錫銀銅無鉛銲錫電遷移之研究

Abstract

摘要 在積體電路的銅或鋁導線中所施加的電流密度若高於106A/cm2，此電流密度在元件操作溫度約攝氏100度的環境下將在陰極造成孔洞與陽極產生凸起。此種電遷移所造成的破壞將對積體電路可靠度造成嚴重的影響。在現今的積體電路設計中每一個覆晶封裝的微接點將承載0.2安培的電流，而在未來電流值將會以倍數成長，而覆晶微接點的尺寸將由100μm減小至50μm，屆時電流密度將到達106A/cm2此電流密度下將對覆晶微接點造成電遷移破壞。此外，在無鉛話的趨勢下傳統錫鉛覆晶微接點將被無鉛覆晶微接點所取代。 本研究主要利用覆晶微接點結構試片與Blech structure來研究無鉛錫銀銅銲錫之電遷移行為。在錫銀銅覆晶微接點研究方面，我們發現破壞發生在陰極的晶片端且造成接點破壞之主要原因為電流擁擠效應所造成，且在較低之電流密度下破壞發生於介金屬與銲錫之界面處。此外UBM中的銅原子在電子流驅動被下形成介金屬化合物堆積於界面中。 在Blech structure研究中我們成功的找出錫銀銅銲錫之門檻電流密度，其值分別為在80 □C下為4.3 □ 104 A/cm2 ， 100 □C下為3.2 □ 104 A/cm2，120 □C下1.4 □ 104 A/cm2，這些數值代表錫銀銅銲錫在該溫度下不會造成電遷移破壞之最高電流密度。錫銀銅銲錫之電遷移活化能在80 □C到120 □C區間為0.45eV。 而其有效電荷系數Z*在80℃，100℃，120℃ 分別為-27 ， -33，-23。 此外試片經過退火後發現晶格擴散與晶界擴散具明顯溫度界限。其門檻電流密度值分別為在80 □C下為4.6 □ 104 A/cm2 ， 100 □C下為3.9 □ 104 A/cm2，120 □C下2.2 □ 104 A/cm2 。電遷移活化能在100 □C到140 □C區間為0.8eV。

Electromigration-induced failure of SnAg3.8Cu0.7 flip-chip solder joints was investyigated with Ti/Cr-Cu/Cu under-bump metallization (UBM) on the chip side and Cu/Ni(P)/Au pad on the BT board side. Electromigration damage was examined under the current density of 2 □ 104 A/cm2 at 100 ℃ and 150℃. The failure was found to be at the cathode/chip side and voids were observed at intermetallic compound/solder interface at the cathode chip side. Copper atoms were found to move in the direction of electron flow to form intermetallic compounds at the interface of the solder and the pad metallization on the substrate side due to current stressing. In addition, eutectic SnAg3.8Cu0.7 solder stripes was investigated in the vicinity of the device operation temperature of 100 □C by using the edge displacement technique. Measurements were made for relevant parameters for electromigration of the solder, such as drift velocity, threshold current density, activation energy, as well as the product of diffusivity and effective charge number (DZ*). The threshold current densities were estimated to be 4.3 □ 104 A/cm2 at 80 □C, 3.2 □ 104 A/cm2 at 100 □C, and 1.4 □ 104 A/cm2 at 120 □C. These values represent the maximum current densities that the SnAg3.8Cu0.7 solder can carry without electromigration damage at the three stressing temperatures. The electromigration activation energy was determined to be 0.45eV in the temperature range of 80℃ to 120℃. The measured products of diffusivity and the effective charge number, DZ*, were -1.8´10-10 cm2/sec at 80℃, -5.0´10-10 cm2/sec at 100℃, and -7.2´10-10 cm2/sec at 120℃. In the pre-annealing specimens, grain boundary and lattice diffusion have a different temperature dependence range from 80℃ to 140℃. The threshold current densities were measured to be 4.6 □ 104 A/cm2 at 80 □C, 3.9 □ 104 A/cm2 at 100 □C, and 2.2 □ 104 A/cm2 at 120 □C. The measured activation energy was 0.8eV for the temperature ranges from 100 to 140℃.