For sub-22nm device generations, device manufacturers are likely to adopt PDMAT precursor for ALD-TaN barrier films for copper interconnect structures.

BY LEIJUN HAO, RAVI K. LAXMAN and SCOTT A. LANEMAN, Digital Specialty Chemicals, Toronto, Ontario, Canada.

At sub-micron device technology, copper is the interconnect metal of choice because of low resistivity, 1.7μΩ-cm, high current densities and excellent thermal conductivity. These characteristics of copper are increasingly important for supporting sub-22nm lines with high device density and speed. Deposition of copper lines can be achieved by a variety of techniques. A standard method generally involves physical vapor deposition (PVD) and electrochemical deposition (ECD). Because copper diffuses into silicon, silicon dioxide, and other low k dielectric materials, which can “poison” the device, Ta/TaN films are used as copper diffusion barriers. Copper integration schemes at sub-22nm use low-k dielectric PVD Ta/TaN barrier/ PVD copper seed/ ECD-Cu material stack [1].

Conventionally, tantalum nitride has been deposited by physical vapor deposition. As conformality becomes crucial for interconnect applica- tions PVD or CVD processes are challenged to achieve aspect ratio of 5:1, due to directional limitations and form “pinch off” resulting in a void formation. When aspect ratio exceeds 5:1 a large amount of effort is focused in the formation of void and seam-free thin layers.

These difficulties with PVD and CVD motivate TaN-atomic layer deposition (ALD) process. At very tight geometries ALD is preferred over other techniques due to excellent conformality [2]. ALD of a tantalum nitride barrier layer involves sequen- tially pulses of pentakis(dimethyl-amino)tantalum (PDMAT), a tantalum nitrogen-containing precursor followed by reaction with ammonia to a process chamber. Deposition of ALD TaN films is also reported [3] using a mixed remote hydrogen (H2) and ammonia (NH3) plasma to reduce PDMAT at 275°C. As the (CD) features are scaled below 22nm BEOL, along with integration challenges, tight precursor composition and manufacturing of ultrahigh purity ALD precursors are desired for successful metallization.

ALD deposition for very thin conformal barrier films (<25 Å) could potentially reduce via resis- tance and does not impact electromigration of copper. Low via contact resistance at the contact area is also dependent on several factors such as trace impurities in the precursors, morphology of the barrier, deposition process, film nucleation and surface interface. In one study, a 10Å PEALD TaN reduced via resistance by 50% compared to the PVD TaN [3]. ALD growth rate also varies with the underlying layers, W < ULK< Cu. From a film property characterization point of view, the density and resis- tivity of PEALD TaN film is about 11.6 g/cm3 and ~2000 μΩ-cm. Ta-rich PVD TaN has a higher density of about 15.0 g/cm3 and a resistivity around 250 μΩ-cm. It was observed that density of thermal ALD TaN using PDMAT is ~90% of the density of PEALD TaN (thermal ALD TaN < PEALD TaN < PVD TaN). With appropriate precursor, ALD TaN also supports desired -Ta and suppresses more resistive -Ta nucleation [5].

FIGURE 1. Improved methods of purification have been used to produce microcrystalline pale yellow PDMAT with purity of >99.99995% (determined by trace metals and other spectroscopic methods) with extremely low chloride (<10ppm), low oxygen and total trace metals.

At film thickness of 10-25Å trace metal impurities in the precursors play detrimental role in the density, resistivity and nucleation properties. Among the impurities tight control of oxygen concentration in the precursors and deposited films are extremely important. Especially, in the case of tantalum metal, being an electropositive metal it forms strong bonds with oxygen; the resulting “Ta-O” behaves as a capacitor in deposited films rather than a conductive barrier with low resistance. Hence low oxygen concentrations in precursor play an important role in ALD film properties.

The manufacturing of extremely high purity in high volume poses several challenges. PDMAT is extremely sensitivity to oxygen and water. For this and other reasons, the chemical synthesis of the PDMAT involves important sequence of purification technologies under inert conditions. We have developed novel purification methods and successful production of highly oxygen and moisture sensitive ALD precursor in High Volume Manufacturing (HVM). The yellow crystalline end product is shown in FIGURE 1, and TABLE 1 lists common properties.

Challenges in manufacture of PDMAT

PDMAT is synthesized in HVM by well-estab- lished metathesis reaction between TaCl5 and LiNMe2 as reported in the literature (equation 1).

TaCl5 + 5LiNMe2 5LiCl + Ta(NMe2)5

PDMAT is relatively stable. However, high conversion to Ta(NMe2)5 presents several challenges because the reaction tends to give a mixture of Ta(NMe2)5, TaCl(NMe2)4, and Ta2(μ-Cl)2(NMe2)6Cl2 due to the equilibrium between them. Unfortunately, the use of large excess of LiNMe2 causes the formation of Me2NCH2N(H)Me; this results in the formation of the byproduct Ta(NMe ) (2-MeNCH NMe ). 2422

On the other hand, due to the abundant chloride ions available in the reaction solution, Ta(NMe2)5 can undergo chloride metathesis to produce byproducts such as TaCl(NMe2)4. Due to this complication of the by-products and the equilibrium between them, the isolated yield of PDMAT from TaCl5 and LiNMe2 is low at ~50%. Digital Specialty Chemicals has studied exten- sively the synthesis of PDMAT by metathesis between TaCl5 and LiNMe2. At optimized reaction conditions, in situ PDMAT yield, as high as 99% can be achieved. Typically, the crude PDMAT isolated from the reaction has a purity of 90-96%. Sublimation of the crude material under high vacuum gave orange crystalline solid with a purity of only 95-96% as determined by 1H NMR spectroscopy.

Isolation and purification of PDMAT presented another challenge because PDMAT is extremely sensitive to both air and trace oxygen. Organometallic precursor with low halide content is required because halides in the barrier layer may attack the copper layer and cause corrosion. PDMAT obtained by crystallization method usually contains high levels of chloride (>80ppm), lithium (>40ppm) and other metals. Although PDMAT has an adequate vapor pressure and can be sublimed, sublimation can only reduce chloride, lithium, and other impurities to a certain degree, because some of the impurities also co-sublime. Purification can be performed by a series of sequential steps and ultra-high purity PDMAT (>99.6% as determined by 1H NMR spectroscopy, FIGURE 2) was achieved by following these processes in an in-house developed reactor. Our manufacturing processes produce micro crystalline pale yellow crystalline solid with purity of >99.99995% (determined by trace metals and other spectroscopic methods) with extremely low chloride (<10ppm), low oxygen and total trace metal analysis. Ultra high purified pentakis(dimethylamido) tantalum having less than about 10 ppm of chloride and extremely low oxygen content can be used as an effective barrier layer for copper. In our high volume production we have developed unique purification processes that allow production of highly crystalline material.

FIGURE 2. 400 MHz 1HNMR (X10) spectroscopy indicates the ultra-high purity of the materials, <99.6%.

Crystalline material is extremely useful in controlling the carryover of small amorphous particles through carrier gas stream. PDMAT is typically introduced as a vapor dissolved in a carrier gas by flowing a carrier gas through a canister containing solid precursor. The canister is heated uniformly to allow clean evaporation of precursor dissolved in the carrier gas (e.g. nitrogen, helium). Optical characterization of PDMAT vapor in an ALD pulse process was recently published [6], the low PDMAT partial pressure is due to low PDMAT vapor pressure and loss of heat of vaporization of the PDMAT powder, and also low carrier gas saturation. High carrier gas mass flow rates do not necessarily result in a higher mass transport of precursor. A microcrystalline PDMAT may help in better contact and resonance time for cleaner and less particulate delivery.