應用於發光二極體抗靜電防護之薄膜變阻器研究

Abstract

近年來，氮化鎵基LED在發光效率上有很大的進展，不過在靜電防護上卻還有很大的改善空間。一般LED靜電防護的作法有兩種：第一種是將LED與兩個背對背的齊納二極體並聯；第二種是利用LED只在逆偏時有靜電防護的問題，將LED與一個二極體反向並聯來達到靜電防護的功效。第一種方法的缺點是必須還要另外製作齊納二極體、第二種方法的缺點是額外製作的二極體會佔據LED晶圓的發光面積，使得成本提高。因為薄膜製程比起傳統變阻器用陶瓷製程相對簡單許多，而且鍍膜製程容易整合至半導體製程中，所以本論文基於此原因，試著研究薄膜式變阻器。 在研究薄膜變阻器的過程中，發現僅鍍上氧化鋅薄膜於藍寶石基板上似乎沒有發現電阻隨電壓呈現非線性變化的傾向。本論文研究了(1)氧化鋅薄膜沉積在藍寶石基板、(2) 矽薄膜沉積在藍寶石基板、(3)Si/ZnO沉積在藍寶石基板等三種結構。研究中發現由Si/ZnO的薄膜搭配組合，再經過適當地熱退火處理，可以呈現出類似於變阻器的效果。若比較經熱退火1100℃秒之後的樣品的電流-電壓特性曲線與傳統300μm × 300μm的LED晶片於20mA的電流下的電性。發現其在V<0時，樣品的電流約在-5V時導通；而在V>0時，樣品的電流約在5V時導通；與LED相比，此樣品在大電壓時呈現小電阻，而在小電壓時呈現大電阻。若將此樣品與LED電路並聯，似乎有機會能夠提供另一條電流路徑，提供了LED的靜電防護機制可能性。在做完透光率量測後，發現此薄膜變阻器在經過熱退火後，透光率在紅光波段約增加了3倍。若將此薄膜變阻器並聯製作在LED上，看起來不僅有製程簡單的便利性，而且由電流-電壓特性的比較中發現，在靜電放電防護上似乎會有不錯的效果。

In recent years, there is a large improvement on the luminous efficiency of LEDs. However, there are some problems in LED ESD protection. Generally, there are two technologies to do the LED ESD protection. One is parallel LED with the head-to-head Zener diodes, the other is parallel LED with a reverse connected diode which is due to the property that LEDs only have troubles in the reverse-bias operation. The disadvantage of the first technology is needed extra works to fabricate the Zener diodes. The disadvantage of the second technology is that the extra fabricated diodes will occupy some emitting areas on the LED wafer. So we want to investigate thin film varistor, since thin film process is simpler than traditional ceramic process, and it is easy to integrate into semiconductor process. In the experiment, we sputter Si/ZnO on sapphire substrate, and do the I-V measurement, optical transmittance measurement and post-annealing process. It is found that the conductance and the optical transmittance of thin film become higher after doing the post-annealing process. It is note that the Si/ZnO thin film demonstrate the “varistor-like” characteristic after the post annealing process at 1100℃ for 30 secs in air condition. The optical transmittance of thin film will increase about three times after the post annealing process. Comparing the I-V characteristic of the sample on which the post annealing is carried out with that of regular LED chip at 20mA, it is found that the sample will turn on at ±5V. Relatively, the samples have low resistance at high voltage, and they have high resistance at low voltage. It seems to have “varistor-like” behavior and it provides another current path for electro-static discharge. It is a possible method to do the LED ESD protection.