High-k dielectric

In semiconductor technology, a high-k dielectric is a material that has a higher dielectric constant than conventional silicon dioxide (ε _r = 3.9) or silicon oxynitride (ε _r <6). ${\ displaystyle {\ varepsilon} _ {\ mathrm {r}}}$

The term “ high-k ” is borrowed from English, where the dielectric constant is often denoted by ( kappa ), or k in the absence of this symbol . ${\ displaystyle \ kappa}$

Reasons for using high-k dielectrics

Comparison of a conventional gate layer stack with silicon dioxide as the dielectric with that of a gate layer stack manufactured using high-k + metal gate technology (simplified illustration).

In order to improve the properties of integrated circuits, for example to reduce the power consumption of highly integrated circuits and memories or to achieve higher switching speeds, the structures are reduced in size. Due to the progressive miniaturization of microelectronic components, the semiconductor industry is increasingly reaching its physical limits and is confronted with higher leakage currents due to quantum mechanical effects. The tunnel current increases sharply as the gate dielectric thickness is reduced below 2 nm. Large capacities (for storing the status between refresh cycles ) with low leakage currents (power loss) are particularly important for memory production . The capacitance of a simple plate capacitor is, for example ${\ displaystyle C}$

{\ displaystyle C = {\ frac {\ varepsilon _ {0} \ varepsilon _ {\ mathrm {r}} A} {d}}.}

It is the plate separation, the area of the capacitor plates , the permittivity of vacuum and the material constant , the relative permittivity of the insulating layer. ${\ displaystyle d}$ ${\ displaystyle A}$ ${\ displaystyle {\ varepsilon} _ {0}}$ ${\ displaystyle {\ varepsilon} _ {\ mathrm {r}}}$

Accordingly, by using high-k materials (larger ) the thickness of the insulator layer in metal-insulator-semiconductor structures ( often also called MOS due to SiO ₂ ) can be increased while maintaining the same capacitance, which drastically reduces leakage currents through the thicker insulator . For the comparison, the (capacitive) properties of such layers are often summarized in one parameter, the equivalent oxide thickness (EOT, dt. "Equivalent oxide thickness "). ${\ displaystyle {\ varepsilon} _ {\ mathrm {r}}}$

In contrast to this are the low-k dielectrics , which are used as an insulator between the interconnects and reduce the parasitic capacitances that arise due to their low dielectric constant .

materials

Dielectric constant of selected materials ${\ displaystyle \ varepsilon _ {\ mathrm {r}}}$
material	${\ displaystyle \ varepsilon _ {\ mathrm {r}}}$	Band gap in eV	Crystal structure
thermal SiO ₂	3.9	8.9	amorphous
Si ₃ N ₄	7th	5.1	amorphous
Al ₂ O ₃	9	8.7	amorphous
Y ₂ O ₃	15th	5.6	cubic
ZrO ₂	25th	7.8	mono. , tetrag. , cubic
HfO ₂	25th	5.7	mono., tetrag., cubic
La ₂ O ₃	30th	4.3	hexagonal , cubic
Ta ₂ O ₅	26th	4.5	orthorhombic
TiO ₂	80	3.5	tetrag. ( Rutile , anatase )

Different material systems are investigated, such as amorphous oxides of metals (e.g. Al ₂ O ₃ , Ta ₂ O ₅ ), transition metals (e.g. HfO ₂ , ZrO ₂ ) and mixed oxides of hafnium silicate and zirconium silicate . Strontium titanate and barium titanate also provide higher permittivity. Crystalline oxides of rare earths (e.g. Pr ₂ O ₃ , Gd ₂ O ₃ and Y ₂ O ₃ ) represent a further approach , which enable lattice-adapted growth and thus a perfect interface (very low number of lattice defects ) between semiconductor and insulator .

Coating

Both physical (PVD) and chemical vapor deposition (CVD) processes can be used to produce thin layers . For example, atomic layer deposition can be used for very thin layers in the thickness range of a few nanometers . As with all other coating processes, the deposition conditions are initially determined empirically in the process development through larger variations of the system parameters (pressure, gas flows, precursor gases, etc.) on the respective production system and then optimized for production in a statistical test planning . The process parameters determined in this way are system-specific and generally only very rarely transferable to other systems; Due to the very high demands on the coating process or the layer, this often also applies to systems of the same construction. In research, a so-called material screening is added to process development or optimization , in which the deposition of a desired layer is examined with regard to the starting gases used (in CVD processes).

The starting material for all of the above oxides and mixed oxides are known complexes of the respective “metals”, which are relatively easy to produce under strictly anaerobic conditions. An important condition for the use of the complexes in semiconductor production is a sufficient vapor pressure of the compound at a moderate temperature (approx. 300–600 ° C). In most cases, precursors that are liquid at room temperature are preferred. In individual cases - HfCl ₄ for HfO ₂ separation - solids with a sufficiently high sublimation pressure are also used.

The starting materials for oxide deposition are produced by specialized fine chemical companies or by manufacturers of catalysts for organic synthesis or plastics production (e.g. Ziegler-Natta catalysts ). The semiconductor industry obtains the small quantities it needs mostly from the laboratory chemicals trade or from suppliers that work with it. The price level for the connections is high to extremely high.

literature

GD Wilk, RM Wallace, JM Anthony: High-κ gate dielectrics: Current status and materials properties considerations . In: Journal of Applied Physics . tape 89 , no. 10 , 2001, p. 5243-5275 , doi : 10.1063 / 1.1361065 (good review article on high-k dielectrics).
J. Robertson: High dielectric constant gate oxides for metal oxide Si transistors . In: Reports on Progress in Physics . tape 69 , no. 2 , 2006, p. 327-396 , doi : 10.1088 / 0034-4885 / 69/2 / R02 .
Samares Kar et al. (Ed.): Physics and Technology of High-K Gate Dielectrics (several volumes). The Electrochemical Society, 2003-2008.

Web links

www.intel.com : "High-k and Metal Gate Research" - Nanotechnology site from Intel with various high-k articles

Individual evidence

↑ H. Huff, D. Gilmer: High Dielectric Constant Materials. VLSI MOSFET Applications . Springer, Berlin 2004, ISBN 3-540-21081-4 , pp. 263 .

[1] H. Huff, D. Gilmer: High Dielectric Constant Materials. VLSI MOSFET Applications . Springer, Berlin 2004, ISBN 3-540-21081-4 , pp. 263 .