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(12) United States Patent 

Waddell, Jr. 	et ai. 

(54) 	 CHEMICAL MODIFICATION OF 

SUBSTRATES BY PHOTO-ABLATION 

UNDER DIFFERENT LOCAL 

ATMOSPHERES AND CHEMICAL 

ENVIRONMENTS FOR THE FABRICATION 

OF MICROSTRUCTURES 

(75) 	 Inventors: Emanuel A. Waddell, Jr., 

Gaithersburg, MD (US); Timothy J. 

Johnson, Gaithersburg, MD (US); Gary 

W. Kramer, Gaithersburg, MD (US); 

Laurie E. Locascio, North Potomac, 

MD (US) 

(73) 	 Assignee: National Institute of Standards and 

Technology, Washington, DC (US) 

( *) Notice: 	 Subject to any disclaimer, the term of this 

patent is extended or adjusted under 35 

U.S.c. 154(b) by 0 days. 

(21) 	 Appl. No.: 09/905,567 

(22) 	 Filed: Jul. 13, 2001 

(65) 	 Prior Publication Data 

US 2001/0053495 A1 Dec. 20, 2001 

Related U.S. Application Data 

(60) 	 Provisional application No. 60/256,654, filed on Dec. 19, 

2000. 

(51) 	 Int. CI? ........................... G03C 5/00; B23K 26/14 


US006703189B2 

(10) Patent No.: US 6,703,189 B2 

(45) Date of Patent: Mar. 9,2004 

(52) 	 U.S. CI. ....................... 430/322; 430/269; 430/945; 

430/394; 219/121.68; 219/121.69 

(58) 	 Field of Search ................................. 430/322-269, 

430/945, 394; 219/121.68-121.69 

References Cited 

FOREIGN PATENT DOCUMENTS 

wo WO 98/23957 * 6/1998 ......... G01N/33/543 


* cited by examiner 

Primary Examiner-Mark F. Huff 

Assistant Examiner-Daborah Chacko-Davis 

(74) Attorney, Agent, or Firm-Butzel Long 

(57) ABSTRACT 

A method for simultaneously forming microstructures in 

substrates and altering their chemical character. The method 

involves exposing a surface portion of a substrate to light 

source, which is strong enough and of the appropriate 

wavelength to cause ablation of the substrate. The ablation 

of the substrate is controlled to form microstructures therein, 

such as channels. The ablation is conducted under a chemical atmosphere, which causes a change in the chemical 

functionality of the microstructures. The chemical atmosphere can be a gas, liquid or solid that is provided on the 

substrate surface. The method can be used to fabricate or 

modify microfiuidic systems. 

22 Claims, 4 Drawing Sheets 

(56) 

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METHANOL OXYGEN NITROGEN AIR 

u.s. Patent Mar. 9,2004 Sheet 1 of 4 US 6,703,189 B2 

LASER BEAM 

/UQUID 

FIG.1 A 

/GAS PHYSICAL AND CHEMICAL 


EFFECTS OCCUR AT 


INTERFACE 


FIG.1 B 
u.s. Patent Mar. 9,2004 Sheet 2 of 4 US 6,703,189 B2 

FIG.2A FIG.2B FIG.2C 

u.s. Patent Mar. 9, 2004 Sheet 3 of 4 US 6,703,189 B2 

FIG.3A 


Oxygen 

v ' ~,~::4 

FIG.3C 


Water 

FIG.4A 


Methanol 

FIG.4B 

u.s. Patent Mar. 9,2004 Sheet 4 of 4 US 6,703,189 B2 

0.0006 

-en 0.0004 

<:. 

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METHANOL OXYGEN NITROGEN AIR 

FIG.5 
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US 6,703,189 B2 


1 

CHEMICAL MODIFICATION OF 


SUBSTRATES BY PHOTO-ABLATION 


UNDER DIFFERENT LOCAL 


ATMOSPHERES AND CHEMICAL 


ENVIRONMENTS FOR THE FABRICATION 


OF MICROSTRUCTURES 


RELATED APPLICATION 


This application is based on U.S. Provisional Patent 

Application Serial No. 60/256,654 filed Dec. 19, 2000, the 

complete disclosure of which is expressly incorporated 

herein by reference. 

TECHNI CAL FIELD 

The present invention relates to the fabrication of microstructures. More particularly, the present invention is 15 

directed to a one-step method of forming microstructures in 

substrates using photo-ablation under a chemical atmosphere that forms the microstructure while altering the 

chemical functionality of the microstructures. 

BACKGROUND ART 

A number of competing technologies are currently used 

for the fabrication of microstructures in polymers including 

reactive ion etching, plasma etching, x-ray 

photolithography, chemical etching and template imprinting. 25 

However, with each technology there exists an associated 

disadvantage. In the case of reactive ion and plasma etching, 

highly specialized equipment, reactive gases, and high vacuums are required. X-ray photolithography (or LIGA) is 

extremely expensive and requires a synchrotron radiation 

source, which are in limited number throughout the world. 

Currently there are no reports of controlling the chemical 

functionality of a polymer surface during the fabrication of 

microstructures. At most, fabrication is completed in one 

step and functionalization is completed in a subsequent step. 35 

In most instances, functionality (or lack thereof) is considered an uncontrollable byproduct of the utilized fabrication 

process. 

DISCLOSURE OF THE INVENTION 

According to various features, characteristics and 

embodiments of the present invention that will become 

apparent as the description thereof proceeds, the present 

invention provides a method of forming a microstructure in 

a substrate, which involves: 45 

exposing a surface portion of the substrate to a light 

source while providing a chemical atmosphere at the 

surface portion, so that the light causes ablation of the 

substrate while the chemical atmosphere effects a 

change in the chemical functionality of the substrate in 

the ablated portion thereof. 

The present invention further provides a method of fabricating a micro fluidic system having fluid flow channels, 

which involves: 

providing a substrate; and 55 

exposing a surface portion of the substrate to a light 

source while providing a chemical atmosphere at the 

surface portion, so that the light causes ablation of the 

substrate and formation of micro channels therein while 

the chemical atmosphere effects a change in the chern ical functionality of the substrate in the ablated portion 

thereof. 

BRIEF DESCRIPTION OF DRAWINGS 

The present invention will be described with reference to 65 

the attached drawings, which are given as non-limiting 

examples only, in which: 

2 

FIGS. 1a and 1b are diagrams, which depict the manner 

in which photo-ablation is conducted under controlled atmospheres. 

FIGS. 2a-2c are cross-sectional views of channels which 

are formed by photo-ablation according to the present invention. 

FIGS. 3a-3c are fluorescent images of channels that 

depict the effects of different liquids on surface charge. 

FIGS. 4a-4b are fluorescent images of channels that 

depict the effects of different on surface charge. 

FIG. 5 is a graph that depicts electroosmotic mobility of 

polycarbonate under different ablation conditions. 

BEST MODE FOR CARRYING OUT THE 


INVENTION 


The present invention provides a method for fabricating 

microchannels, microvials, and other microstructures in 

different substrates which utilizes photo-ablation techniques. Moreover, the present invention involves controlling 

the atmosphere and chemical environment local to the site of 

photo-ablation, in order to alter the chemical functionality as 

well as to control the charge density on the substrate surface. 

The ability to functionalize a substrate during the fabrication 

step allows one to construct devices in which micro fluidic 

flow and chemical specificity may be tailored towards 

specific applications. 

The present invention provides a one step photo-ablation 

process that is performed under different gas and liquid 

atmospheres to simultaneously pattern microchannels and 

functionalize the surface in various polymer substrates. 

During the course of the present invention it has been 

discovered that by changing the local atmosphere surrounding the ablation site it is possible to alter the chemical nature 

and surface charge on polymer surfaces during fabrication. 

This allows two time consuming steps to be integrated into 

one rapid step. 

Potential commercial applications of the techniques of the 

present invention include the fabrication of micro-totalanalytical systems (ft-TAS) which are also referred to as 

lab-on-a-chip---(LOC) devices. These systems/devices are 

used for the analysis of chemical compounds and bio-arrays. 

For example, because of the ability to fabricate microstructures in channels, it is possible to tailor devices that incorporate multiple features (e.g., posts of packed beds). 

Additionally, the fabrication techniques of the present invention allow one to produce micro-vials that are content 

specific. Because the process is not limited to a particular 

class of materials, it can be used in the fabrication of devices 

in substrates in use today as well as future composites and 

hybrid materials. 

Photo-ablation occurs when a sufficiently energetic beam 

of photons of sufficient energy and appropriate wavelength 

is directed onto a surface and consequently chemical bonds 

are broken resulting in a thermal expansion and pressure 

ejection of the material. 

According to one example of the present invention, a 

248-nm KrF pulsed laser was used to ablate polymer substrates. The work surface on which the polymer substrates 

were placed was a two-dimensional motion-controlled stage 

that was part of a laser micro machining system (Potomac 

Systems LMT-4000). The use of a two-dimensional motioncontrolled stage allows the formation of linear and/or nonlinear channels without moving the laser beam. Alternatively of course, the laser beam could be moved in any 

desired pattern. In addition to moving the stage, various 
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US 6,703,189 B2 


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gases can be directed to flow over the substrate during the 

ablation process to provide a desired chemical or reactive 

environment. In some instances, liquids can be placed on the 

top surface of the substrate and photo-ablation can be 

achieved by passing the light through the liquid. It is also 

possible according to the present invention to ablate through 

a solid on the surface of the substrate to change the functionality of the substrate beneath. 

FIGS. la and Ib are diagrams that depict the manner in 

which photo-ablation is conducted under controlled atmospheres. In FIG. la the substrate 1 which is targeted for 

photo-ablation has a liquid 2 on an upper surface thereof. 

The laser light beam 3 passes through the liquid 2 and 

ablates the underlying surface of the substrate 1. The liquid 

can be selected to provide a chemical atmosphere, which 15 

reacts with the ablated surface of the substrate 1. FIG. Ib is 

similar to FIG. la except that instead of providing a liquid 

on the surface of the substrate 1, a gas 4 is directed across 

the surface of the substrate 1. 

When using any of the above processing techniques of the 

present invention, the dimensions of the micro fabricated 

features produced are limited by the spot size of the beam 

and the physical characteristics of the substrate, such as its 

absorption at the photon frequency. The present inventors 

have discovered that the aspect ratios attainable with the 25 

photo-ablation technique are comparable to other techniques 

such as X-ray micromachining, anisotropic wet etching, and 

silicon template imprinting. However, the amount of time 

required to produce a structure according to the present 

invention is greatly reduced due to the rapidity of the 

technique. 

FIGS. 2a-2c are cross-sectional views of channels, which 

are formed by photo-ablation according to the present invention. The channel produced in FIG. 2a involved photoablating a substrate that was covered with water. The chan­ 35 

nel produced in FIG. 2b involved photo-ablating a substrate 

that was covered with methanol. The channel produced in 

FIG. 2c involved photo-ablating a substrate that was blanketed with a stream of nitrogen gas. 

Changes in chemical functionality produced by the techniques of the present invention can be assessed using 

chemical mapping. Chemical mapping is a technique in 

which group-specific fluorescent probes are used to image 

functional groups on a polymer surface. During the course 45 

of the present invention, ablation effects and chemical 

functionality were accessed utilizing a modified fluorescein 

dye to image functional groups on the surface of polymer 

substrates (e.g., carboxylate groups on PMMA) before and 

after laser ablation at 248 nm. Additional chemical moieties 

such as amine or alcohol groups may be probed with 

differently modified fluorophores. The intensity of the fluorescence correlated to the density of specific functional 

groups on the surface before and after ablation. 

FIGS. 3a, 3b, and 3c are fluorescent chemical mapping 55 

images of a PETG channel ablated under oxygen, nitrogen, 

and air, respectively. These figures demonstrate changes in 

chemical functionality produced according to the present 

invention. FIGS. 4a and 4b are fluorescence images of a 

polymer (PETG) microchannel ablated under a) methanol 

and b) water and then chemically mapped as described. 

These figures demonstrate changes in chemical functionality 

produced according to the present invention under a liquid 

atmosphere. FIGS. 3a-3c and 4a and 4b identify the change 

in carboxyl groups and demonstrate that the process of 65 

changing functionality according to the present invention 

can be verified by chemical mapping. 

In addition to chemical mapping, micro fluidic channels 

produced according to the present invention were characterized by their electroosmotic mobility, which is directly 

related to the surface charge that resides on the channel 

walls. This characteristic is important in the construction of 

microfluidic devices because it dictates a channel's ability to 

move plugs of solution. The electroosmotic mobility was 

measured by two methods. The first technique utilized was 

a current monitoring method which measures the amount of 

time required to displace a solution in a channel with one of 

another concentration. In the flow imaging method, a caged 

fluorophore in solution is released by a pulsed laser and 

imaged by fluorescence microscopy. By monitoring its 

migration under the influence of an electric field, the electroosmotic mobility of the channel can be ascertained. 

Chemical mapping and the measurement of the electroosmotic mobility demonstrate that it is possible to vary the 

charge density and the distribution of functional groups by 

controlling the atmosphere and local chemical environment 

during photo-ablation. Thus, the present invention provides 

a process by which microfluidic channels and microvials 

may be constructed in various polymers under different 

localized chemical atmospheres to create devices that are 

specific towards different applications. 

Using the photo-ablation technique of the present 

invention, various polymers were ablated under five different local atmospheres. The polymers studied included 

polycarbonate, polystyrene, poly (ethylene terephthalate 

glycol), poly (methyl methacrylate), and poly (vinyl 

chloride). These polymers were ablated under argon gas, 

nitrogen gas, oxygen gas, liquid methanol, and liquid water. 

As opposed to other techniques that functionalize an entire 

surface, the photo-ablation technique allows one to functionalize areas that are defined by the dimension of the 

photon area imaged on the surface. Consequently, it is 

possible to place different functional groups adjacent to each 

other during fabrication B 

FIGS. 3 and 4 fluorescent images of chemically mapped 

channels that depict the effects of different atmospheres on 

surface charge. The substrates depicted in FIGS. 3 and 4 

were sonicated in a mild phosphate buffer (pH=7.0) after 

channel ablation to remove debris, soaked in fluorescein and 

EDAC for 8 hours, and rinsed in phosphate buffer. The 

fluorescence in the FIGS. 3 and 4 is indicative of surface 

bound, labeled carboxylate groups. The substrate in FIG. 3b 

was photo-ablated under oxygen. The substrate in FIG. 3b 

was photo-ablated under nitrogen. The substrate in FIG. 3a 

was photo-ablated under air. The substrate in FIG. 4a was 

photo-ablated under water. The substrate in FIG. 4b was 

photo-ablated under methanol. 

FIG. 5 is a graph that depicts electroosmotic mobility of 

polycarbonate under different ablation conditions. As seen 

from FIG. 5, microchannels ablated under methanol had the 

lowest electroosmotic mobility, whereas micro channels 

ablated under oxygen had the highest electroosmotic mobility. This indicates that the oxygen-ablated channels had a 

higher surface charge density that those ablated under 

methanol. 

The present invention allows the fabrication and surface 

modification of polymer microdevices to be integrated into 

a one-step process. In addition, the localized process allows 

for different functional groups to be placed in close proximity on the same device. Atmospheres under which photoablation takes place may consist of any chemical in a solid, 

liquid or gas phase that may be placed in contact with the 

substrate surface. The chemical environment needs to exist 
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only where photo-ablation occurs. Using appropriate wavelengths of light will enable the process to be applied towards 

fabrication and surface functionalization of any material, 

including polymeric, glass, silica, ceramics, composites, and 

hybrid materials. 

Although the present invention has been described with 

reference to particular means, materials and embodiments, 

from the foregoing description, one skilled in the art can 

easily ascertain the essential characteristics of the present 

invention and various changes and modifications can be 

made to adapt the various uses and characteristics without 

departing from the spirit and scope of the present invention 

as set forth in the attached claims. 

What is claimed is: 

1. A method of forming a microstructure in a substrate 15 

which comprises: 

providing a substrate having an exposed surface; and 

exposing a portion of the exposed surface of the substrate 

to a light source while providing a chemical atmosphere at the portion of to exposed surface, so that to 

light causes ablation of the exposed surface of the 

substrate while the chemical atmosphere effects a 

change in the chemical functionality of the exposed 

surface of the substrate in the ablated portion thereof. 25 

2. A method of forming a microstructure in a substrate 

according to claim 1, wherein the substrate is a member 

selected from the group consisting of polymeric substrates, 

glass substrates, silica substrates, ceramic substrates and, 

composites thereof. 

3. A method of forming a microstructure in a substrate 

according to claim 2, wherein the substrate comprises a 

polymeric substrate. 

4. A method of forming a microstructure in a substrate 

according to claim 3, wherein the substrate is made from a 35 

polymeric material selected from the group consisting of 

polycarbonate, polystyrene, poly (ethylene terephthalate 

glycol), poly (methyl methacrylate), and poly (vinyl 

chloride). 

5. A method of forming a microstructure in a substrate 

according to claim 1, wherein the substrate comprises a 

microfluidic system. 

6. A method of forming a microstructure in a substrate 

according to claim 1 wherein the chemical atmosphere 

comprises an inert atmosphere. 45 

7. A method of forming a microstructure in a substrate 

according to claim 1, wherein the atmosphere comprises a 

reactive atmosphere. 

8. A method of forming a microstructure in a substrate 

according to claim 7, wherein the atmosphere is selected 

from the group consisting of oxygen, nitrogen, air, methanol, 

and water. 

9. A method of forming a microstructure in a substrate 

according to claim 1, wherein the ablation of the substrate 

forms a channel therein. 

10. A method of forming a microstructure in a substrate 

according to claim 9, wherein the channel is linear. 

11. A method of forming a microstructure in a substrate 

according to claim 9, wherein the channel is non-linear. 

12. A method of forming a microstructure in a substrate 

according to claim 1, wherein said chemical atmosphere is 

one of a solid, a liquid and a gas. 

13. A method of forming a microstructure in a substrate 

according to claim 1, wherein the change in chemical 

functionality comprises a change in surface charge. 

14. A method of fabricating a micro fluidic system having 

fluid flow channels which comprises: 

providing a substrate having an exposed surface; and 

exposing a portion of the exposed surface of the substrate 

to a light source while providing a chemical atmosphere at the portion of the exposed surface, so that the 

laser light causes ablation of the exposed surface of to 

substrate and formation of microchannels therein while 

the chemical atmosphere effects a change in the chemical functionality of the exposed surface of the substrate 

in to ablated portion thereof. 

15. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

substrate is a member selected from the group consisting of 

polymeric substrates, glass substrates, silica substrates, 

ceramic substrates, and composites thereof. 

16. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 15, wherein the 

substrate comprises a polymeric substrate. 

17. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

substrate is made from a polymeric material selected from 

the group consisting of polycarbonate, polystyrene, poly 

(ethylene terephthalate glycol), poly (methyl methacrylate), 

and poly (vinyl chloride). 

18. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein said 

chemical atmosphere is one of a solid, a liquid and a gas. 

19. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

chemical atmosphere comprises an inert atmosphere. 

20. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

chemical atmosphere comprises a reactive atmosphere. 

21. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

microchannels comprise at least one of linear and non-linear 

channels. 

22. A method of fabricating a micro fluidic system having 

fluid flow channels according to claim 14, wherein the 

change in chemical functionality comprises a change in 

surface charge. 

* * * * *