Silicium nitride, LPCVD and PECVD deposition. Stress control
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In semiconductor technology, silicon nitride layers are used as dielectrics, passivation layers or mask materials. They are suitable as etch stoppers (e.g. in dual Damascene technology) and as diffusion blockers (e.g. for nitride ions). Additionally, there are several applications in micromechanics, for example as membrane material. In solar technology, Si3N4 is used as antireflection layer. Deposition is either carried out
|LPCVD reactor||triode PECVD reactor|
LPCVD nitride can easily be deposited in a reproducible, very pure and uniform way.
This leads to layers with good electric features, very good coverage of edges, high thermal stability and low etch rates.
However; high temperatures are necessary for deposition and reaction rate is slower.
Deposition is performed in several steps: gas transport to the surface - adsorption - reaction of surface reactants (without involvement of substrate atoms) - desorption of byproducts. Since the reaction at the surface at given temperatures determines the deposition rate(this is called reaction controlled), depletion of reaction material in the gas phase by consumption, and therefore the induction of fresh gas, only plays a subordinate role. Hence it is possible to process many wafers, which are arranged side by side in the gas flow, without great problems. The formation of silicon usually results from dichlorosilane (DCS) and ammonia at 700-850░C.
|3 SiH2Cl2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2|
LPCVD systems are available from the company Koyo Thermo Systems. Several vertical furnaces and horizontal furnaces can be used.
PECVD Nitride allows a faster deposition, which therefore allows thicker layers. Stoichiometry and stress can be adjusted. Edge coverage is good and etch rates are comparatively high. PECVD nitride is very suitable for passivation layers. Usually, silane and ammonia are used as feedstock. Deposition can take place at temperatures below 400░C.
|3 SiH4 + 4 NH3 → Si3N4 + 24 H2|
PECVD deposition systems.
Due to different lattic spacings of substrate and silicon nitride layers, as well as stacking faults in the crystal structure, pin holes or interstitial atoms, tensions in deposited layers can occur. One differentiates between tensile and compressive stress. Additionally, stress caused by temperature changes is possible.
Tension in the nitride layer can be modified by various items:
By thermal silicon deposition primarily stoichiometric silicon nitride with low tensile stress is formed. By increasing the gas flow of dichlorosilane silicon-rich silicon nitride is formed. The process can be adjusted so that this nitride is almost stress free. In combination with the excellent properties of thermal silicon nitride it is well suited for micromechanical applications. However many particles are formed during such a process and it requires a lot of experience to keep failure-free operation. With our vertical furnaces from Koyo Thermo Systems we can offer a process for stress free siliconnitride.
Through plasma deposition of silicon nitride, hydrogene is easily integrated and thinner layers which have tensile stress are formed.
If silane percentage and ion bombardment are increased, a silicon rich silicon nitride is formed. It is denser and underlies lower stress or even compressive stress.
Low deposition temperature stabilizes the nonstoichiometric Si3+xN4-y. Chemical composition can be measured by the refraction index of the layer.
If plasma is used for deposition of nitride, the feed materials are cracked and reactive radicals are created. Therefore; deposition at lower temperature is possible. Depending on the polarization and excitation frequency of the plasma reactor, molecules or radicals with different velocity and energy strike the surface. This way, layer tension between tensile and compressive stress can be shifted. Additionally, deposition of multilayers which alternately underlie tensile and compressive stress is possible.
For better adjustment of the layer tension, a triode configuration of the plasma reactor, also known as double-frequency-PECVD, is used.
A RF-frequency of 13,56 MH is impressed on the upper electrode, while the sample holder acts upon 360 kHz. The reaction chamber itself is grounded.
A high plasma density can be set by the high frequency generator, while through the low frequency generator an acceleration of the ions to the substrate is attained.
A Frequency lower than 1 MHz enables ions to follow the polarization change of the plasma - at 13,56 MHZ only electrons can do that.