OBRAS SUSPENDIDAS
MONTO DE TRÁMITES PAGADOS GRAN TOTAL DE INGRESOS
The fabrication of structures such as shown in Fig. 4.1 involves the following processes: vacuum deposition of thin films, grating pattern definition by interference lithography (IL), grating pattern metallization via dry etching (reactive ion etching (RIE)) and lift-off. The schematic of the main steps involved are shown in Fig. 4.2; the details for each step are as follows.
Figure 4.2: Schematic of the main steps in the sample fabrication process.
The samples are prepared on BK7 glass substrates (microscope cover slips 20 mm × 20 mm × 0.20 mm). After solvent cleaning (acetone, methanol, and IPA), a layer of Ag, followed by a layer of SiO2, are deposited on the
substrates by means of radio frequency (RF) sputtering using an Edwards Auto-500 magnetron sputtering system in Ar+gas environment. The process
parameters are shown in Table 4.1.
Table 4.1: Process parameters for RF sputtering of Ag and SiO2
Target Power Rate Temperature Process Pressure
Ag(99.9%) 50W 0.1nm/s 25oC ∼ 30oC 3×10−3mBar
SiO2 (99.995%) 200W 0.01nm/s 25oC ∼ 65oC 6×10−3mBar
The thickness of the deposited Ag films are 30 – 40 nm and the thickness of the SiO2 layer ranges from 180 nm to 220 nm, depending on the structure
design. The surface roughness of the films, characterized by the Atomic Force Microscope (AFM) (Digital Instruments 3100), is around 1 nm rms (root- mean-square) with a maximum peak-to-peak range of height under 10 nm. An AFM scan of a planar Ag film surface on glass substrate is shown in Fig. 4.3.
Step 2: Deposit a 4-layer imaging system.
A 4-layer imaging scheme, which is prepared for the late steps of grating pattern definition (lithography) and metallization (lift-off), is deposited on top of the SiO2 after sputtering thin films. This involves:
1. deposition of a 100-nm-thick PMMA layer by first spin coating (3000rpm, 1 minute) a PMMA/Chlorobenzene solution (2.5% w/w) on the substrate, oven bake at 185oC for 30 minutes, then cool down
§ 4. EOT Through Planar Metal Films Coupled to Metallic Gratings 58
Figure 4.3: AFM scan of a planar Ag film surface (top) and the trace through the marked line (bottom).
to room temperature;
2. spin coating (5000rpm, 1 minute) of a 100-nm-thick bottom anti- reflection coating (ARC) material (Clariant AZr BARLir II);
3. RF sputtering of a 30-nm-thick SiO2 film;
4. spin coating (3000rpm, 1 minute) of a 150-nm-thick i-line photoresist (Clariant AZr HiRTM1075 diluted 1:3 with methyl amyl ketone) layer
on top.
The grating patterns are firstly defined onto the photoresist (PR) layer by IL; this then serves as a photomask for the subsequential metallization of the desired patterns (lift-off). For a successful lift-off, it is desirable to have an
undercut profile in the photomask. However, the sinusoidal intensity profile of the interference fringe patterns implies that this is difficult to achieve in a single layer resist. In addition, as there exists a highly reflective Ag layer on the substrate and a bottom ARC layer is necessary in order to suppress the light reflection during the IL process. Due to the above constraints, we choose the multilayer imaging system in the sample fabrication, in which the SiO2
layer serves as a hard etch mask for pattern transfer from the top resist layer to the ARC/PMMA layers underneath. A hard mask is required because the chemical system used to etch ARC/PMMA also etches photoresist, resulting in the erosion of the resist during the opening of ARC layer. The purpose of the additional PMMA bottom layer over the conventional tri-layer system is for easy stripping of the ARC during the final lift-off process.
Step 3: Define grating pattern via interference lithography. Interference lithography (IL) utilizes the interference of the coherent wavefronts to create periodic patterns. A schematic of the principle of the IL technique is illustrated in Fig. 4.4. The spatial period, P , of the fringe patterns is determined by the wavelength of light source λ and the half-angle of intersection of two coherent beams θi via
P = λ
2 sin θi
. (4.1)
The width of the grating lines defined on the resist layer is governed by the exposure dose (optical power × exposure time) onto the resist layer. In addition to the classical one-dimensional line structures obtained by a single exposure of photoresist, more complex two- and three-dimensional patterns can also be generated by using multi-exposure or multi-beam schemes [144] [145].
§ 4. EOT Through Planar Metal Films Coupled to Metallic Gratings 60
Figure 4.4: Schematic of the principle of the two-beam interference lithography technique.
Lloyd’s mirror interferometer, to define the grating patterns. An Argon-ion laser (Spectra-Physics Beamlok 2080-25S) operating at the 364 nm spectral line is employed as the source. The intensity of the light incident on the substrate is 300 µW/cm2. For grating patterns with periodicity of 450 nm
to 550 nm and duty cycle of 40 − 60%, the required time of exposure ranges from 40 seconds to 50 second. After exposure, samples are developed in undiluted AZr300 MIF developer (2.18% tetra-methyl ammonium hydroxide
(TMAH)) for 7 seconds. The pattern formed in the photoresist layer acts as a photomask in the followed steps. The AFM image of a 500 nm period grating in photoresist is shown in Fig. 4.5.
Step 4 & 5: Pattern transfer through SiO2 and ARC/PMMA
layers by RIE.
Grating patterns in the photoresist are transferred through the resist stack via two separate RIE process using an Oxford Instrument’s Plasmalab
Figure 4.5: AFM image of a 500 nm period grating in photoresist 80 Plus etcher. The first stage uses CHF3 to etch SiO2, which produces a
hard mask for the following O2 plasma etch of ARC/PMMA.
The RIE process is a combination of ion bombardment and chemical reaction, from which a good control over etch profiles can be achieved through adjusting a number of the parameters, such as pressure, temperature, plasma power and etchant chemistry. For a successful lift-off, an anisotropic steep resist sidewall profile with slight undercut is essential. The process parameters for etching SiO2 and ARC/PMMA are given in Table 4.2.
A SEM image showing the profile of the resist stack after the O2 etch
step for a 500 nm period grating is illustrated in Fig. 4.6, where the desired undercut profile has been achieved on the sample.
Step 6 & 7: Deposit Ag via thermal evaporation and lift-off. Definition of the Ag grating involves two steps. Firstly, a Ag film is
§ 4. EOT Through Planar Metal Films Coupled to Metallic Gratings 62
Table 4.2: RIE recipes for pattern transfer through SiO2 and ARC/PMMA
layers (Sample area: 22 mm × 22 mm).
Material Gas Etch
Rate (nm/min) Pressure (Torr) Gas Flow (sccm) Temp. RF Power SiO2 CHF3 3.2 0.03 15 295 oK 150 W ARC/PMMA O2 30 0.001 5 173 oK 100 W
deposited onto the sample by thermal evaporation; secondly, the resist and all metal not adhering directly to the SiO2 spacer is removed with a solvent
– called lift-off.
Thermal evaporation is a highly directional deposition process, i.e., the evaporant arrives at the sample is greatest on the horizontal surface and least on the vertical surface. Therefore, it is a preferred method for pattern metallization. The apparatus used in this process is a Balzers 510-A evaporator system. The process parameters for Ag evaporation are shown in Table 4.3.
Table 4.3: Process parameters for thermal evaporation of Ag.
Target Power Rate Temperature Process Pressure
Ag pelets(99.9%) 3.5 (on dial) 1nm/s N.A. 4×10−3mBar
After the thermal evaporation, the final step is to leave the sample in 60oC
acetone for 30 minutes to dissolve the remaining PMMA on the bottom of the imaging stack. This process removes all metal not adhering directly to the SiO2 spacer from the the sample and the pattern in the image stack is
Figure 4.6: SEM image showing the profile the resist stack after ARC/PMMA etch step for a 500 nm period grating.
transferred into metallic gratings with an inverted profile. Fig. 4.7 shows a SEM image of a fabricated 500 nm period Ag grating on top of the sample. The AFM scan of the sample is displayed in Fig. 4.8.
§ 4. EOT Through Planar Metal Films Coupled to Metallic Gratings 64
Figure 4.8: AFM scan of the fabricated Ag grating shown in Fig. 4.7.