3.1 Fabrication process combined with SIL/SSIL and nano-aperture.
The general fabrication flowchart of near-field pick-up with the self-alignment techniques is shown in Fig. 3.1.
（a）
The Al, Ti, and Au film is deposited on the silicon substrate, respectively.
（b）
The nano-aperture and opening ring is patterned with FIB.
（c）
The sacrificial layer Al is etched by Al etchant.
（d） （e）
The AZ-4620 is patterned by mask and backside exposure.
（f） （g）
The PR is reflowed thermally.
（h）
After depositing Cr layer, the SU-8 is patterned by mask#2.
（i）
Opening the Cr layer on the SIL by CR-7T.
（j）
Etching sacrificial layer Al for releasing.
Fig. 3.1 The general fabrication flowchart of near-field pick-up head.
3.2 Process of the nano-aperture
As shown in Fig. 3.1（a）, 4-in glass wafers is applied to RCA clean. Next, Al film 1.5um as sacrificial layer is deposited by thermal evaporating on glass wafer. Ti films 20nm as adhesion layer and Au 200nm as pedestal layer are then deposited by sputtering. After metal deposition, the 4-in wafer is sawed to 15mm×15mm square dices by dicing saw system.
In next step, nano-aperture and opening ring are patterned on the 15mm×15mm square dice with FEI NOVA 200 FIB tool which is a dual beam system（Scanning electron microscope &
Focused ion beam）, with the result as illustrated in Fig. 3.1（b）. The illustration of FIB system is shown in Fig. 3.2. It is noticeable that the ideal etching depth of aperture and opening ring is controlled to just stop on the Al film, nevertheless, over-etching is needed to ensue completely removing the Au/Ti films. Because aperture and opening ring is patterned in the same step, they are a concentric circle structure. Following FIB patterning, Al etchant
（80%H3PO4+5%HNO3+ 5%CH3COOH+10%H2O） is used to undercut the Al layer until the glass in the opening ring is exposed（Fig. 3.1（c））. The next step is to fabricate SIL/SSIL.
Fig. 3.2 The photograph of FEI NOVA 200 FIB tool.
SEM
FIB
3.3 SIL formation and self-alignment process
In order to fabricate SIL/SSIL easily, positive photoresist AZ-4620 is chosen as the material of SIL/SSIL. After undercutting Al film, SIL is formed to integrate with the nano-aperture, which can improve the light throughput with good resolution. At first, photoresist (AZ-4620) 7 mµ is spin-coated and patterned with mask#1 and backside exposure with using Au pedestal layer as the mask is performed next. The parameters of photolithography are listed in Tab.3.2. Seeing that UV light can pass through the glass wafer and opening ring, the photoresist around the opening ring will be exposed. After development, only photoresist on the pedestal layer will be remained, as shown in Fig. 3.1（d）.
Because the columnar photoresist and pedestal are concentric, the SIL and aperture can be combined without misalignment after reflowing thermally at 190℃ for 12hr（Fig. 3.1（f））.
Following SIL formation, Cr cover layer is deposited by physical sputtering. The Cr film has two functions, one is protect solvent from permeating into the SIL during SU-8 coating and softbake, and the other is protect the SU-8 development from etching SIL during development. Then negative photoresist SU-8 as a support and connection between SIL and other part of pick-up head is patterned with mask#2（Fig. 3.1（h））. In order to make light source can pass through the SIL, the Cr film is etched with SU-8 as mask, with the result as illustrated in Fig. 3.1（i）. Finally, the Al sacrificial layer is released by KOH（Fig. 3.1（j））.
Tab. 3.1 Basic parameters of coating AZ-4620 7 mµ .
Process step parameters equipment
photoresist AZ-4620 －
Exposure time 12 sec JAPAN K-310P-100S
Development time 120-130 sec 【FHD-5】 －
It should be noted that after coating thick photoresist, the edge of die will remain thicker photoresist film, called “edge bead” effect. If the edge bead is not removed, it will cause incomplete contact between the photomask and the photoresist, and errors occurred in transferring the patterns from mask to photoresist due to light diffraction. Therefore, in order to get the best resolution using contact lithography, it is a necessary step to remove the
“edge bead”. A common method eliminating “edge bead” effect is using Acetone to removing the edge photoresist film on the 15mm×15mm square dice before UV exposure.
3.4 SSIL formation and self-alignment process
The self-alignment process of “Backside Exposure Method” is not suited to fabricate SSIL combined with aperture. It is only suited to form the integrated structure of SIL and aperture owing to the light diffraction effect during backside exposure step. After development, there is some pattern loss of the columnar photoresist which make the diameter of photoresist has a gap with the boundary of pedestal. Therefore, even though the patterned photoresist is thick enough, the standard SSIL might not be easy to form. This reason might be that the surface tension around the boundary of columnar photoresist does not form SSIL easily under a pattern loss.
Here, a self-alignment process called “Surface Tension Modulation Method” is proposed to avoid the misalignment between SSIL and aperture. Similar to backside exposure method, UV exposure with mask#3 and backside exposure is performed after coating photoresist 10μm-thickness. The parameters of photolithography are listed in Tab.
3.2. In comparison between the two self-alignment methods, the major difference is dose quantity of backside exposure. About surface tension modulation method, for want of dose quantity enough in backside exposure step, which is much smaller than that of backside exposure method, only the photoresist exposed with mask#3 and backside exposed in opening ring region will be removed after development. The remained photoresist is as the result illustrated in Fig. 3.1（e）. Besides, backside exposure method performs self-alignment in backside exposure step, whereas surface tension modulation method performs that in thermal reflowing step. Even though a little misalignment between the pedestal and the columnar photoresist is occurred in columnar photoresist defining step, the surface tension may drag the liquid-like photoresist to right position and correct the final shape of SSIL during heat treatment. Therefore, the integrated structure combined with SSIL and nano-aperture will be fabricated without misalignment（Fig. 3.1（g））.
Tab. 3.2 Basic parameters of coating AZ-4620 10μm.
Process step parameters equipment
photoresist AZ-4620 －
Exposure time 12 sec JAPAN K-310P-100S
Development time 120-130 sec 【FHD-5】 －