Strem Chemicals’ offers new La-FMD ALD precursor for future leading edge logic and memory products

Strem Chemicals’ offering of La-FMD is one of the most promising metal-amidinate ALD precursors for lanthanum (La) based ALD thin-films which are potentially strong candidates for high-k gate dielectric in the next generation of CMOS technology.

Rare earth elements have entered high volume manufacturing for advanced logic devices since the 32 nm node (IBM, Samsung and Globalfoundries – Chipworks 2010). Especially for Lanthanum (La) — the eponym of the lanthanide series in the periodic table has been implemented as a dopant in the high-k metal gate stack. Lanthanum oxide (La2O3, dielectric constant ~ 27), for example, has been explored for two decades as a high-k gate dielectric for the replacement of conventional silicon dioxide (SiO2) gate dielectric in the next generation transistors in logic as well as in dynamic random access memories (DRAMs).

Keyword segmentation of patent applications the last 20 years for Lanthanum and “Atomic Layer Deposition” [Patbase search 15 November 2018]

Atomic layer deposition is the most promising method for growing ultra-thin-films of La based gate dielectrics and has therefore been under extensive research and filing of patent applications in the last 20 years. The R&D effort has been focused on fields relating to dielectric and high-k dielectric applications in the semiconductor industry (see keyword segmentation above). The atomic layer-by-layer film growth facilitated by self-limiting surface reactions in ALD provides atomically precise film-thickness control, good uniformity across a large area substrate, and excellent conformality in case of high aspect ratio structures like modern FinFETs and memory capacitor type pillar structures. However, to work flawlessly it requires the ALD precursors that have specific properties (LINK): 

1. Sufficiently volatile (at least ~ 0.1 Torr equilibrium vapor pressure at a temperature at which they do not decompose thermally).

2. Rapidly vaporizing and at a reproducible rate (conditions that are usually met for liquid precursors, but not for solids).

3. Not self-reacting or decomposing on the surface or in the gas phase (for self-terminating surface reactions).

4. Highly reactive with the other reactant previously attached to the surface, which results in relatively fast kinetics and thus lower ALD temperatures and cycle times.

5. Volatile byproducts that can be easily purged in order to prepare for the subsequent half-cycle.

6. Non-corrosive byproducts to prevent non-uniformities due to film etching and corrosion of the tool.

In 2007, Intel Corporation incorporated HfO2 into high-k gate dielectric stack at 45 nm technology node. However, pure HfO2 suffers from low-k interface layer problem with Si, limiting lower equivalent oxide thickness (EOT) values. It also readily crystallizes at temperatures as low as ~500°C. Therefore, amorphous dielectrics with high thermal stability are still sought after for no intrinsic defects (e.g. grain boundaries), provided they still offer the advantages of HfO2, such as high dielectric constant, wide band-gap, and low leakage current. Lanthanum-based ternary oxides, such as lanthanum scandate (LaScO3) and lanthanum lutetium oxide (LaLuO3), deposited by ALD process involving metal amidinate precursors reportedly exhibit desirable structural and electrical properties. In fact LaLuO3 is potentially the best amorphous phase gate dielectric with dielectric constant k~32. It doesn’t form low-k interfacial layers with Si which enables effective oxide thickness (EOT) values < 1 nm with significantly low leakage current. Another factor contributing to the low leakage current across ALD grown thin LaLuO3 gate dielectric is the large band-offset (2.1 eV) with respect to Si; the symmetric conduction and valence band offsets result into equal leakage currents in electron driven NMOSFETs and hole driven PMOSFETs. It stays amorphous and doesn’t form alloys with Si or Ge after respective source/drain activation anneals.  

As a very recent example of an actual high aspect ratio application on 300 mm wafers requiring all ALD precursor characteristics described above (1 to 6) we can see the paper that Imec presented at this famous IEDM conference, on using a LaSiOx layer as a dipole inserted in the HKMG stack. Imec succeeded in stacking the complete FinFET front end module on top of a "standard" bulk silicon FinFET module demonstrating also good threshold voltage tuning, reliability and low-temperature performance. Presumably it has most likely been deposited by an ALD process since it will have to conformally coat the fins and ensure precise thickness control and uniformity : IEDM2018 Paper #7.1, “First Demonstration of 3D Stacked FinFETs at a 45nm Fin Pitch and 110nm Gate Pitch Technology on 300mm Wafers,” A. Vandooren et al, Imec [LINK].

As in this case and many more, the stringent qualifications for ALD precursors put them in the category of high quality specialty chemicals — the performance or function specific materials or molecules of choice. The deposited film properties are strongly influenced by the physical and chemical properties of a single molecule or a formulated mixture of molecules as well as its chemical composition. Therefore, it puts a lot of pressure on the manufacturer and supplier of the high purity specialty chemicals in terms of quality, purity, documentation procedures, customer service etc.

Strem Chemicals, Inc. headquartered in Newburyport, Massachusetts, USA is an employee-owned fine chemicals supplier. They’ve been manufacturing and marketing high-quality specialty chemicals for more than half-a-century. Strem provides custom synthesis and FDA inspected current good manufacturing practice (cGMP) services to its clients, which include academia, industry, and government R&D labs. What helps to distinguish Strem from the other chemicals suppliers is their commitment to quality products. In fact, Strem has been listing color and form, a key quality indicator, for every product since their first catalog. Strem is an ISO 9001:2015 Certified company.  For additional details about the more than 5,000 products offered by Strem, visit their website, www.strem.com.

Tris(N,N'-di-i-propylformamidinato)lanthanum(III), (99.999+%-La) La-FMD is one of the metal amidinate precursors in Strem’s product catalog (catalog number 57-1200, CAS number 1034537-36-4) for La ALD. The material is a white to off-white powder. The chemical formula and the molecular weight of La-FMD are C₂₁H₄₅LaN₆ and 520.53, respectively. Rohm and Haas Electronic Materials LLC (subsequently Dow Chemical) reports La-FMD as the most volatile La precursor known so far. The vapor pressure at a given temperature imparted by La-FMD is higher than that by La(Cp)₃ and La(thd)₃. Moreover, Roy G. Gordon from Harvard University reports that the amidinate precursors are thermally more stable than their amide counterparts because of the chelating amidinate ligand and the absence of M-C bond. La amidinates are highly reactive with Si-H bonds yielding much smaller surface saturation time and in turn quick self-termination of ALD half-reaction; thus shortening the ALD cycle time. Also, an excellent surface coverage is provided by La amidinate precursors on Hydrogen terminated Si. 

---
Promotional Blog for Strem Chemicals, Inc.
Researched, produced & written by BALD Engineering AB, Stockholm, 2019-01-23
Abhishekkumar Thakur, Jonas Sundqvist
www.baldengineering.com