單原子針氣體場離子源特性與其針形對離子發射效應模擬

Abstract

聚焦離子束系統廣泛的應用於奈米科學與先進半導體製程領域中，常見的離子源有三大類：感應耦合電漿離子源（ICP, inductively coupled plasma ion sources）、液態金屬離子源（LMIS’s, liquid metal ion sources）與氣體場離子源（GFIS’s, gas field ion sources）。然而，電漿態離子源之聚焦光斑尺寸為次微米級故無法提供高精準度加工。液態金屬離子源則會在操作時對樣品造成金屬殘留污染。而以氣體場離子源為光源的聚焦離子束系統聚焦點小於奈米尺度，亦不對樣品造成金屬污染。氣體場離子源發展於20世紀中，具備極小的能量分散（energy spread）。單原子針之針尖末端僅有一顆原子，由於單原子造成局部電場增益，使得整個針尖上僅有此單原子能發射氣體場離子。因此離子光源亮度極高，適合作為聚焦離子束系統相關應用。熱穩定態覆銥鎢單原子針具有優良抗化學腐蝕特性，已驗證適合發射多種氣體場離子（氦、氖、氬、氫與氧氣）。在本研究中使用覆銥鎢（111）單原子針發射高亮度與高穩定性氙氣體場離子源，量測由溫度150 K至309 K的場離子電流對電壓作圖，經由計算在氙氣分壓1 × 10-4 torr時，簡化亮度（reduced brightness）遠高於電漿態離子源與液態金屬離子源。為了提高氣體場離子源之離子電流，根據氣體場離子電流理論進行了模擬，了解到提升氣體場離子源的條件：大針尖曲率半徑、小針柄錐角以及對成像氣體預先降溫。而模擬也顯示當針尖軸向上有一小凸起結構時，該凸起結構越大則有效捕獲面積越小，此結果暗示了單原子針為能兼具大電流與高亮度的場離子電流發射極。分析實驗上不同發射極發射離子電流比例與理論計算預測比例數值相仿。最終實驗上得到當針座溫度29 K與氦氣分壓3.7 × 10-3 torr時，能發射氣體場離子電流2,000 pA的發射極。

Focused ion beam (FIB) systems are widely used in nanoscience and advanced semiconductor manufacturing. Ion sources applied in conventional FIB systems can be divided into three main categories: inductively coupled plasma (ICP) ion sources, liquid metal ion sources (LMIS’s), and gas field ion sources (GFIS’s). Among these sources, an ICP ion source has a submicrometer spot size; therefore, a system with such a source cannot be used in high-precision milling. Moreover, an LMIS engenders metal contamination in the target sample during operation. By contrast, a GFIS has a subnanometer spot size and does not engender postoperative metal contamination. The first GFIS was developed in the mid-20th century and has a very small energy spread. The apex of a single-atom tip (SAT) has only one atom. Gas ions are field emitted from this atom because of the local field enhancement caused by the atom. Therefore, the brightness of an SAT-GFIS is extremely high, rendering it an appropriate ion source for FIB system applications. An Ir/W(111) SAT is thermally stable and thus chemically inert, which is advantageous for GFIS emission. The characteristics of several gas species (He, Ne, Ar, H2, and O2) emitted from Ir/W(111) SAT’s have been reported. This study presents a system with an Ir/W(111) SAT that can emit Xe+ ion beams with high brightness and high current stability. The Xe+ ion emission current was analyzed against the extraction voltage at temperatures ranging from 150 to 309 K in order to determine the optimal emitter temperature for maximum Xe+ ion emission. The optimal emitter temperature for maximum Xe+ ion emission was approximately 150 K; moreover, at a Xe gas pressure of 1.0 × 10−4 Torr, the reduced brightness observed for the presented system was considerably larger than that observed for an ICP ion source or LMIS. The simulations were performed based on the theory of gas field ion current. Several conditions were identified to boost the GFIS current: a larger radius of curvature, a smaller shank cone angle, and precooling image gas. Furthermore, if the emitter apex has a small protrusion, increasing the protrusion diameter can reduce the effective captured area. This implies that developments in SAT’s may enable both high-ion-current and high-brightness ion sources. The simulations could enable qualitative analyses of experimental data. The experimentally derived field ion current ratios for different emitter shapes were similar to the ratios calculated using the simulation. Finally, the experimental results revealed that the gas field ion current emitted from the Ir/W(111) tip at a tip holder temperature of 29 K and a He gas pressure of 3.7 × 10−3 Torr was 2,000 pA.