NANO MODIFICATION OF THE W(100)/ZrO ELECTRON EMITTER TIP USING REACTIVE ION ETCHING

Transcription

1 NANO MODIFICATION OF THE W(100)/ZrO ELECTRON EMITTER TIP USING REACTIVE ION ETCHING Miroslav HORÁČEK, František MATĚJKA, Vladimír KOLAŘÍK, Milan MATĚJKA, Michal URBÁNEK Ústav přístrojové techniky AV ČR, v. v. i., Královopolská 147, CZ Brno Abstract The W(100)/ZrO electron emitter tip is typically prepared from a tungsten single-crystal shaft of a diameter of 125 µm using electrochemical anodic etching. In order to prepare an emitter for e-beam writer with a shaped beam it is desirable to etch the tip with a radius around 100 nm. Despite the anodic etching is precisely controlled using dedicated software, the desired final form shape of the emitter tip is not achieved in every case. The correcting anodic etching is not possible due to the technology principle of the etching itself. We present in this contribution the procedure that modifies/repairs the tungsten tip shape in a nanoscale region using a reactive ion etching (RIE) in CF 4 + O 2 gaseous mix in a barrel type reactor at the radio frequency of 13,56 MHz and the working pressure of 1000 Pa. The change of the geometry after the RIE process is checked using a high resolution scanning electron microscope. The influence of the tip modification of the activated thermal-field W(100)/ZrO electron emitter on its emission characteristics is also presented. Key words: Electron emitter, reactive ion etching, electron emission, thermal field, W(100)/ZrO. 1. INTRODUCTION The ZrO/W electron emitter, a <100>-surface tungsten covered with ZrO X, has become increasingly popular as a source of electrons for high-performance electron-beam instruments. Today, the high-temperature field emission source is the most commonly used electron source for a variety of electron beam instruments requiring a high-brightness cathode like scanning and transmission electron microscopes and electron beam lithography (pattern generators). The emitter is often referred to as a Schottky emitter, due to its use of the Schottky effect for electron emission (alternatively this type of electron sources is also referred to as a field emission cathode) [1]. Schottky emission regime is located between the thermal emission regime and the cold field emission regime. The thermal emission is increased in Schottky regime due to decreasing of potential barrier under the influence of a high electric field. The typical operation conditions are the tip temperature 1800 K, electric field V/nm and vacuum < 10-6 Pa. The activated tungsten single-crystal etched tip is typically used for Schottky electron emitter. The top of the tip is made of one of the chemically active facets, nowadays commonly (100). The aim of the activation is to increase the total emission current in comparison with the low temperature field emitter. The activation means to cover the cathode tip with atoms of a suitable element to reduce the work function. A typical activation process uses Zr and O influence under high temperature ( K). Such a tip is referred to as W(100) ZrO or ZrO/W emitter. The activated cathode have to be operated under increased temperature to prevent unwanted adsorption of other elements or molecules which in reverse can increase the work function. The tip radius is purposely increased after the activation up to 1 µm. Hence, the common Schottky cathode has tip radius from 0.3 to 1.0 µm. As a consequence, the electric field intensity on the tip is lower, the potential barrier is wider thus the electron tunnelling is lower. Ergo, the electron energy spread is nearly as low as for the field emission regime. Next, the higher radius results in the - 1 -

2 Fig. 1. SEM pictures of the emitter tip of new cathode: before (left) and after (right) the RIE process. larger emission surface; more atoms of the surface emit the electrons so the noise is lower in comparison with the field emission regime. The spherical surface of the tip made after the etching is changing during activation process and next during the stabilization of the cathode emission. The low-index facets are made on the tip with (100) facets on the top. The angular current density of the Schottky emitter mounted in the typical electron gun is approximately ma/sr. 2. MODIFICATION OF THE ELECTRON EMITTER 2.1 E-beam writing system A Schottky electron emitter with a layer of ZrO x is used in our e-beam writing system working with a rectangular-shaped electron beam of variable dimension [2] [3]. The size of the shaped beam can be from 33 to 6300 nm independently in both directions [4]. The variable shaped beam brings the high exposure throughput that allows the production of large area structures (e.g. optical diffractive elements) [5]. The higher exposure throughput is the main advantage over the Gaussian electron beam writer systems. The electron-optical column of the system is based on the point projection of a spot electron source. The low radius of the tip less than 300 nm after cathode activation is necessary for the proper function of the electron-optical system based on the principle of point projection. 2.2 RIE modification of the electron emitter The W(100)/ZrO electron emitter tip is typically prepared from a tungsten single-crystal shaft of a diameter of 125 µm using electrochemical anodic etching. In order to prepare an emitter for e-beam writer with a shaped beam it is desirable to etch the tip with a radius around 100 nm. Despite the anodic etching is precisely controlled using dedicated software, the desired final form shape of the emitter tip is not achieved in every case. The correcting anodic etching is not possible due to the technology principle of the etching itself. At the same time it is evident the surface treatment and the shape of the tip determine the emission characteristics significantly [6] [7] [8]. We used the RIE (reactive ion etching) process for the tip radius modification, for regeneration of the active surface of the tip and repair of the incorrectly made tips. Next, we studied the influence of both the surface treatment and the change of the shape of the tip on the emission characteristics. A radio frequency (13,56 MHz) RIE in barrel type reactor at Pa and 200W with gaseous mix of CF4 and 16vol% O 2 were applied. The rate of the etching was only a few nm/minute at the starting room temperature. The SEM images of the new cathode just after anodic etching before and after RIE taken in JEOL JSM 6700F and demonstrating the radius repairing are shown in Fig

3 Fig. 2. SEM pictures of the emitter tip of aged cathode: before (top left) and after (top right) the RIE process and overlapped image of both. The SEM images of the aged cathode after long-term period of operation before and after RIE are shown in Fig. 2. The Zr reservoir was deposited on the emitter shaft before RIE. 2.3 Modification of the electron emitter during oxygen processing Further, we studied the behaviour of the emitter tip during a next technological step, the oxygen processing. First, the emitter temperature was elevated from 300 K to ~1730 K. Next the emitter was exposed to oxygen at a partial pressure of Pa for a 210 minutes. The SEM images of the cathode before and after oxygen processing taken in JEOL JSM 6700F are shown in Fig. 3 and Fig. 4. By comparision of detailed SEM images we can see the significant shortening of the tip of ~ 7 µm. We suppose the mechanism of shortening is the production of the WO 2 and WO 3 and subsequent physical procedures. WO 2 sublimates already at 1073 K (at pressure 100 kpa), WO 3 has melting point 1746 K and boiling point 1973 K. 2.4 Evaluation of the RIE process in the experimental apparatus The influence of the RIE process on the emission characteristic was studied in an apparatus which allows cathode activation, debugging, testing and observation (Fig. 5). The original electron gun of our e-beam writing system with emitter was attached in the apparatus

4 Fig. 3. SEM pictures of the emitter tip: before (top left), after (top right) the oxygen processing and overlapped image of both. Fig. 4. Detailed SEM pictures of the emitter tip: before (top left) and after (top right) the oxygen processing and overlapped image of both

5 The standard cathode-extractor distance is 3 mm and the extractor voltage is also the working voltage of the e-beam writing system column. The apparatus consists of emitter (cathode), suppressor, extractor (anode), anode aperture and metallized scintillator. The measurement of a beam current from electrons bombarding scintillator is possible using external picoammeter. The apparatus is equipped with two inspection glasses for visual process monitoring. The first one is positioned on the optical axis of the apparatus behind the scintillator and it allows the observation of the cathode emission pattern. The second one is positioned radially and it allows the observation of the space between the suppressor and the anode. A camera can be attached to both glass windows. Fig. 5. Apparatus for cathode debugging and testing. C-cathode, W-suppressor, A-extractor (anode), AP-aperture, S-scintillator, HV-high voltage power supply, P-picoammeter, SG-inspection glass, F-flange, V-gas dosing valve, CM-camera, O-ocular. 3. RESULTS In order to evaluate the influence of the RIE treatment on the emitter tip, the voltage/current characteristics were measured and simultaneously the emission pattern images projected on the screen were recorded by a camera. The following conditions were kept for all measurements: cathode operating temperature ~ 1650 K, vacuum Pa, the cathode-extractor distance 3 mm, and suppressor bias 100 V. The comparison of the voltage/current characteristics of the tip is in Fig. 6. The comparison of the emission pattern images gives Fig. 7. We can see low brightness electron emission of the original cathode even after new oxygen processing (Fig. 7 left). After RIE the emission increased significantly, but not in the central part of the emission pattern where it is the most important for shaped e-beam writing system (Fig. 7 center). Only after RIE followed by oxygen processing the high brightness electron emission from the top of the tip started (Fig. 7 right). ACKNOWLEDGEMENT This work was partially supported by European Commission and Ministry of Education,Youth and Sports of the Czech Republic (project No. CZ.1.05/2.1.00/ ALISI), by institutional support RVO: and by TA CR project No. TE

6 scintillator current (na) , Brno, Czech Republic, EU cathode - extractor voltage (kv) before RIE after RIE after activation Fig. 6. Scintillator current versus cathode-extractor voltage for the emitter: before RIE, after RIE and after both the RIE process and the oxygen activation. Fig. 7. The scintillator images of the emission pattern from the emitter: before RIE (left), after RIE (center) and after RIE and oxygen activation (right). Cathode-extractor voltage 8 kv, angular current density 4,5 A/sr (left) and 51 A/sr (right). REFERENCES [1] SWANSON L. W., SCHWIND G. A.: Rewiew of ZrO/W Schottky cathode. Handbook of Charge Particle Optics, Second Edition, Edited by Jon Orloff, CRC Press (2008). [2] DELONG A., KOLAŘÍK V.: Field-emission gun for microengineering application. J. Phys. E: Sci. Instrum., 22 (1989) 612. [3] DELONG A. et al.: Electron beam lithography system with a field emission source and variable spot shaping. 10th ICEM, Hamburg (1982) Proc [4] MATĚJKA F. et al.: Reducing the size of a rectangular-shaped electron beam in E-beam writing system. 8 th MCM 2007, Prague (2007). Proc [5] HORÁČEK M. et. al.: Thin metallic layers structured by e-beam lithography. 21st Metal 2012, Brno, Czech Republic, May 23-25, Proceedings. [6] MATĚJKA F. et al.: Modification of the Schottky FE ZrO/W electron emitter. 17th IFSM IMC 17, Rio de Janeiro (2010) Proceedings. [7] MATĚJKA F. et al.: Thermal-field electron emission W(100)/ZrO cathode: facets versus edges. 13th Seminar in Recent Trends in CPOptics and SPI, Brno, Czech Republic, June 25-29, Proceedings, pp [8] BOK J., KOLAŘÍK V.: Measurements of the current density distribution in e-beam writer. 13th Seminar in Recent Trends in CPOptics and SPI, Brno, Czech Republic, June 25-29, Proceedings, pp