WO2002084832A1

WO2002084832A1 - Protection of reticles from electrostatic charges

Info

Publication number: WO2002084832A1
Application number: PCT/US2002/010167
Authority: WO
Inventors: Lawrence B. Levit
Original assignee: Ion Systems, Inc.
Priority date: 2001-04-11
Filing date: 2002-03-28
Publication date: 2002-10-24

Abstract

A device such as a reticle (9) including conductive surface patterns on an insulating substrate is protected from detrimental static charges and electrostatic fields by apparatus and method including a source of radiation (15) disposed to form ions in an ionizable gas contiguous the conductive surface patterns to promote discharge paths and conductive neutralization of isolated accumulations of static charges.

Description

CONTROL OF ELECTROSTATIC

CHARGES ON RETICLES

Field of the Invention:

[0001] This invention relates to reticles for optically forming circuit

patterns on semiconductor wafers, and more particularly to apparatus and

method for reducing electrostatic charge and associated particulate

contaminants and pattern damage on complex fine conductive structures on

insulators such as contained in reticles, flat-panel display plates, integrated

circuit patterns on wafers, and the like.

Background of the Invention:

[0002] Certain known semiconductor fabrication processes rely

upon forming circuit patterns upon semiconductor wafers using pattern masks,

or reticles, to optically form the desired circuit patterns on prepared surfaces of

the wafers. A reticle containing a desired circuit pattern is commonly formed

with an optically opaque, thin conductive layer of chromium on a quartz

substrate, and such thin conductive layer is vulnerable to damage from

electrostatic discharges. Such discharges are commonly attributable to static

charges that accumulate on nearby objects and on isolated segments of the

circuit pattern to form electrostatic fields of sufficient field strength to cause

ionization of ambient air and spark discharge that erodes or melts the thin

chromium layer. And isolated surface charges attract and retain contaminant

I particles with significant static attractive force, with resultant optical

reproductions on a prepared surface of a semiconductor wafer of the reticle

containing defects attributable to discharge erosions and adhered particles. In

addition, a protective cover, or pellicle, for the reticle is commonly provided to

enclose the reticle and to protect the reticle from particulate contaminants and

surface scratching. Such pellicles are commonly formed of non-conductive

materials that may also support isolated surface charges which, with any

charges on an enclosed reticle, may produce combined electrostatic fields

sufficiently high to promote spark discharge and associated damage to the

reticle.

Summary of the Invention:

[0003] In accordance with embodiments of the present invention,

static charges on the reticle are significantly reduced using alpha radiation to

produce high density ions adjacent the reticle to make ambient air, or other gas

contiguous the reticle, slightly conductive for forming conductive paths

through which static charges may be neutralized or otherwise dissipated.

Alpha radiation sources produce ion pairs through collision with molecules in

atmospheric gas, or air, and these ions serve as free charge carriers which can

neutralize surface charges on the reticle. Seed ions are formed in the

atmosphere adjacent the surface of the reticle that effectively lower the

threshold voltage for static discharge of charged regions. [0004] In one embodiment, a foil containing Polonium 210 is

disposed near the reticle as a source of alpha radiation that ionizes the air, and

a portion of a guard ring 15 provides a ground path for conducting static

charges away from the reticle.

Brief Description of the Drawings:

[0005] Figures 1A and IB are perspective views of a reticle and

pellicle assembly including an alpha radiation source in accordance with one

embodiment of the present invention; and

[0006] Figure 2 is a perspective view of an environment surrounding

typical segments of a reticle to be protected from static charge buildup.

Detailed Description of the Invention:

[0007] In accordance with one embodiment of the invention for

protecting a device having segregated conductive surface patterns on an

insulating sublayer, a reticle 9 as illustrated in Figures 1A and IB, is formed of

optically transparent quartz includes a desired pattern of circuits and

components formed in a thin surface layer of chromium. A pellicle frame 11

surrounds the chromium pattern on the quartz substrate of the reticle 9, and

supports thereon a pellicle 13, typically formed of quartz, in spaced

relationship away from the surface pattern of the reticle 9. Configurations of

reticles and pellicles are known in which the pellicle is supported away from the surface pattern of the reticle in a structure that encloses a certain volume 17

of air.

[0008] In the illustrated embodiment of the invention, a foil 15

containing Polonium 210 is attached to a sidewall of the pellicle frame 11 to

supply alpha radiation into the ambient air within the volume of ambient air

about the surface pattern of the reticle 9. Of course, other mounting schemes

can be used to keep the alpha source close to the chromium pattern, preferably

within a distance typically of less than 1 cm. Such source 15 of alpha radiation

creates ion pairs through collisions with gas molecules in the air within the

volume 17 enclosed about the reticle 9. These ions serve as free charge

carriers which can neutralize static charge on reticle and pellicle surfaces over

an interval typically less than about <20 seconds initially and thereafter

continually to inhibit buildup of surface charge sufficient to prevent discharge

or attract contaminant particles.

[0009] Additionally, the introduction of a source of alpha radiation

into the volume of air surrounding the surface pattern of the reticle 9

significantly increases the number of electrons and ions available to serve as

charge carriers. Thus, electrostatic discharge events caused by electric fields

from external charged object can occur at such lower energy levels without

creating sufficient heat to damage the chromium surface layer of the reticle 9.

The differential voltage induced on adjacent chromium features on the reticle is determined by the electric field from the external charged object and the

geometry of the two features. The energy in a discharge resulting from 50%

lower voltage differential is 75% lower and less damaging, and lower voltage

differential is assured by charge carriers that are present in the vicinity of an

air gap between isolated features or segments of the chromium surface pattern.

Charge-carrying ions thus aid in precipitating a discharge before the

differential voltage becomes high enough to cause a damaging discharge.

Thus, a reticle 9 protected by a source 15 of alpha radiation in accordance with

the illustrated embodiment of Figures 1A and IB may be moved into a region

of electric fields capable of inducing a voltage differential between adjacent,

isolated segments of the chromium surface pattern, and electrostatic discharge

may result, but at much lower voltage threshold and concomitant lower energy

levels. For continual charge-inducing events, many electrostatic discharges

may occur, but each occurring at lower voltage thresholds and lower energy

levels, with resulting negligible damage to micron and submicron features of

the chromium surface pattern of the reticle 9. And the rapidity with which

discharges may recur will depend upon the rate of buildup of voltage

differential between isolated segments of the surface pattern, and such rate of

buildup may be significantly retarded as a result of the alpha radiation source

15 which increases the density of charge carriers in the ambient air or

ionizable gas surrounding the isolated segments. Buildups of differential voltage resulting, for example, from movement of charged objects will

produce multiple discharges each at lower voltage thresholds and lower energy

levels significantly lowering the temperature of discharges, with cooling

intervals between discharges for significant reductions in or elimination of

damage to the chromium surface pattern. A source of alpha radiation is

preferred over sources of beta, gamma or x-ray radiation for the benefit of

confinement of the generation of charge carriers within a limited small volume

of air, but other sources of such forms of radiation may also be used.

[0010] Referring now to Figure 2, there is shown a pictorial

perspective representation of a volume 19 of ambient air surrounding isolated

features 21, 23 of a surface pattern of reticle 9. The volume 19 is shown

surrounding the gap 25 approximately to the limits of force upon an ion

attributable to the electric field associated with a voltage difference across the

gap 25. An ion within these surroundings would likely help initiate a

discharge, but if further away, would not likely influence a discharge. Thus,

the minimum level of ionization required to promote an electric discharge

across a gap 25 can be calculated for such volume 19 of surrounding ambient

air. The source activity that produces an average one ion pair in the volume 19

immediately surrounding a gap 25 in a surface pattern of the reticle is

determined in part by the dimensions of the surrounding volume. By way of

example for a typical 5X reticle (i.e., 5 times larger than the desired pattern on a semiconductor wafer), then 0.2 micron features on the semiconductor wafer

yield typical 1 micron gaps in the reticle. For a volume extending

approximately 0.5 to 1.0 microns in directions about a gap 25, the resultant

dimensions may be about 2 μm wide and 2 μm long and 1 μm high, or about

4 μm about the gap 25. Such gap 25 exists within the larger volume 17

disposed within the reticle/pellicle assembly having dimensions of about

5 inches wide by about 5 inches long by about 0.5 inches high, or about

12.5 inches³ (200 cm³ or 2 x 10^M μm ³). Therefore, in order to supply one ion

pair per 4 μm ³ volume about gap 25 within the total volume of 2 x 10¹⁴ μm ³

requires — - — -₃ — or 5 x 10¹³ ion pairs (assuming uniform distribution of

generated ion pairs within the total volume 17 of the reticle/pellicle assembly).

Since about 1.7 x 10⁵ ion pairs are generated per alpha (in particle or wave

5 x 10¹³ theory), then -p= — r^s ≡ 3 x 10⁸ alphas. (One Curie is defined as 3.7 x 10¹⁰

annihilations or disintegrations per second.) Therefore, in the foregoing

example, an alpha source of about 8 milliCurie disposed within the foil 15

establishes a minimum threshold of about one ion pair per 4 μm ³ volume of

ambient air surrounding a gap 25 in the surface pattern of reticle 9 within the

assembly 9, 11, 13, as shown in Figures 1A and IB. Enhanced rates of

generation of ion pairs may be attained with increased alpha radiation activity

from sources larger than about 8 milliCurie. Of course, similar electrostatic discharges and the attraction of contaminating particle by the electrostatic

fields associated with surface charges can adversely affect reticles without an

enclosing pellicle, and can similarly affect devices generally involving regions

of metallization on insulating sublayers. Such devices include flat-panel,

liquid crystal displays (LCD) and integrated circuits and other semiconductor

devices. Accordingly, introduction of a source of alpha radiation in the

operative proximity of such metallized patterns beneficially reduces or

eliminates damage to such devices attributable to static discharges and static

field attractions of contaminant particles surrounding such metallized patterns.

[0011] Therefore, damage to reticles attributable to the field from

electrostatic charges attracting and retaining contaminant particles, and

attributable to electrostatic discharges across gaps between isolated segments

of reticle circuit patterns, can be significantly reduced by introducing alpha

radiation to generate ion pairs within surrounding ambient air that promote

conduction and neutralization of isolated surface charges.

Claims

What is claimed is:

1. A method for protecting a device having segregated conductive

regions from detrimental effects of electrostatic charges, comprising:

disposing the conductive regions of the device within an ionizable

gas; and

irradiating the ionizable gas to form ion pairs therein in proximity

to the conductive regions of the device.

2. The method according to claim 1 in which the device is a reticle

and a volume of an ionizable gas is contiguous the conductive regions within

an assembly of the reticle and a protective structure therefor.

3. The method according to claim 1 in which irradiating the

ionizable gas includes introducing one of alpha, beta, gamma, and x-ray

radiation into the ionizable gas contiguous the conductive regions.

4. The method according to claim 3 in which the ionizable gas is air

and irradiation thereof is by alpha radiation.

5. The method according to claim 4 in which alpha radiation is

supplied from a quantity of Polonium 210 disposed to irradiate the ionizable

gas contiguous the conductive regions.

6. The method according to claim 4 in which the alpha radiation is at

an intensity sufficient to produce an ion density of approximately one ion pair

per 4 μm of the ionizable gas contiguous the conductive regions.

7. The method according to claim 2 in which a source for irradiating

the ionizable gas is disposed within the assembly to irradiate the ionizable gas

contiguous the conductive regions.

8. The method according to claim 2 in which the protective structure

includes interior surfaces confining the volume of an ionizable gas and a

quantity of Polonium 210 is disposed on an interior surface to irradiate

ionizable gas contiguous the conductive regions with alpha radiation.

9. The method according to claim 8 in which alpha radiation is

supplied with intensity to generate an ion density of not less than about one ion

pair of the ionizable gas per 4 μm ³ in the ionizable gas contiguous the

conductive regions.

10. Apparatus for protecting a device having conductive surface

patterns thereon from detrimental effects of electrostatic charges, the apparatus

comprising:

a structure disposed about the conductive surface patterns of the

device and bounding an ionizable gas contiguous the conductive surface

patterns; and

a source of radiation disposed to form ion pairs in the ionizable

gas contiguous the conductive surface patterns.

11. Apparatus according to claim 10 in which the source provides one

of alpha, beta, gamma, and x-ray radiation in the ionizable gas contiguous the

conductive surface patterns.

12. Apparatus according to claim 11 in which the source provides

alpha radiation at an intensity to produce an ion density of at least about one

ion pair per 4 μm ³ in the ionizable gas contiguous the conductive surface

patterns.