Thermal budget

An important consideration in the sequence of semiconductor devices fabrication is the so-called thermal budget, a measure of both the CVD temperature and the time at that temperature for any given CVD operation. As a rule, the thermal budget becomes lower the farther away a given step is from the original surface of the silicon wafer. This restriction is the result of the temperature limitations of the already deposited materials. 

For instance, doped phosphosilicate glasses used in planarization cannot be heated above their flow temperature of 725°C. Likewise, after a layer of aluminum is deposited, subsequent temperatures cannot exceed 380°C because spiking and the formation of hillocks would occur rapidly (see diffusion barrier in the following chapter). The factor of time at a given temperature is just as important, as it will influence phenomena, such as diffusion and dissolution. In the planning of a CVD process, the sequence of events and the thermal budget are essential considerations. 

CVD is a maj or process in the production of thin films of all three categories of electronic materials semiconductors, conductors, and insulators. In this chapter, the role of CVD in the fabrication of semiconductors is reviewed. The CVD production of insulators, conductors, and diffusion barriers is reviewed in the following chapter. The major semiconductor materials in production or development are silicon, germanium, ni-V and II-VI compounds, silicon carbide, and diamond. 

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Seebauer, C. G., and Ditchfield, R., Fixing Hidden Problem with Thermal Budget, Solid State Technology, pp. 111-120 (Oct. 1997)... 

The bolometer loses a power G (rbol — Ts) to the heat sink at temperature Ts, through the thermal conductance G. Hence, the thermal budget is given by ... 

Although current processing temperatures for A1PO dielectrics remain above the threshold for plastic substrates, they are well within the thermal budget of several other substrates of interest for large-area applications. 

To summarize, thermal silicon oxides are superior to anodic oxides for most applications. However, for special requirements, for instance if a very low thermal budget or a homogeneous oxide thickness on polysilicon layers is required, anodic oxides offer some benefits. 

Excess Point Defects and Low-Thermal-Budget Annealing. Submicrometer VLSI (very-large-scale integration) technologies require low thermal budgets (the product of dopant diffusivity and diffusion time) to limit the diffusional motion of dopants. Two options exist to reduce the thermal... 

It is clear that the development of vapor phase epitaxy reactor at atmospheric pressure is booming. The process leads to good quality material and is compatible with the photovoltaic industry criteria of efficiency and reduced production costs. Indeed, the active layer deposition step represents the main cost of a thin cell process. Any innovation to reduce the thermal budget of this operation is of major interest. 

Radiation and thermal budget of arbitrary oriented surface,... 

The passivation can be achieved in several manners. The most frequently used one is a thermal passivation [33-36], by the simple exposure of the Si surface to an 02 ambience at elevated temperature. In contrast, an electrochemical passivation can be performed at room temperature, which is beneficial when a low thermal budget is desired. Both the thermal and the electrochemical treatment result in a consumption of Si from the surface, that is the Si02 grows into the surface (about 1/3). This can be critical for very large scale integration (VLSI) structures. If consumption of Si is not acceptable, a deposition of Si02 by chemical or physical vapor deposition (PVD/CVD) techniques is necessary. 

Another important use of thermodynamic data in atmospheric applications is the evaluation of the exothermicity of chemical reactions which may play a role in the thermal budget of the atmosphere. Consider, for example, the addition of atomic oxygen to molecular oxygen ... 

The most abundant carbon-containing compound in the stratosphere and mesosphere is carbon dioxide (CO2). By interacting with infrared radiation, this gas plays an important role in the thermal budget of the atmosphere, and the 30% increase in its concentration resulting mainly from fossil fuel burning has provided a significant forcing to the climate system of about 1.5 Wm 2 (IPCC, 2001). Carbon dioxide does not play any substantial role in the chemistry of the atmosphere except in the lower thermosphere, where its photolysis is an important source of carbon monoxide (CO). This latter gas, which is also released at the Earth s surface by incomplete combustion (pollution) and is partially transported to the stratosphere, is converted to CO2 by reaction with the hydroxyl radical (OH). 

Most of the interest in OFETs stems from the low thermal budget required to fabricate these devices and their high degree of mechanical flexibility. These characteristics follow from two basic properties of organic semiconductors ... 

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