WO2003055791A2 - Improved etch process for etching microstructures - Google Patents

Abstract

A two-step method of releasing microelectromechanical devices from a substrate is disclosed. The first step comprises isotropically etching a silicon oxide layer sandwiched between two silicon-containing layers with a gaseous hydrogen fluoride-water mixture, the overlying silicon layer to be separated from the underlying silicon layer or substrate for a time sufficient to form an opening but not to release the overlying layer, and the second step comprises adding a drying agent to substitute for moisture remaining in the opening and to dissolve away any residues in the opening that can cause stiction.

Description

IMPROVED ETCH PROCESS FOR ETCHING MICROSTRUCTURES

This application claims the priority of Provisional

Application 60/344,497 filed October 17, 2001.

This invention relates to a method of releasing

microelectromechanical devices from a substrate using a

gaseous etchant. More particularly, this invention relates to a method of releasing silicon-containing devices using a two-

step method.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (hereinafter MEMS) require

controllable, partial separation of device parts from a

substrate. Compliant silicon-containing microstructures are

etched so as to completely release them from an underlying

silicon-containing substrate. For example, an intermediate

silicon oxide layer is etched to separate at least a portion

of a silicon-containing layer from a substrate.

A simple MEMS device is shown in Fig. 1. A device part,

or beam 10, is partially isotropically etched from a substrate

12, leaving a support or connector 14 between them, that allows the part 10 to move, e.g., up and down, with respect to

the substrate 12.

The etchant of choice heretofore for isotropically etching silicon oxide is aqueous hydrogen fluoride (HF) .

A major problem with processing such parts is that as

etching proceeds, adherent residues form as by-products on the

substrate, and capillary, van der Walls and electrostatic

attraction between the etched part 10 and the substrate 12

causes collapse of the part 10. In effect the beam 10 of Fig.

1 under this attraction bends down toward the substrate, and

sticks to it, generally permanently. This phenomenon is known

as stiction. In addition, etch by-products and contaminants in rinse waters also precipitate out of solution during drying

steps, and cause adhesion bonding between the device part and

the substrate that is even stronger than the electrostatic

bonding, and interferes or prevents release of the final

structure from the substrate .

Several ways of minimizing stiction have been proposed,

including wet etching with HF, rinsing the residues away with

a solvent, and drying the parts with a liquid that has no or little surface tension, such as supercritical carbon dioxide.

An alternative etch is anhydrous HF, which does not leave

residues. However, because it is a very strong acid, special

equipment is required to handle it.

Since other steps in the formation of MEMS devices use dry, rather than wet methods, and large multichamber units can

be used to transfer a substrate from one processing step to

another without requiring that the substrate be exposed to the

atmosphere, it is undesirable to mix wet and dry processes

when forming such devices. The use of rinse solvents to remove

moisture from a microstructure causes as many problems as it

solves; the use of supercritical carbon dioxide requires a

complex and difficult setup, and thus adds to the expense of

manufacture .

The possibility then, of using anhydrous HF as the

etchant, appears to be advantageous because it is easy to implement in a multi-chamber processor, it is an efficient,

isotropic etch for silicon oxides, and it does not require

mixing wet and dry processing. However, the etch rate is lower

than when using aqueous HF. Further, anhydrous HF is a very

powerful etchant, and can etch the materials used for making

the processing chambers as well as the substrate to be etched.

Thus damage to the processing chamber occurs which must be

repaired, adding to the cost of manufacture.

Generally, semiconductor processes using semiconductor

materials, particularly silicon and silicon oxide, are used to

make MEMS devices. Because of their varying water content, doped silicon oxides, which have a high moisture content, etch

faster than undoped oxides.

When an anhydrous HF etch is used to etch a silicon

oxide, the amount of water present can vary depending on the

water content of the silicon oxide to be etched away. Doped

silicon oxides, which are hygroscopic, absorb water from the

atmosphere to form internal hydroxyl groups, and thus have a

high water content. Dense silicon oxides, such as thermal,

undoped thermally densified TEOS and high temperatures oxides,

have a lower moisture content because their water absorption

is limited to the surface layer of the oxide. However, as will

be further explained below, since water initiates and promotes

the etch reaction between HF and silicon oxides, the presence

of some water is necessary to maintain an adequate etch rate.

The overall etch reaction is

4HF + Si0₂ SiF₄ + 2H₂0

Thus water molecules are formed on the surface of the oxide

during the etch step. High water content silicon oxides

initiate the etch reaction rapidly and come to a steady etch

rate rapidly as well. On the other hand, the etch rate of low

water content silicon oxides begins slowly, i.e., there is an

initiation period, and the etch rate thus increases over time. However, overall the etch rate of these oxides remains low.

Other reactions between HF and silicon oxides are also

possible :

2) 6HF + Si0₂ → H₂SiF₆ + 2H₂0

The silicon fluoride compound can decompose to form either

silicon tetrafluoride, as

3) H₂SiF₆ → 2HF + SiF₄,

which does not leave a residue, and wherein the reaction

products are in the gaseous phase; or to form a silicate and

more HF, as

4) 2 H₂SiF_s + 3 H₂0 → H₃Si0₃ + 6 HF .

This latter reaction does leave a residue which can cause

stiction. Thus this reaction should be avoided to prevent

deposits on the surface of the structure or feature being

formed.

Further, the initial etching reaction also leaves a

residue, and thus a rinse is necessary at completion of the

anhydrous HF etch to remove the residue; this etch then is

difficult to integrate into a multichamber or cluster tool

that otherwise uses dry processes.

In efforts to solve the stiction problems, it has also

been suggested to use anhydrous HF alone; but since water initiates the etch reaction, particularly for thermal oxides

with a low moisture content, the etch rate for anhydrous HF

alone is low. Etching with anhydrous HF can take up to 10

hours to form complex microstructures .

The addition of methanol to anhydrous HF as a substitute

for water has been suggested. This would be advantageous

because capillary forces are reduced, and no residue is

observed on some oxides when methanol is used. However, again,

the etch rate is low initially until sufficient water is

generated in the reaction, which leads to a low yield;

further, the unknown initiation time hinders determination of

the time required for release.

The addition of acetic acid to anhydrous HF also has been

suggested as a catalyst for the etch reaction with anhydrous

HF, since acetic acid repels water vapor. However, the etch

rate here is low as well .

Thus using anhydrous HF as the etchant results in a dry,

isotropic, non-plasma etch method that does not leave a

residue on the etched surface, that does not cause stiction,

and that can be used in a cluster tool. However, the etch rate

is too low for commercial applications.

Prior art workers have tried a combination of anhydrous HF and methanol, using an etch chamber of aluminum coated with

tetrafluoroethylene. A mass flow controller regulated the flow

rate of anhydrous HF and a mass spectrometer regulated the

flow rate of methanol in a nitrogen carrier gas. Polysilicon cantilevers having a thickness of 2 microns, a width of 10

microns, a length of 1000 microns and a gap between the

polysilicon and the substrate of 2 microns, were fabricated

without stiction. The detachment length is much higher than

when conventional wet release etching is performed. The etch rate however can only be estimated, at about 10-15 microns/hr

at an HF partial pressure of 15 torr and a methanol partial

pressure of 4.5 torr. Thus the etch rate remains low, and

about 100 hours was required to etch a cantilever beam about 1000 microns long.

Thus the problem remains that by using anhydrous HF, the

total time needed for release of a microstructure is long, and

the etch rate cannot be known with certainty because it

depends on the type of silicon oxide employed and the amount of water generated in the reaction.

An improved and more reliable method of releasing a

feature from a MEM device has thus been sought that will

maintain high etch rates. SUMMARY OF THE INVENTION

The process of the invention comprises two steps that can

be cycled.

The first step uses a gaseous HF-H₂0 etchant mixture to

etch a silicon oxide to form an opening between two silicon-

containing layers. This step is continued for that amount of

time sufficient to form structures that will not collapse

during a subsequent drying step, but one that does not

completely release the structure either. This first step does

produce a residue however, as an excess of water is used to

increase the etch rate of the silicon oxide to an acceptable

level . Thus the timing for this first step is chosen to be such that the amount of etching is limited to that length of a

device that cannot contact the underlying substrate during a

subsequent step. Thus stiction is avoided by limiting the

amount of etching that occurs during this first step.

In the second step, a second solvent or drying agent is

added, one that will repel or substitute for the water present in the opening during the first step. Thus the second solvent

substitutes and displaces the water remaining in the opening

under conditions that vaporize the moisture. The second

solvent also must be able to dissolve the residue produced by the etching reaction, thereby preventing future stiction

problems. These two steps can be repeated or cycled until the

desired features are formed and released.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a cross sectional view of a simple MEM device.

Fig. 2 is a cross sectional view of a simple trilayer

substrate to be processed in accordance with the invention.

Fig. 3 is a cross sectional view of a substrate after

performing step 1 of the present process.

Fig. 4 is a cross sectional view of a substrate while

performing step 2 of the present process.

Fig. 5 is a cross sectional view of a partially released substrate after performing step 2 of the present process .

Fig. 6 is a schematic cross sectional view of a chamber

suitable for carrying out the inventive steps.

DETAILED DESCRIPTION OF THE INVENTION

Figs. 2-5 illustrate the steps of the present process.

Referring to Fig. 2, a simple substrate 20 for making a

MEM device is shown comprising a sacrificial silicon oxide

layer 22 between two silicon-containing layers 24 and 26.

Suitable silicon-containing materials can include polysilicon, crystalline silicon, doped silicon, a silicon wafer, and the like. The two layers 24 and 26 can be the same or different.

Various methods are well known to deposit various silicon

oxides on a silicon-containing substrate. For example, silicon

oxide can be deposited by chemical vapor deposition (CVD) ; by

plasma-enhanced chemical vapor deposition (PECVD) ; by high

^• temperature chemical vapor deposition (HTO) ; by low pressure

chemical vapor deposition (LPCVD) and the like. Suitable

silicon oxides can be deposited from silane or

tetraethoxysilane. The silicon oxides can be variously doped

or can be undoped. Typical useful silicon oxides include

phosphosilicate glass (PSG) ; borophosphosilicate glass (BPSG) ;

silicon oxide deposited from tetraethoxysilane (TEOS) ,

including dopants such as boron and phosphorus; and can be

hygroscopic or dense. Differently doped or made silicon oxides

vary as to the amount of moisture absorbed or adsorbed on

their surface .

In a first step of the present process, as shown in Fig.

3, the oxide layer 22 is partially etched away with a water-HF

mixture for a first timed interval to form an opening 21, in

which some of the silicon oxide layer 22 is removed, leaving

some aqueous solution 27 and a residue 28 in the opening 21.

This timed interval only partially releases the MEM device. The addition of excess water for the reaction increases

the etch rate of the initial step, so that the amount of

moisture in the oxide to be etched becomes immaterial . About

1-10% by weight of HF of water can be added, preferably from

1-5% by weight of the HF .

In the second step of the present process, as shown in

Fig. 4, a drying agent is added to the etchant. Suitable

drying agents are polar solvents, and include methanol,

ethanol, isopropyl alcohol, acetic acid and the like. The

drying agent is added both to remove or substitute for the

water on the surface of the etched oxide, and to dissolve and

rinse away the residue 28 produced in the first step that

causes stiction.

The polar drying agent dissolves and removes the residue

28, and, as shown by the arrows, replaces the water 27 present

initially. Thus the drying agent acts to dissolve the residue,

to at least partially replace the water present, and thus to

dry the opening 21.

Fig. 5 is a cross sectional view of the substrate at the

end of step 2 of the present process. A partial opening 21

having a length "d" has been made in the silicon oxide layer

22, which opening is now dry and residue-free . By cycling these two steps, the oxide etch rate remains

high due to the water present initially, and the residue which

causes stiction is continually removed. The growing opening is

also repeatedly dried with the polar solvent to prevent

moisture buildup in the opening. The oxide layer 22 is thus

removed using a non-plasma process wherein water is

continually removed so that the present process can be

integrated into a cluster tool used to form the structures,

and to release them from the substrate.

The above two steps can be repeated or cycled as needed

to etch away sufficient oxide for full release of the desired

device part .

The above two-step process can be carried out in an

apparatus as described below in Fig. 6.

Referring to Fig. 6, a remote plasma source chamber 610

is commercially available. A plurality of gas sources are

connected to suitable lines 612, 614, 616, 618 and 620 to feed

one or more gases such as oxygen, ammonia, nitrogen

trifluoride, argon and nitrogen, as examples, through valves

612a, 614a, 616a, 618a, and 620a respectively. Such plasma

precursor gases can be used to clean or ash residues that

build up in the main chamber 624. The plasma is then fed through a line 622 into an etch chamber 624. A valve 626

adjusts the pressure in the remote plasma chamber 610 and

passes plasma to the chamber 624.

A plurality of gas lines 626, 628, 630, 632, 646, 648,

650 and 652, supply various etch and reaction gases to the chamber 624 through lines 634 and 635 using valves 626a, 628a,

630a, 632a, 646a, 648a, 650a and 652a respectively.

The chamber 624 includes a mount 636 for the substrate to

be etched 638. The mount 636 is connected to a temperature

control means 639, which can be a resistance heater as shown,

that maintains the temperature of the substrate generally

between about 10 and 40°C during the etch reaction. The

temperature control means 639 can also be an array of lamps,

or a water cooled jacket. The chamber 624 is suitably maintained at about room temperature during the etch. However,

the temperature can be elevated somewhat to ensure that

moisture is removed from the substrate 638, but without

bringing the drying agent to the boiling point. Temperatures

that will condense the drying agent are to be avoided as well.

The drying agent should remain in the liquid phase to dissolve

the residues that cause stiction, and the HF-water mixture

should be in the gaseous phase during the reaction. An exhaust line 640 maintains a suitable pressure in the

chamber 624 by means of a valve 642.

Separate gas lines are required in some instances to

avoid reactions in the gas lines, rather than in the chamber

624, as for example, a reaction between xenon difluoride and

water.

In order to reduce damage to the chamber interior walls

and fixtures caused by the use of anhydrous HF, a layer of

nickel can be applied to those lines and surfaces that come in

contact with HF .

The chamber 624 is also capable of depositing a

passivation layer over the etched feature to protect it and to

prevent stiction during or after etching.

Additional lines and valves can be provided as needed for

other process steps, such as cleaning the chambers or

depositing a protective film over the released part in known

manner .

The invention will be further described in the following

example, but the invention is not meant to be limited by the

details set forth therein.

Example

A layer of silicon dioxide deposited over a silicon layer and in turn having a polysilicon layer to be released

deposited thereon, was etched using a mixture of 1.0 standard

liters per minute (slm) of 5% by weight aqueous HF at a

pressure of 100 millitorr and a temperature of about 40°C to

form a small opening in the silicon oxide layer. About 33 ml

of liquid methanol were then added to the opening to dissolve

any residue and rinse the opening.

The resultant opening was free of residues and methanol

had replaced much of the water generated during etching.

These etch and rinse steps were repeated to form the

desired device.

Although the invention has been described in terms of

particular materials, other materials used to make MEMS

devices can be substituted, and other reaction conditions and

processing equipment can be substituted, as will be known to

those skilled in the art. Thus the invention is meant to be

limited only by the scope of the claims appended hereto.

Claims

We Claim :

1. A method of releasing a microelectromechanical structure

comprising a silicon oxide layer sandwiched between two

silicon-containing layers, a first, substrate layer and a

second, overlying layer to be released from the silicon oxide

layer comprising, in sequence,

a) isotropically etching a partial opening in the silicon

oxide layer using a gaseous aqueous hydrogen fluoride etchant;

and

b) adding a polar drying agent to replace the moisture

remaining in the partial opening, and dissolve away residues

remaining therein.

2. A method according to claim 1 wherein steps a) and b) are

repeated.

3. A method according to claim 1 wherein steps a) and b) are

cycled until complete release of the structure is achieved.

4. A method according to claim 1 wherein the initial isotropic

etch is continued for a time period that only partially

removes the silicon oxide layer such that the second layer

cannot touch the substrate layer and adhere to it.

5. A method according to claim 1 wherein from about 1 to 10%

by weight of HF of water is added.

6. A method according to claim 1 wherein the drying agent is

methanol .

7. A method according to claim 1 wherein the drying agent is

acetic acid.

8. A method of forming a microelectromechanical feature

comprising

a) isotropically etching a silicon oxide layer between

two silicon-containing layers with gaseous aqueous hydrogen

fluoride for a time period that provides an opening in the

silicon oxide layer, but that does not allow the overlying

silicon layer to collapse onto and adhere to the underlying

silicon-containing layer; and

b) adding a quantity of a drying agent so as to

substitute the drying agent for water remaining in the opening

and to dissolve residues remaining in the opening; and

c) repeating steps a) and b) until complete release of

the feature to be made is achieved.