwiched between Cu and PAE-2 serve as effective barriers against Cu penetration during a 30 min thermal annealing at temperatures of up to 400 and 450℃, respectively. A failure mechanism related to the outgassing induced gaseous stress of PAE-2 in the structure of Cu/Ta/PAE-2/Si is proposed to explain its premature barrier degradation. The TaN barrier does not suffer from this gaseous stress problem because of its stronger adhesion to PAE-2 than that of Ta to PAE-2, leading to a better barrier capability against Cu diffusion. Two species of newly developed organic spin-on aromatic polyether polymers, AlliedSignal’s FLARE™ (k=2.8) and Dow Chemical’s SiLK™ (k=2.7), are investigated with regard to their dielectric and barrier properties. Both of these low-k polymers exhibit acceptable thermal stability with respect to a thermal annealing at 400℃ for eight hours in an N2 ambient. Moreover, they also exhibit a good dielectric barrier property against Cu penetration under bias- temperature stress (BTS) at 150℃ with an effective applied field of 0.8 MV/cm. However, an anomalous instability is observed in the C-V curve under BTS, which is explained by a proposed model of stress induced dielectric polarization charges within these organic aromatic polymers. To evaluate the feasibility of low-k CVD dielectrics for BEOL IMD applications, the physical and electrical properties of two species of inorganic low-k CVD oxides, F-doped FSG (k=3.5) and methylsilane (1MS)-doped OSG (k=2.9), are investigated and compared. FSG has a higher thermal stability (>600℃) than 1MS-doped OSG (500℃), based on the results of thermal annealing for 30 min in an N2 ambient. The degradation of the low-k property in 1MS-doped OSG is mainly due to the thermal decomposition at temperatures above 500℃ of methyl groups, which are introduced to lower the density and polarizability of OSG. For the Cu-electrode oxide-sandwiched low-k dielectric MIS capacitors, Cu penetration is observed in both FSG and 1MS-doped OSG with the MIS capacitors under BTS at 250 and 150℃, respectively, with an effective applied field of 0.8 MV/cm. However, Cu appears to drift more readily in 1MS-doped OSG than in FSG, presumably because 1MS-doped OSG, which is deposited at a low deposition temperature of 17℃, has a more porous and less dense structure than FSG. The physical and electrical properties of another species of inorganic C-doped CVD oxides, trimethylsilane (3MS)-doped OSG (k=2.7), are also investigated and compared with those of 1MS-doped OSG. Probably due to the distinct chemical bonding schemes, 3MS-doped OSG has a higher thermal stability temperature (600℃) than 1MS-doped OSG (500℃), based on the results of thermal annealing for 30 min in an N2 ambient. However, Cu permeation is also observed in 3MS-doped OSG after BTS stressing the Cu-electrode oxide-sandwiched 3MS-doped OSG MIS capacitor at 150℃ with an effective applied field of 0.8 MV/cm. Although Cu penetration is observed in both 1MS- and 3MS-doped OSG, Cu appears to drift more readily in 1MS-doped OSG than in 3MS-doped OSG, as evidenced by the results of their negative flat-band voltage shift in C-V curves and SIMS measurements. The distinct difference between the observed Cu drift behavior in 1MS- and 3MS-doped OSG is presumably due to the more porous and less crosslinked structure of 1MS-doped OSG than 3MS-doped OSG, which is grown at a higher deposition temperature of 350℃. The leakage mechanism in the single level Cu damascene structure with 1MS-doped OSG as the IMD is investigated using a comb/serpentine structure. The leakage current between Cu lines is presumed to be dominated by the Frenkel-Poole emission in 1MS-doped OSG for the structure using a 50 nm thick SiC etching stop layer (ESL). In the structure using a 50 nm thick SiN ESL, the leakage current is larger than that using a SiC ESL because the leakage component through SiN also makes a considerable contribution to the total leakage in addition to the bulk leakage from field-enhanced thermal excitation of trapped electrons in 1MS-doped OSG. Moreover, reducing the thickness of SiN ESLs further exacerbates the total leakage. Furthermore, the passivation of Cu surface in the damascene structure is required to prevent the damascene Cu feature from serious oxidation during thermal annealing processes, and thus greatly minimizing the leakage. The passivation of Cu surface against oxidation in air is investigated, using BF2+ doping by the technique of ion implantation. With BF2+ implantation at the optimum conditions (35~40 keV, 1~8 ´ 1014 cm-2), Cu films are capable of resisting oxidation at temperatures of up to 250℃ because boron atoms of peak concentration are projected near the Cu surface; thus the diffusion paths of oxidizing species, such as oxygen and Cu atoms, are efficiently blocked to avoid Cu oxidation. Finally, to further strengthen the oxidation resistance of Cu films, the effects of Mg doping on the oxidation resistance of Cu are investigated using Ar+ implantation through a multilayer structure of SiO2 (100nm)/Mg (20nm)/Cu/SiO2/Si. It is found that the implantation at 130 keV to a dose of 5 ´ 1015 cm-2 significantly enhances the oxidation resistance of Cu at temperatures of up to 375℃. At this energy, a small number of Mg atoms are knocked out, leading to the formation of an impervious MgO barrier layer on the Cu surface, which effectively suppresses the oxidizing diffusion paths and prevents Cu from oxidation.
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