燈源加熱之垂直單晶片處理爐內混合對流流場觀測及晶圓等溫性研究

Abstract

本論文研究分為兩個部分，第一部份主要研究垂直8吋單晶片快速升溫反應爐爐內混合對流流場結構，而第二部分我們也提供一個可在晶圓背面通入冷卻氣體之支撐座，以改善在快速升溫爐降溫過程中晶圓之均溫性並且提高晶片降溫速率。 第一部份研究中，我們採用兩種不同結構之多孔板，一個為2410孔直徑2mm之等孔多孔板，另一個為中央直徑1mm外圍直徑3mm的不等孔多孔板；而研究的焦點放在晶圓表面溫度、爐體內壓力、進氣流量對於爐體渦流流場結構的影響；本實驗中我們發現，在低浮慣比(buoyancy-to-inertia ratio)時，爐內流場結構主要為衝擊塞柱流(plug flow)，流體直接衝擊到晶圓表面而且整個結構並沒有渦流之產生。對於非等孔板而言，在低浮慣比時流體易從較大直徑的孔流出，因此在爐體中央會形成一個很大的對稱渦流(mixed flow 1)。而當我們提高晶圓溫度時，在爐體兩側會有渦流產生(mixed flow 2 或mixed flow)，並且隨著晶圓溫度漸漸增高(浮慣比增高)，此渦流會漸漸變大，最後佔據整個爐體，流場結構完全由浮力驅動流(buoyancy-induced flow)所支配。 在第二部分的研究中，我們設計一個可在背面通入冷卻流體的晶圓支撐座，藉由增加入口流量與背面冷卻空氣之流量以增快晶圓之降溫速率；研究中也發現，適當的入口及背面冷卻空氣流量可明顯的提高晶圓的均溫性；但太高的流量卻會破壞其均溫性。因此適當的流量不但可以提高晶片的均溫性，也可以直接提高晶圓的降溫速率，以減低製程所需要的時間。

A two-part experimental study is conducted here to unravel the vortex flow patterns and to improve the wafer temperature uniformity in a single wafer vertical rapid thermal processor (RTP) for IC processing. Flow visualization is carried out in the first part to investigate the vortex flow characteristics in an experimental lamp heated, vertical 8-inch single wafer rapid thermal processor. While in the second part of the study the possible improvement of the wafer temperature uniformity during the ramp-down period by placing an air cooled copper plate right below the silicon wafer is explored. In the flow visualization experiment two different showerheads are tested. One is the uniformly perforated showerhead with 2410 holes of 2 mm in diameter and the other is the nonuniformly perforated showerhead with the hole diameter varying from 1 to 3 mm. A thick copper plate is used to replace the actual silicon wafer, intending to have a nearly isothermal boundary condition on the wafer without too complicate power control of the heating lamp unit. The physical parameters in the visualization experiment include the temperature difference between the wafer and inlet air, inlet air flow rate, and pressure of the processing chamber. The flow photos taken from the processor installed with the uniformly perforated showerhead indicate that at a lower buoyancy-to-inertia ratio, the plug flow dominates in the processing chamber and no vortex roll appears above the wafer. While at a high buoyancy-to-inertia ratio the chamber is occupied by a big buoyancy induced circular vortex roll. We noted a mixed vortex flow consisting of both plug flow and a buoyancy induced vortex roll at an intermediate buoyancy-to-inertia ratio. Reducing the chamber pressure is found to be rather effective in eliminating the buoyancy induced vortex roll. When the processor is installed with the nonuniformly perforated showerhead, we can have two circular vortex rolls above the wafer at a certain buoyancy-to-inertia ratio. Moreover, the plug flow only exists for an unheated wafer. Hence the nonuniform showerhead does not improve the flow distribution over the wafer. In fact, the resulting flow distribution is poorer. We also provide empirical correlations to delineate the ranges of parameters for the appearance of various vortex flow patterns. In the second part of the experiment, the air cooling of the copper plate placed right below the wafer is shown to significantly improve the wafer temperature uniformity in the ramp-down period for low and intermediate backside cooling air flow rates. Raising the input air flow rate to the processor also effectively improves the wafer temperature uniformity for the air flow rate up to some intermediate level. Besides, increasing the gas flow rate directly input to the processor or the backside air flow rate for the copper plate cooling also substantially speeds up the ramp-down rate of the wafer temperature.