A study into the formation and characterisation of electroless nickel silicide for laser doped solar cells
In this paper we will study the formation and characteristics of nickel silicide compounds and the subsequent specific contact resistivity (Ω.cm2) during the formation of an ohmic contact in solar cells. Using Transmission Line Method (TLM) we will analyse the electrical characteristics of the formed nickel silicides at different sintering temperatures and durations. In each sintering condition tested, the relationship between the total resistance and the distance between the contact pads on the fabricated test structures is measured and graphed. From these graphs the contact resistance (Ω) and specific contact resistivity (Ω.cm2) of the metallisation stack is calculated and tabulated. We conclude the paper by showing the optimum sintering condition of 350°C for three minutes for laser doped solar cells which yields negligible contact resistance.
Development of a Full Silicidation (FUSI) Process for Nickel Silicide
Suicides have been long used to solve the problem of poly depletion effects in the CMOS circuits. The depletion layer in the poly-gates increases the total effective gate-dielectric thickness causing a poor device performance. Many advantages are promised in replacing the Polysilicon gates with metal gates, which include improved sheet resistance of the gates, decreased equivalent electrical thickness of gate dielectric by eliminating the Polysilicon gate depletion effect, and dual work functions, higher than n+ poly work function for NMOS and lower than p+ poly for PMOS in a single full Silicidation (FUSI) of Polysilicon gates. In the ever reducing gate dimension and junction depths, nickel Suicide has become one of the promising candidates for silicides applications in submicron CMOS devices. Nickel silicides has many advantages over other metal silicides due to its one step low temperature formation, low resistivity (l4~2O /.t≤2cm), low Si consumption, and its nature of not suffering from resistivity degradation on narrow lines or gates. However, nickel suicides have some problems associated with them such as the large junction leakage current and sheet resistance degradation due to oxygen contamination and rough interface between NiSi and Si. Nickel Silicide was formed on doped Polysilicon to simulate the effects on gate CMOS regions. Sheet resistance will be measured on silicided Polysilicon and results will be introduced in this report as well.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.