METHOD OF PATTERNING ULTRA-SMALL STRUCTURES Copyright Notice

[0001] A portion of the disclosure of this patent document contains material

which is subject to copyright or mask work protection. The copyright or mask

work owner has no objection to the facsimile reproduction by anyone of the patent

document or the patent disclosure, as it appears in the Patent and Trademark

Office patent file or records, but otherwise reserves all copyright or mask work

rights whatsoever.

Related Applications

[0002] This application is related to U.S. Patent Application

No. 10/917,571, filed on August 13, 2004, entitled "Patterning Thin Metal Film by

Dry Reactive Ion Etching," which is commonly owned at the time of filing, and

the entire contents of which is incorporated herein by reference.

Field Of The Disclosure

[0003] This disclosure relates to patterning ultra-small structures using

pulsed electroplating.

INTRODUCTION [0004] Electroplating is well known and is used in a variety of applications,

including the production of microelectronics. For example, an integrated circuit

can be electroplated with copper to fill structural recesses such as blind via

structures.

[0005] In a basic electroplating procedure, samples are immersed in a

suitable solution containing ions, typically cations, but anionic solutions are also

known. An appropriate electrode is also immersed in the solution and a charge is

applied that causes the deposition of metals ions from the solution onto the sample

surface via an ionic reaction.

[0006] The current density, established by adjusting the electrode potential,

controls the reaction rate at the sample surface. At high current density, the

reaction rate becomes limited by diffusion of ions in the solution. Pulsed

electroplating, also known as pulse plating, alters the current or voltage applied to

the sample according to a predetermined waveform. The shape of the waveform

pattern depends upon the required surface characteristics of the final plated

structure.

[0007] Pulse plating can permit the use of simpler solutions containing

fewer additives to achieve the plated surface. Pulse plating is well known as a

method of improving coating density, ductility, hardness, electrical conductivity,

wear resistance and roughness. In addition, pulse plating provides more uniform

plating than other plating methods.

[0008] The production of compact, nonporous and smooth vertical surfaces

is difficult with existing electroplating techniques. Porous structural morphologies

produced in a controlled and predictable manner can also be advantageous to

device designers.

[0009] Ultra-small structures encompass a range of structure sizes

sometimes described as micro- or nano-sized. Objects with dimensions measured

in ones, tens or hundreds of microns are described as micro-sized. Objects with

dimensions measured in ones, tens or hundreds of nanometers or less are

commonly designated nano-sized. Ultra-small hereinafter refers to structures and

features ranging in size from hundreds of microns in size to ones of nanometers in

size.

[0010] Catalysts, sensors, and filters represent a non-exhaustive list of

examples of devices that can be fabricated or enhanced with structures of a porous

morphology. The ability to create three-dimensional structures with a designed

predictable structural morphology offers designers a method to realize new

devices.

[0011] Ultra-small three-dimensional surface structures with sidewalls can

be fabricated with coating techniques such as evaporation or sputtering. In both

these techniques, a negative of the desired surface structures is created, usually

using photolithographic techniques well known in the art. The patterned surface is

then placed into a vacuum chamber and coated with the final material. After

coating, the residual resist is removed.

[0012] However, with these techniques, patterned structures are known to

cause a shadowing effect to occur during coating such that material deposition is

uneven across the breadth of the patterned surface. In addition, the angles

between the sidewalls created and the substrate surface often have angles other

than the desired 90° orientation. The ability to create smooth, dense sidewalls

oriented at a 90° angle relative to the substrate surface is desirable for the

fabrication of a variety of ultra-small devices.

[0013] The ability to build significantly larger three-dimensional structures

with smooth dense sidewalls employing the similar processing offers advantages

to device designers. For example, smooth, dense sidewalls increase the efficiency

of optical device function. It may also be beneficial in some microfluidic

application.

[0014] Recent emphasis in the arts relates to the production of dense,

smooth contiguous coatings using pulse-electroplating techniques. For example,

in U.S. Patent Publication No. 20040231996Al, Web et al. describe a method of

plating a copper layer onto a wafer with an integrated circuit including depressions

using a pulse plating technique. In the preferred embodiment disclosed, the wafer

is rotated during deposition. The rate of rotation affects the quality of layer

produced. Depressions within the circuit layer are filled during the plating

process.

[0015] Electroplating dendritic three-dimensional surface structures using

electroplating techniques is also known. For example, U.S. Patent No. 5,185,073,

to Bindra et al., includes a description of forming dendritic surface structures using

pulsed electroplating techniques. The structures produced are dense, but have

angled sidewalls.

[0016] In 1995, Joo et al ("Air Cooling OfIC Chip With Novel

Microchannels Monolithically Formed On Chip Front Surface," Cooling and

Thermal Design of Electronic Systems (HTD-VoI. 319 & EEP-VoI. 15),

International Mechanical Engineering Congress and Exposition, San Francisco,

CA, Nov. 1995, pp. 117-121) described fabricated cooling channels using direct

current electroplate of nickel. The smoothness and microstructure of the walls

created was not an issue. Direct current electro-plating processing tends to

produce non-uniform structures across a die or wafer.

BRIEF DESCRIPTION OF FIGURES

[0017] The invention is better understood by reading the following detailed

description with reference to the accompanying drawings in which:

[0018] FIG. 1 is a schematic of a typical apparatus.

[0019] FIGS. 2(a)-2(b) are plots of typical voltage waveform according to

embodiments of the present invention.

[0020] FIG. 3 is an electron microscope photograph illustrating sample

representative dense ultra-small structures.

[0021] FIG. 4 is an electron microscope photograph illustrating a sample

representative structure with a varying morphology.

[0022] FIG. 5 is an electron microscope photograph illustrating a sample

representative porous ultra-small structures.

[0023] FIGS. 6(a)-6(f) are electron microscope photographs illustrating

various exemplary structures produced according to embodiments of the present

invention.

[0024] FIGS. 7(a)-7(b) depict exemplary shapes and patterns made

according to embodiments of the present invention.

DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS OF

THE INVENTION

[0025] Figure 1 is a schematic drawing of a configuration of an example

coating apparatus according to embodiments of the present invention. A computer

such as personal computer 101 is connected to a function generator 102, e.g., by a

standard cable such as USB cable 103. Personal computer 101 is also connected

to analog input-output card 105, e.g., by standard USB cable 104.

[0026] Waveform functions on the personal computer 101 are drawn using

a standard program included with function generator 102. After the personal

computer 101 downloads the waveforms to function generator 102, the function

generator sets characteristics such as amplitude, period, and offset of its output

electrical signal. The output of function generator 102 is sent to the current

amplifier 108 along cables 106 and 107. The cables 106 and 107 may be, e.g.,

standard USB cables.

[0027] In cases where the output current of the function generator is

insufficient to carry out the plating, an amplifier 108 can be introduced between

the function generation and the plating bath 112. Amplifier 108 increases the

output current of the function generator 102, making it sufficient to carry out the

plating without experiencing a voltage drop. Current amplifier 108 maintains an

appropriately constant voltage in plating bath 112 as deposition occurs. Any DC

voltage offset introduced by an imperfect amplifier can be corrected by

programming an opposite DC offset from the function generator.

[0028] Time between pulses is controlled via a program that triggers the

function generator output. This program is also used to start and stop the plating.

[0029] The output signal from the current amplifier 108 is provided to

electrode switch 111 on cable 109. Analog input-output (I/O) card 105 sends a

signal to electrode switch 111 via cable/line 114. Analog input-output card 105 is

controlled by an output signal from the computer 101.

[0030] Electrode switch 111 generates an output signal that is sent to timer

116 (via cable 115). The signal output from timer 116 is connected to anode 117

in the plating bath 112. In currently preferred embodiments, the anode is a silver

(Ag) metal plate, but there is no requirement that the anode consist of silver, and

other materials, including (without limitation) copper (Cu), aluminum (Al), gold

(Au) and platinum (Pt) may be used and are contemplated by the invention.

[0031] A second output signal is sent from current amplifier 108 via cable

110 (which may be, e.g., a USB cable) to sample 113 (which comprises the

surface / substrate to be coated/plated by the metal on the anode 117). Sample 113

is the cathode. In presently preferred embodiments, substrates are rectangular and

are about 1 cm by 2 cm. There is no requirement that the substrate be any

minimum or maximum size.

[0032] An agitation mechanism such as agitation pump 118 is attached to

plating bath 112. Agitation of the liquid in the bath 112 speeds up the deposition

rate. The pump 118 agitates the solution, thereby moving the solution around the

plating bath 112. The plating bath 112 is preferably large enough to permit even

flow of the solution over the substrate.

[0033] The effect of agitation depends on the size and shape of the device

being plated. In some cases, agitation reduces the plating time to thirty seconds on

some of the smaller devices and down to ninety seconds on larger ones. Agitation

also facilitates uniform thicknesses on all the devices across the substrate leading

to higher yields. There are other known ways of agitation, including using an air

pump to aerate the solution. For some applications, agitation may not be preferred

at all.

[0034] An appropriate plating solution is placed into plating bath 112. In

presently preferred embodiments, a silver plating solution is used. In the currently

preferred embodiment the solution is Caswell's Silver Brush & Tank Plating

Solution.

[0035] To ensure that the plating bath 112 is getting the desired period and

amplitude, an oscilloscope 121 can be connected directly to the plating bath.

[0036] In one presently preferred embodiment, sample 113 is prepared by

evaporating a 0.3 nanometer thick layer of nickel (Ni) onto the surface of a silicon

(Si) wafer to form a conductive layer. The artisan will recognize that the substrate

need not be silicon. The substrate in this example is substantially flat and may be

either conductive or non-conductive with a conductive layer applied by other

means. A 10 to 300 nanometer layer of silver (Ag) is deposited using electron

beam evaporation on top of the nickel layer. Alternative methods of production

can also be used to deposit the silver coating. The presence of the nickel layer

improves the adherence of silver to the silicon. In an alternate embodiment, a thin

carbon (C) layer may be evaporated onto the surface instead of the nickel layers.

Alternatively, the conductive layer may comprise indium tin oxide (ITO) or

comprise a conductive polymer.

[0037] The now-conductive substrate is coated with a layer of photoresist.

In current embodiments, a layer of polymethylmethacrylate (PMMA) is deposited

over top of the conductive coating. The PMMA may be diluted to produce a

continuous layer of 200 nanometers. The photoresist layer is exposed with a

scanning electron microscope (SEM) and developed to produce a pattern of the

desired device structure. The patterned substrate is positioned in an electroplating

bath 112. A range of alternate examples of photoresists, both negative and

positive in type, exist that can be used to coat the conductive surface and then

patterned to create the desired structure.

[0038] In the plating process, the voltage applied on the sample 113 is

pulsed. Figures 2(a)-2(b) show a plot of a typical voltage waveform. In

Figures 2(a)-2(b), the percentage of the total voltage applied on the sample is

plotted versus time. In this waveform, a positive voltage pulse of between five

and six volts is applied on the sample, and after some rest time, the voltage is

reversed to a negative voltage. Plating occurs as the voltage applied on the

substrate is negative-referenced to the counter electrode. Accordingly, the y-axis

in Figures 2(a) and 2(b) corresponds to percentages of negative voltages. It has

been noticed that if the pulsed length is increased the plating pushes on the

photoresist, creating slightly larger features.

[0039] During the intervals when the voltage is positive, material is

removed from the structures. The optimum values of parameters such as peak

voltage, pulse widths, and rest times will vary depending upon the size, shape and

density of the devices on the substrate that are being plated, temperature and

composition of the bath, and other specifications of the particular system to which

this technique is applied.

[0040] Figure 3 shows an image of a typical ultra-small feature fabricated

using the example waveforms shown in Figures 2(a)-2(b). In the fabrication of

these structures, the time between pulses was ten microseconds. The time between

pulses can be varied to achieve a variety of structure morphologies.

[0041] After the devices are plated onto the surface, the conductive surface

may be removed between the devices. Many methods of surface removal exist

that can be applied in these circumstances. In one currently preferred exemplary

surface removal method, if the ultra-small structures are comprised of silver, a thin

layer of nickel is plated over the silver structures to mask them during a reactive

ion etching process. A thin hard mask of other materials might also be used,

including but not limited to a thin layer of silicon dioxide (SiO2) of silicon nitride

(SiNx).

[0042] The reactive ion etching method is described in commonly-owned

U.S. Patent Application No. 10/917,511 (Davidson, et al, filed August 13, 2004),

the entire contents of which are incorporated herein by reference. If the

conductive layer consists of carbon, then the layer may be removed easily with

oxygen. If the conductive surface is removed, the devices are insulated from each

other. Material deposited over remaining photoresist is removed along with the

photoresist using standard processing methods.

[0043] The desired characteristics of the ultra-small structures depend upon

the application for which the structures are intended. Altering the nature of the

voltage waveform can vary the characteristics of the ultra-small structures

fabricated using this pulse-electroplating process.

[0044] For example, in the case of heat sinks, a very dense silver film

contacting the substrate is required to draw the heat out of the chip. Then it might

be better to have a less dense, larger surface area to conduct heat to the air or

liquid cooling media. Figure 4 shows an image of sample representative ultra-

small structures with this desired morphology. In this alternative example, a

negative voltage of two (2) volts was applied, with a time between pulses of fifty

milliseconds. A wide range of morphologies can be achieved by altering

parameters such as peak voltage, pulse widths, and rest times.

[0045] In some embodiments, a series of plating pulses including at least

one positive voltage pulse and at least one negative voltage pulse, are applied.

Preferably each voltage pulse is for an ultra-short period. As used herein, an

"ultra-short period" is a period of less than one microsecond, preferably less than

500 ns, and more preferably less than or equal to 400 ns. Preferably there is a rest

period between each of the pulses in the pulse series. In presently preferred

embodiments of the invention, the series of plating pulses is repeated at least once,

after an inter-series rest time. Preferably the inter-series rest time is

1 microsecond or greater. In some embodiments of the present invention, the

inter-series rest time is between 1 microsecond and 500 ms. As used herein, the

term "ultra-short voltage pulse" refers to a voltage pulse that lasts for an ultra-

short period - i.e., a voltage pulse (positive or negative) that lasts less than one

microsecond, preferably less than 500 ns, and more preferably less than or equal to

400 ns.

[0046] In some presently preferred implementations, the temperature used is

25° C to 29° C, with 28° C being preferable. In these preferred implementations,

the distance from the cathode 113 to the anode 117 is 8 mm and a typical substrate

size is about 1 cm x 5 mm, ±2mm.

[0047] Implementations of the current technique have been used to plate

features 30nm by 60nm, up to 80nm by 700nm, with heights in the range 150nm

to 350nm. Those skilled in the art will realize that as the shapes (areas) become

smaller, plating time decreases. Pulses in these preferred implementations ranged

from 1.5 microseconds to 400 nanoseconds, with the thinner pulses being used for

the smallest features. In one implementation of the present invention, for larger

structures, 500nm by 1 micron and 350nm high, longer pulses, on the order of

1.5 micron, were used.

[0048] Ultra-small porous structures may be fabricated as well. Such

structures can be used, for example, as filters, sensors or gas separation media.

Figure 5 shows an image of sample representative of ultra-small structures having

a porous structure. In this alternative example, a positive voltage pulse of between

four and five volts is applied, and after some rest time, the voltage is reversed to a

negative voltage.

[0049] FIGS. 6(a)-6(f) are electron microscope photographs illustrating

various exemplary structures produced according to embodiments of the present

invention.

[0050] FIGS. 7(a)-7(b) depict exemplary shapes and patterns made

according to embodiments of the present invention. These shapes range from

circular cavities to square cavities, as well as single and multiple cavities. Note

that these drawings are not necessarily to scale.

[0051] While the invention has been described in connection with what is

presently considered to be the most practical and preferred embodiment, it is to be

understood that the invention is not to be limited to the disclosed embodiment, but

on the contrary, is intended to cover various modifications and equivalent

arrangements included within the spirit and scope of the appended claims.