Low-k dielectric

In semiconductor technology, a low-k dielectric is a material that has a lower dielectric constant than SiO ₂ ; H. ε _r <3.9. Nowadays, so-called ultra-low-k materials with a dielectric constant of less than 2.4 are aimed for.

The term “Low-k” is borrowed from English, where the dielectric constant (relative permittivity ) is often referred to as ( kappa ), sometimes just k . In contrast to this are the high-k dielectrics , which are used as gate insulators and, thanks to their high dielectric constant, allow a thicker insulation layer and thus contribute to reducing leakage currents (see tunnel effect ). ${\ displaystyle {\ varepsilon} _ {\ mathrm {r}}}$ ${\ displaystyle \ kappa}$

background

In order to improve the properties of integrated circuits, for example to reduce the power consumption of the large-scale integrated circuits or to achieve higher switching speeds, the structures are made smaller. Due to the advancing miniaturization of microelectronic components, the semiconductor industry is increasingly reaching its physical limits. One effect of miniaturization is the reduction in the spacing between the metallization levels (conductor track layers for wiring the components) on the chip. This reduction in the insulator thickness between two interconnects increases the influence of the parasitic capacitances . They disrupt the function of the circuit and reduce, for example, the maximum switching speed.

Parasitic capacitances arise, for example, when two conductor tracks cross on different levels or when two conductor tracks run parallel to each other. The crossing area or the adjacent interconnects resemble a simple plate capacitor . The capacity of a plate capacitor is calculated according to: ${\ displaystyle C}$

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

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

It can be seen that the reduction of the distance d , the capacitance C increases. In order to compensate for this, it is necessary to reduce the plate area A (results from the conductor track width; on conductor track or parallel conductor tracks) or the dielectric constant . The conductor track cross-section and thus the plate area of the parasitic capacitances are hardly reduced. The current density in the conductor tracks must not increase and smaller cross-sections increase the electrical resistance due to the greater influence of the interface scattering of the electrons. All that remains is therefore to develop new insulating layers with a lower dielectric constant, the low-k dielectrics. ${\ displaystyle {\ varepsilon} _ {r}}$

In principle there are two ways to reduce the dielectric constant:

Lowering the polarizability (dipole strength) by using substances with less polar bonds such as
- Carbon-carbon (C – C)
- Carbon-hydrogen (C – H)
- Silicon fluorine (Si – F)
- Silicon-carbon (Si-C), e.g. B. Applied Materials " Black Diamond I "
Reduction of the material density (dipole density) by creating free volume or the formation of locally limited pores (microporous layers)

materials

Low-k materials currently used in the semiconductor industry include microporous SiO and SiOC layers deposited by CVD or spin-on processes (so-called spin-on dielectrics , SOD). The base materials used are organosilicon compounds ( silicones ), which are also widely used in the building materials and coating sectors. Typical low-k precursors are tetraethyl orthosilicate (TEOS) - an industrially produced organosilicon compound that melts at −77 ° C and boils at 168.5 ° C - and the methyl-substituted silanes , tetramethylsilane and trimethylsilane .

Silane and functionalized silanes are manufactured in particular by the German companies Evonik Industries (in the Chemicals Business Area , formerly Degussa) and Wacker-Chemie, and by the American Dow Corning Inc. on an industrial scale. The American company Silar , for example, produces specialty silanes with challenging organic substituents . In addition, silanes can be obtained from laboratory and chemical wholesalers.

Microporous low-k layers - the model here are the silicon aerogels known since the 1930s - can be produced, for example, by adding oxidizing agents and emulsifiers to the low-k precursor.

Other low-k materials are, for example, plastics , but they do not always have the mechanical strength required for use in semiconductor technology.

Intensive research and development is currently underway in the entire area of low-k materials. This quickly expands the spectrum of the low-k materials discussed. However, as a thin layer, the materials must also meet the current industrial requirements with regard to leakage current density (<10 ⁻⁹ A / cm) and breakdown field strength (EBD> 3 MV / cm).

Candidates for low-k dielectrics with relative permittivities
Material class	material	ε _r	Deposition technology
inorganic	amorphous silicon dioxide	3.9 ... 4.5	CVD
	fluorinated silicate glass (FSG, SiOF)	3.3 ... 4.0	CVD
	Hydrogen silsesquioxane (English hydrogen silesquioxane , HSQ or HSSQ)	2.9 ... 3.2	SOD
	amorphous carbon (English diamond-like carbon , DLC)	2.7 ... 3.4	CVD
	Carbon-doped silicon oxide (engl. Carbon-doped oxide , CDO)	2.8 ... 3.2	CVD
Hybrids (inorganic / organic)	Si-OC polymers (e.g. MSQ )	2.0	SOD
organic	Polyimides	3.1 ... 3.4	SOD
	Parylene-N	2.7	CVD
	Benzocyclobutenes (BCB)	2.6 ... 2.7	SOD
	fluorinated polyimides	2.5 ... 2.9	SOD
	aromatic polyethers ( poly (arylene ether) , PAE)	2.7 ... 2.9	SOD
	Polyaryls	2.6 ... 2.7	SOD
	Parylene-F4	2.4 ... 2.5	CVD
	Fluoropolymers (e.g. PTFE )	1.9 ... 2.1	SOD / CVD
porous
	porous organic materials	2.1 ... 2.2	SOD
	porous CDO	2.0 ... 2.5	CVD
	silicate xerogels ( silica gel )	2.0 ... 2.5	SOD
	silicate aerogels	1.8	SOD
	mesoporous organosilicates	1.8 ... 2.2	SOD
	porous HSSQ / MSSQ	1.5 ... 2.2	SOD
	mesoporous silicate glasses (SiO ₂ )	1.3 ... 2.6	SOD

history

Low-k dielectrics were first used around 2002 with the introduction of the 130 nm technology node in industrial production. B. AMD Athlon 64 and Opteron. Since then, low-k and ultra-low-k materials have increasingly become the standard dielectric for the first metallization levels, at least for products in 65 nm technology and below.

Individual evidence

↑ Mikhail Baklanov, Martin Green, Karen Maex: Dielectric films for advanced microelectronics . John Wiley & Sons, Chichester 2007, ISBN 978-0-470-01360-1 , pp. 35 .
↑ Christof Windeck: Low-k dielectrics are widely used in chip production. In: Heise-Online. February 5, 2004, accessed April 10, 2009 .

[1] Mikhail Baklanov, Martin Green, Karen Maex: Dielectric films for advanced microelectronics . John Wiley & Sons, Chichester 2007, ISBN 978-0-470-01360-1 , pp. 35 .

[2] Christof Windeck: Low-k dielectrics are widely used in chip production. In: Heise-Online. February 5, 2004, accessed April 10, 2009 .