GB2390684A

GB2390684A - Non-destructive observation of profile roughness

Info

Publication number: GB2390684A
Application number: GB0216232A
Authority: GB
Inventors: Cher Liang Randall Cha; Kin Chung Kong; Siew Ling Chan; Lin Hui Tan; Kian Hin Victor Teo; Geok Ing Ng; Yee Loy Lam
Original assignee: DenseLight Semiconductors Pte Ltd
Current assignee: DenseLight Semiconductors Pte Ltd
Priority date: 2002-07-12
Filing date: 2002-07-12
Publication date: 2004-01-14
Also published as: GB0216232D0

Abstract

There is provided a method of examining the surface roughness of the topological features of a micron or sub-micron level device. Features that can be studied include: the bottom of wet etched trenches, and the sidewall roughness of FC channels, V-grooves and etched waveguides. The method comprises the steps of: coating a surface of the device with silicone elastomer (for example polydimethylsiloxane - PMDS); peeling the coating from the surface; and, examining the surface roughness of the coating. In a variation on the method not one, but two silicone elastomer coatings are deposited and between them a release layer (e.g. a layer of gold) is provided.

Description

NON-DESTRUCTIVE OBSERVATION OF

PROFILE ROUGHNESS

Field of the Invention

5 The present invention relates to a method of examining the surface roughness of topological features of micron or sub-micron level devices.

Background to the Invention

Polydimethylsiloxane (PDMS) is traditionally a material for integrated circuit 10 (IC) packaging (encapsulation) and medical applications, and has been used in micro-electromechanical systems (MEMS) for making disposable devices for DNA analysis (e.g. sample boats), and as a gaspermeable membrane for cell cartridges.

However, recently, the material has also been popularly utilized in microfluidics technology. The main reason lies in its convenience and ease to replicate any 15 morphology, or pattern when it is applied over- they can become mold replicates.

With very low surface energy (a 2.1 mJ), it creeps' and fills up every detail found on the patterned substrate or device (or master), which was poured or flooded with PDMS, and this is achievable under room temperature conditions. After some amount of curing, usually at 65 C for around 2 hours, this vulcanizing elastomer 20 cures and can detach itself very easily from the master. The detachment process does not damage any chemical or physical property of the master. The PDMS mold will have a pattern that is of exact morphology from the master. Consequently, one can emboss the PDMS mold onto any material of their interest, and after some treatment and detachment of the PDMS mold, a replicate of the master of other 25 material is attainable.

PDMS is durable, optically transparent, and inexpensive. Rapid prototyping methods of polymer/metallic microfluidic circuits (,uFCs) and high aspect ratio microstructures (HARMs) using PDMS replication in low cost mass production are getting increasingly popular. PDMS also found some degree of success for 30 employment as 'stamps, to transfer monolayers of molecules between itself and the substrate that it contacted. No application, has so far, reported that it uses PDMS to study surface morphology along features, e.g., bottom of wet etched trenches, sidewall roughness of FC channels, V-grooves and etched waveguides.

Summary of the Invention

According to the present invention, a method of examining the surface roughness of topological features of a micron or sub-micron level device comprises the steps of: 5 coating a surface of the device with silicone elastomer; peeling the coating from the surface; and, examining the surface roughness of the coating.

The present invention employs the unique characteristics of silicone elastomers to measure the surface roughness either qualitatively or quantitatively of 10 topological features in microstructural (micron level) devices and ultra-large scale integration (ULSI) submicron level, devices. The silicone elastomer can be easily peeled away from the surface, without damaging it, and provides a mold of the exact morphology of the surface which can easily be studied. The method is particularly useful for shdying concave surfaces such as V-grooves, the sides of which are 15 difficult to examine directly.

Preferably, the method includes the step of coating the molded surface with a metallic layer, preferably by sputtering. This makes the replica surface easier to study. Preferably, the silicone elastomer is polydimethylsiloxane (PDMS).

20 Preferably, the elastomer is premixed with a curing agent at a ratio of approximately between 8:1 and 12:1, more preferably 10:1.

Preferably, the method includes a step of pre-mixing the elastomer and a curing agent and stirring for between 1 and 5 minutes, preferably 2 minutes. This duration of stirring is optimum for providing a good mixture but without producing an 25 excessive number of bubbles in the mixture which will adversely affect the integrity of the mould.

Preferably, the curing includes a first step of curing for between 8 and 15 minutes, preferably 10 minutes, at substantially room temperature. Any longer period of curing results in increased brittleness of the final film, making it difficult to 30 peel away from the surface in one piece.

Preferably, the curing includes a second step of curing at between 60 C and 100 C, preferably 80 C, for between 60 and 120 minutes, preferably 80 minutes.

This produces a film which is not too rubbery and which will leave silicone oligonomers on the surface, and also does not contain too many bubbles, and is not 35 too brittle such that the film cracks.

In another preferred embodiment, the method includes the furler steps of:

depositing a release layer onto the molded surface of the silicone elastomer layer; depositing a second layer of silicone elastomer onto the release layer; and, detaching the second layer.

5 This method is particularly useful for evaluating the surface morphology of protruding features such as etched waveguides or laser ridges. The second silicone elastomer layer will look identical to the master with the ridges, and can be sliced and examined in profile under a microscope.

Preferably, the release layer is metallic. However, other alternatives like 10 dielectrics and photo resist films can work as well. For metallic layer deposition, one can employ evaporation, sputtering or even chemical vapour deposition (CVD) techniques For dielectrics, one can employ a number of different CVD techniques, ion beam deposition, spin on, sputtering, and thermal growth. For photoresist films, one can either use wet resist or apply a dry resist.

Brief Description of the Drawings

Examples of the present invention will now be described with reference to the accompanying drawings, in which: Figures 1A to 1 D illustrate a first embodiment of the present invention; 20 Figures 2A to 2C illustrate PDMS molds made in accordance with the first embodiment; and Figure 3 illustrates a method in accordance with a second embodiment of the present invention.

25 Detailed Description

A method of evaluating the surface roughness of a V-groove sloped sidewall will be described. This method allows the surface roughness of the sloped sidewalls, or any particles/dirt that is residing along to be clearly seen. The PDMS mold registers the exact morphology of any features it spreads over. The exactness 30 and vividness of the recorded morphology depends heavily on the composition and treatment of the PDMS. Optimum PDMS treated conditions will be described.

Figures 1A to 1D illustrate the method of examining the surface roughness of the sloped side walls of a V-groove 1 formed in a silicon substrate 2. First, the substrate adajcent to the V-groove 1 is masked with a mask 3. Then a mixture of 35 resin and curing agent is applied to form a layer 4 which fills the V-groove 1. Then the layer 4 is cured, after which it can be peeled away from the surface to produce a mold 5 including a replica 6 of the V-groove 1. As PDMSis hydrophobic and does

not adhere well on dielectrics, it is easily peeled away from the surface. To observe the surface roughness of the mold 5, a thin layer of metal, preferably gold, can be sputtered onto the surface of the mold 5, and the surface can be viewed with an X ray scanning electron microscope (XSEM) or by atomic force microscopy (AFM).

5 Figures 2A to 2C illustrate molds. Figure 2A shows a PDMS replica of a series of V-grooves 6 with different widths and depths. As the V-grooves are etched into the substrate, the resulting replicas 6 of the V-grooves protrude from the PDMS layer. Figure 2B shows a close up view of the Vgroove replicas 6 showing the V groove bottoms 7 and the V-groove sloped side walls 8. Figure 2C shows a close up 10 view of the side wall 8 in which the surface roughness 9 can be seen.

To evaluate the surface morphology of protruding features, e.g., etched waveguides or laser ridges, we still can adopt the PDMS approach but with some additional process steps. This is shown in Figure 3. From the first PDMS mold 5, which will have trenches 6 at the positions whereby ridges are located, a thin layer 15 of gold 10, or any other metal (a 200A) is sputtered uniformly across it. Then on the same side where we have the sputtered gold, a second layer of PDMS 11 is deposited (by pouring or using a coaler) and the same curing treatment is performed for the second PDMS layer. After cooling down, the second PDMS layer can be detached easily from the first PDMS layer in one whole piece. The second PDMS 20 layer 11 will have the exact opposite features as the first PDMS layer, which implies it will look identical to the master with the ridges 12. We can slice the second PDMS layer (using a micro-slicer) to see the sidewall of the ridge, which is of interest to us under a XSEM or an AFM. This technique allows us to study the sidewall profiles of the ridges, which can be expensive to build, under a non-destructive manner.

25 The best recipe to prepare the PDMS is as follows: À Composition = 10: 1 (resin: curing agent; by weight) À Mixed by stirring for 2 mins (longer - excessive bubbles in films) À Leave to cure for 10 mins (longer increased brittleness) À Cured at 80 C for 80 mins (cured at 60C for 2hrs - rubbery and leave Si 30 oligonomers on master; cured at 100C for 30mins - bubbles, brittle/cracks)

Claims

1. A method of examining the surface roughness of topological features of a micron or sub-micron level device, comprising the steps of: 5 coating a surface of the device with silicone elastomer; peeling the coating from the surface; and, examining the surface roughness of the coating.

2. A method according to any preceding claim, including the step of coating the 10 molded surface with a metallic layer, preferably by sputtering.

3. A method according to claim 1, in which the elastomer is premixed with a curing agent at a ratio of approximately between 8:1 and 12:1, more preferably 10:1.

15

4. A method according to claim 2 or 3, in which the curing includes a first step of curing for between 8 and 15 minutes, preferably 10 minutes, at substantially room temperature.

5. A method according to claim 4, in which the curing includes a second step of 20 curing at approximately 60 C to 100 C, preferably 80 C, for between 60 and 120 minutes, preferably 80 minutes.

6. A method according to any preceding claim, including the step of coating the molded surface with a metallic layer, preferably by sputtering.

7. A method according to any preceding claim, further including the steps of: depositing a release layer onto the molded surface of the silicone elastomer layer; depositing a second layer of silicone elastomer onto the release layer; and, 30 detaching the second layer.

8. A method according to claim 7, in which the release layer is a metallic, a dielectric or a photoresist film.