Injected red dye
6.3 Results and discussions
Glass is easily etched by HF producing gaseous SiH4 and H2O.
SiO2 (s) + 4 HF (aq) SiH4 (g) + 2 H2O (l) (1)
On the other hand polycarbonate does not react with HF. Thus HF ink along with the polycarbonate mold would be an ideal case where the patterns in the polycarbonate mold could be transferred to glass by etching with HF. One could argue that other polymers that
Chapter 6 Patterning Sub-micron Structures on Glass by Chemical Etching 92
are non-reactive with HF could also be used, which is probably true. We have however stuck to polycarbonate mold because it is commercially available and uniform sub-micron scale parallel line patterns are present in the molds. Figure 6.3 represents a typical optical micrograph of a polycarbonate disk that has been used as the mold in the present work.
As clear from the picture, the mold consists of parallel lines (“hills”) of about 0.7 µm thick that are separated by about 0.8 µm gaps (“valleys”).
Figure 6.3: Optical micrograph of the polycarbonate piece that was used as a mold for the present method (obtained from a compact disc of SAMSUNG –make)
When the mold was inked with 6% (v/v) aqueous HF solution and pressed onto the glass substrate reproducible parallel line patterns could be transferred to the glass surface.
The parallel line patterns were similar to those in the mold. In Figure 6.4 we show optical micrographs of three such slides containing etched parallel lines on the glass surface as imprinted by the present method. As evident from the figures continuous long range parallel alternate dark and light colored lines can be seen running from one end to the other. This shows that the present method could transfer the patterns of the mold onto a glass surface by etching with HF.
0.70µm
0.80µm
Polycarbonate mold
Figure 6.4: Optical micrographs of typically etched glass slides. The slides were etched by HF solution using polycarbonate disk of a CD as the mold. Bar is 10µm.
Chapter 6 Patterning Sub-micron Structures on Glass by Chemical Etching 94
The width of each light colored line is about 0.7 µm that is nearly equal to that of CD mold, while the width of each dark line is about 0.8 µm and that is nearly equal to the separation of two parallel lines of the CD mold. The imprints of parallel lines via chemical etching using the mold was further confirmed by scanning electron microscopy (SEM).
In Figure 6.5 we show the SEM micrographs of two such slides. The dimensions of the lines representing etched channels and non-etched crests match with those of the positive of the mold.
Figure 6.5 Scanning Electron Microscopy (SEM) images of parallel lines obtained by etching glass employing the present method. The micrographs are from two different samples.
_ _ _ _ _ _ _ 1 µ m
______________2µm
Production of micro arrays of spots on glass substrate was also achieved by using the same principle. In Figure 6.6, we show the optical micrograph of a cross pattern generated on a glass surface as a result of stamping twice in sequence using HF – inked mold. In this case, at first parallel line patterns were generated as in Figure 6.4 by stamping the inked mold onto a microscope glass slide. The slide was then washed with water and dried.
Figure 6.6: Optical micrographs of cross patterns generated by etching the surface twice in sequence at an angle using polycarbonate mold. Bar is 10µm.
This was followed by stamping for a second time at an angle to the previous one. The rest of the procedure was followed as before. Here also distinct features of the stamp–etched
Chapter 6 Patterning Sub-micron Structures on Glass by Chemical Etching 96
the slide while the brighter areas are the channels formed due to etching. A typical dark spot here has an area of about 0.64 (µm)2 as expected from the dimensions of the mold.
We also wanted to learn how the etching takes place using these nanoscopic patterned polymer discs. But before that let us see how the mold will look like in two dimensions. It will contain “hills” and “valleys” as shown in Figure 6.7. There are two possible mechanisms by which the patterns could be generated. The first is by only the
“hills” of the mold upon inking generated these patterns by chemical etching of glass. On the other hand there is a possibility of aqueous HF solution from the swab draining into the channels thus filling them in addition to inking the “hills”. Thus the etching process would take place both by the “hills” of the mold and the filled channels. This might seem to inhibit fulfilling our aim of producing nanoscopic lines and instead would produce an etched surface with no features. As this was not the case, the reason for obtaining the channels and cross patters as we did is as follows. Both the “crests” and the filled channels etch the glass plate. Since the channels have much more volume of etching solution than the “crests” could possibly have the regions of glass plates in contact with the filled channels would be etched more than the regions in contact with the “crests” thus producing the patterned channels.
Figure 6.7: Diagram showing polycarbonate part of the compact disk (CD) in two- dimensional plane
To verify this we did the following experiment. We spin coated a glass plate with a thin film of polystyrene. We then cut two widely separated parallel channels on it using a razor blade. We then rubbed a line of the HF “inked” swab against the film in perpendicular to both channels and on the plane of the film. A glass plate was then pressed on this film. In case the HF solution went into the channels it would produce an
“H” letter shaped etching on the glass covering the film; otherwise it would produce a
Hills
Valleys
single line with the dimension of the original ink-line. As shown in Figure 6.8 we observed the formation of an “H” shaped mark on the glass plate thus proving that indeed etching of glass took place by HF present in the “crests” and filled channels of the mold.
Figure 6.8: Photograph of the macroscopic “H” shaped mark on glass plate due to etching of glass. The bar is 1 cm.
In the experiments described above we have used polycarbonate mold and commercially available microscope glass slides as the substrates for generation of submicron scale patterns by chemical reaction etching. However, as can be seen from Figures 6.4 and Figure 6.6 the pattern transfer has not been perfect for long-range imprints. These are probably due to problems with the mold and substrate rather than the method itself. Polycarbonate is a relatively hard mold offering less flexibility at least with respect to making imprints on non-flat surfaces. On the other hand the glass slides that we have used are not atomically flat. Hence they would be susceptible to defects especially when patterns are being transferred from hard molds. On the other hand previous reports on lithography using a soft mold like polydimethoxy silane (PDMS) have indicated defective pattern transfer due to buckling and bending of the mold31. We have shown that under carefully performed pattern transfer defects in the structure could be minimized.
From our observations we noticed that best pattern transfer was possible when the pressure applied was 8.5 Kg/cm2. This condition resulted in good quality pattern transfer shown in Figure 6.4 B, 6.4 C and 6.6 A. Hence it is possible to obtain good quality patterns using the present method.
The next question comes about the way pattern formation take place in the present method. As described above, during the process of inking HF goes inside the channel and
Chapter 6 Patterning Sub-micron Structures on Glass by Chemical Etching 98
the amount of HF inside the channels (“valleys”) would be much more than that on the
“hills” of the mold. Thus there is the possibility of the glass surface getting etched everywhere. The silver lining here is that even though the HF in the “hills” of the mold would compete with those in the “valleys”, because of the sheer volume in the “valleys”, the effective pattern would be due mainly to the “valleys” and hence would serve the purpose of channel formation on the glass surface using the present method.