積體電路奈米銅製程之研究

Abstract

在IC後段金屬製程中，電鍍銅金屬導線現今已成為100奈米圖形以下的小孔填充最主要方法。而在未來隨著元件朝向90到65奈米後，電鍍製程技術的應用面臨三種主要的挑戰：(1)更小尺寸的填充；(2)在孔內更低電阻的銅金屬；與(3)在整合議題上，要有更高的可靠度。因此，本研究首先針對在高深寬比奈米尺寸鑲嵌結構中得到無孔洞的沈積，主要是取決於電鍍銅溶液中，添加劑的選擇；在這一方面需找出有效添加劑的規則，而有別於商業上使用的添加劑，來達到更好的填充能力和較低的電阻；在另一方面則是加入合金來增加電鍍銅的可靠度。在本研究發現更小尺寸的填充，需滿足四個條件。(1) 鍍液在小孔的孔壁，要有較低的表面張力的鍍液，才能克服孔壁的毛細管現象。(2)整平劑要含有足夠的活化過電壓。(3)要有足夠的抑制梯度，(4) 底部加速能力。 根據上述條件，我們發現綜合兩種不同分子量的PEG有較好的潤濕性，和低消耗且低擴散能力的有機整平劑(leveler)，比如aminobenzothiazole，可使其能應用在0.1μm小尺寸孔洞的填充。如果要能達到100奈米以下小孔洞的填充，除了上述潤濕劑和整平劑之外；加速劑是不可或缺的，因為增加了小孔入口和底部之間的沉積梯度。從另一角度來看，同時間添加3種或更多的添加劑卻會造成導線阻值增加，因此本研究發現一種雙重添加劑(hybrid mode additive)，benzothiazole (BTA)，可同時兼具整平劑和加速劑的功能，且因不含硫元素（因為硫有較高於氮的阻值）；所以有較低的電阻值和0.1μm以下小孔洞的填充。 除此之外，本實驗在第五章探討添加劑與阻值之間的影響。發現阻值主要取決於雜質散射，少部份決定於晶界散射。因此低消耗的添加劑有較低的阻值。在關於脈衝電鍍頻率的效應研究，發現最大填充力和最低阻值出現在100 Hz 的頻率下，這是由於低於臨界頻率，由於過電壓的效應，隨著頻率增加會導致鍍膜更緻密；反之，高於臨界頻率，由於暫態時間的效應，隨著頻率再增加會導致抑制劑補充不足鍍膜更粗糙。 最後未來工作研究中對於電鍍銅膜在可靠度的改善中，本實驗使用硫酸鋁溶液，除了可使電鍍銅膜的優選方位（111）、表面粗糙度、機械強度、抗氧化性得到改善之外；對電阻亦不會有明顯負面的影響。另一方面，由於添加硫酸鋁粉末有細化銅膜晶粒的功能；因此添加微量的合金並不會造成小孔填充能力變差。電鍍銅鋁合金沉積在高深寬比奈米尺寸鑲嵌結構中也能得到無孔洞的沈積，並不會破壞填充能力；但同時可得到比存同有更高的可靠度，且在300~400℃真空退火可得到1.8μΩ-cm 的低電阻。透過本實驗有助於未來深次微米鑲嵌之電鍍銅合金製程提供有用的資訊。

The move to copper still exists many challenges. A successful electroplating process meet 3 basic requirements, such as (1) a void-free deposit, (2) a low resistivity, and (3) a high reliability, which are strongly dependent on the plating chemistry, the plating condition, and the post-treatment, especially for next-generation of 90 → 65 nm. In the first study for a higher filling power of smaller features, we demonstrated the technology of copper electroplating in trenches or vias as small as 0.1 mm with aspect ratio 10. In order to achieve a superfilling behavior for aggressive features, the additive chemistry system obeys 4 rules, (1) a lower surface tension, (2) an adequate activation ability, (3) a sufficient inhibition gradient, and (4) a accelerating ability on the bottom. Therefore, our wetting agent system was consisted of two polyethylene glycols (PEG) with different molecular weights. This novel scheme of wetting agents could transport the electrolyte into the base of a sub-0.1 mm via. Through studying the adsorption, and diffusion reactions of additives by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetric stripping (CVS) measurements, the ratio between the diffusion coefficient and the consumption rate of a leveler was identified experimentally to determine its leveling ability. Excepting that, an important finding, a leveler with a lower diffusion and depletion abilities, had a better leveling performance. By means of those analyses, levelers of heterocyclic compounds containing N, S atoms provided the matching properties. In this dissertation, influences of additives on the impurity incorporation and grain number density in the deposited film were firstly discussed. A leveler with weaker adsorption ability could reduce the depletion reaction during the electroplating process and eventually deposit a film with high quality conductivity. After optimizing the electrolyte as our investigations, the electrolyte with 2-aminobenzothiazole (2-ABT) and two different molecular weights of PEG could obtain a void-free electroplating in 0.1 μm via and the electrical resistance of the deposited copper film was below 2.5 mW-cm. In addition to the investigation of leveling agents, one hybrid-mode additive, BTA, which can simultaneously cause acceleration and inhibition, replacing the original functions of an accelerator and a leveler in the three-additive electrolyte, was demonstrated. An electrolyte with BTA and PEG successfully electroplated 100 nm vias with an aspect ratio of 10:1. BTA at a low concentration level acted as a bottom-up accelerator and increased the rate of deposition at the base of the vias. BTA at a high concentration acted as an effective leveler and enhanced the inhibition of the PEG suppressor at the opening of the vias. Using an electrolyte with the hybrid-mode additive yields a higher deposition gradient within the filled patterns than another recipe that includes only suppressing agents. The filling capacity within deep sub-micron damascenes obtained by using this novel combination of additives is valuable for use in next generation devices. The electrical resistance of electroplated copper is described well by Mattheiesen’s rule. For the electroplating of copper films, the additive chemistry dependence is dominated by an impurity scattering and less affected by grain-boundary scattering. The impurity resistance depends on the impurity type and the content incorporated into deposited films. The defect resistance depends on growth types and boundary numbers. Through XPS and voltage transient analysis, we compared the effect of various additives on the resistances of copper films. In this experiment, the leveling agent, 2-aminobenzothiazole was demonstrated to be an effective additive with low electrical resistance due to the low consumption ability. Finally, Pulse-and DC-plated copper films on wafers were investigated with respect to their applicability as advanced interconnects materials. Pulse frequency was varied to determine its influence on the topography, grain size, and electrical resistivity of electroplated copper films. The maximum filling power and lowest resistance are obtained at 100Hz pulse current frequency for an acid-copper bath with PEG additives. Below a threshold pulse frequency, increasing pulse frequency leads to denser copper films of low resistance owing to the dominating factor-overpotential effects. Above a threshold frequency, due to a weak recovery rate for PEG additives (or dominating factor-transient time effects), the copper morphology becomes rougher and has higher resistance.