Presentation on theme: "In-vacuum radiative heater for 3” silicon wafers John Gregoire."— Presentation transcript:
In-vacuum radiative heater for 3” silicon wafers John Gregoire
Desired Properties Oxygen partial pressure compatibility 100C-1000C temperatures on substrate surface Quick heat-up, cool-down time (~10min) –Low thermal mass, modest thermal contact between substrate and heater body Compatibility with current U.S Inc. Heaters –This is a geometrical consideration with the distance from the substrate surface to the opposite end of the heater being the most important dimmension Use halogen bulbs from Home Depot –(these are simply tungsten filaments in a vacuum-tight seal filled with inert gas and a halogen to help redeposit evaporated tungsten on filament, thus extending bulb lifetime)
Silicon vs. Tantalum on silicon Silicon has a band gap of 1.14eV Tungsten wire is 0.007” diameter and thus a current of –0.5A => Tungsten is at 1000K –7A => Tungsten is at 3000K Assuming heater radiates as blackbody at these temperatures, the below plots show the amount of silicon absorption will vary with current (voltage applied to bulbs) and will always be much lower than desired. As a result, all temperature tests are done on substrates with >1000A of Ta sputtered on. These are blackbody radiation plots at 1000K,2000K, and 3000K. All three plots have different scaling for ease in comparison with Si band gap (vertical line).
Notes on development Single bulb systems considered –A single bulb heater based on a Lesker product was tested and uniformity was found to be poor.
Single-bulb heater test 60V,3A,180W test Variations in excess of 120 deg
Thermodynamics The 3 phenomena to consider are –photons emitted from the filament colliding with the substrate directly (strongly dependant on filament position) –photons emitted from the heater body colliding with the substrate (fairly constant background) –thermal conductivity of the substrate (suppresses thermal gradients).
Modeling Considering a filament to be a line of point radiation sources, geometry and integration can be used to calculate the intensity of radiation experienced at every point on a substrate Modeling of the thermodynamics of direct-incidence photons was performed to investigate improvements on the single bulb heater. The different light bulbs considered in this investigations were PHILIPS 250W, 300W and 500W quartz halogen bulbs. The only quantities specific to the different types of bulbs in this treatment are the length of the filament and the linear (radiated) power density. –The radiated power densities of the bulbs were measured with an applied voltage of 30V
Modeling (cont) More than a dozen different 1-,2-,3-, and 4-bulb configurations were considered With this treatment, contour plots of the radiation intensity provide numerical information on the heating of the substrate due to photons that collide with the substrate directly. The contour plots also allow semi-quantitative analysis of the temperature gradients that would exist with no thermal conductivity in the substrate. Thus, one is able to make educated guesses concerning the extent to which temperature variations resulting from this model will be suppressed by thermal conductivity in the substrate. The final thermodynamics to keep in mind is that the heater design will include a reflector box that will add an approximate uniform radiation intensity to the entire substrate
Model results This model indicated that with a single bulb, the center of the substrate received twice as much radiation intensity as the edge of the substrate. The optimal bulb configurations were 4-bulb ensembles with complicated geometry that would not make for practical manufacturing The only configuration with acceptable contour plot results and possibility for ease in manufacturing are ensembles of 3 parallel 500W bulbs –There are some nice theoretical configurations that involve perpendicular bulbs, but they cannot be realized due to finite bulb thickness.
Parameters of 3-bulb assemblies 3 identical bulbs are parallel with the center bulb at a height a above the substrate, shifted away from center along the axis of the bulb a distance b. The two other bulbs are symmetric about the center bulb a distance c above the substrate, separated by 2d, and shifted away from center a distance f in the opposite direction from the center bulb shift
Design parameters and model results scenarioalphabetachideltaepsilon acenter bulb-substrate separation0.951.2 11.1 bcenter bulb on-axis shift0.522.214.171.124 couter bulb-substrate separation0.750.8 douter bulb-outer bulb separation *0.51.211 1 fouter bulb on-axis shift (direction opposite of b)0.5126.96.36.199 max intensity (energy/s/area) on substrate (arb units)120118 112123 max percent variation in intensity363135 32 ALL DISTANCES IN INCHES The contours on the following slide are of intensity of photons incident directly from filaments onto 3” substrates. The filament positions are depicted by straight lines. It is worth noting that the filament from a 500W lamp is considered to be only the coiled portion of the wire and is thus significantly shorter than the entire lamp assembly.