Déposition des couches anti-réflexion de nitrure de silicium Si3N4 sur des cellules solaires photovoltaïque; dépôt par procédé PECVD.
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Cellules solaires photovoltaïque au silicon cristalline c-Si.

Most solar cells which are used today are based on crystalline silicon. The silicon can be mono-crystalline or poly-crystalline. Monocrystalline material is produced by the Czrochalski-process, while polycrystalline material is usually prepared by molding. In both cases the generated material is cut to wafers by wire saws. The wafers serve then as substrate material for the solar cell.

The solar cells consists mainly of silicon and is called therefore thick film solar cell, in contrary to thin film solar cells where the semiconductor layers are deposited on substrate of a different material. The bulk silicon is usually lightly p-doped, and conductive for positive charge carriers or holes. On the front side a thin heavily n-doped layer has to be formed by doping, which is conductive for negative charge carriers or electrodes. This way a p/n-junction is formed, which can separate the charge carrier pairs, generated by the absorption of sunlight. On the front side and on the back side metallic contacts have to be formed in order to drain the solar current. At the backside a holohedral aluminium layer is deposited, while at the front side silver contact fingers are generated, which allow most of the sunlight to pass into the cell. Finally a silicon nitride antireflection coating ARC is attached to the front side in order to increase absorption of the sunlight. The last production step is the assembly of the solar cells to solar modules.

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Déposition des couches anti-réflexion de nitrure de silicium Si3N4 par procédé PECVD.

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

• at high temperatures in a LPCVD reactor or
• plasma-enhanced in a PECVD system.

LPCVD reactor	triode PECVD reactor

LPCVD Nitride

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.

LPCVD systems are available from the company Koyo Thermo Systems. Several vertical furnaces and horizontal furnaces can be used.

PECVD Nitride

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.

PECVD deposition systems are manufactured by Trion Technology

Tension, stress

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:

• ratio of (DCS or) silane and NH3.
• temperature
• plasma parameter

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 Si_3+xN_4-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.
The triode configuration is available for Trion PECVD models Titan (production system with automatic loading cassette to cassette), Orion (stand-alone system for research and development), and Oracle (Cluster system).

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