NBSIR 76-1019

Notes on the Preparation of

Silicon Density Artifacts

R. M. Schoonover

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

February 1 976

Final

Prepared for

Mechanics Division

Institute for Basic Standards

Washington, D. C. 20234

NBSIR 76-1019

NOTES ON THE PREPARATION OF

SILICON DENSITY ARTIFACTS

R. M. Schoonover

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

February 1 976

Final

Prepared for

Mechanics Division

Institute for Basic Standards

Washington, D. C. 20234

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richard^. S»cfUuy

James A. Baker, III, Under Secntary

Dr. Betsy Ancfcer-Johnson. Assistant Secretary for Science and Tec/inoiogy

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

Notes on the Preparation of Silicon Density Artifacts

R. M. Schoonover

1.0 Introduction

Now that single crystal silicon is in use as a working standard

of density, we at NBS are being asked by potential users just how

a crystal should be prepared for this application. There is a vast technical knowledge available in the semiconductor

manufacturing industry regarding silicon fabrication techniques.

These are largely inapplicable for density artifact manufacture

due to differences in applications. These notes are an attempt

to compile the basic information required for the successful

fabrication of a density artifact working standard.

Many of the techniques described here are standard optical

polishing and grinding techniques. Considerable effort is

expended to decrease the possibility of chipping the artifacts.

However, this occurrence is not disastrous because density

information is the primary characteristic of this type of

artifact. Therefore, chipping of the artifact changes its mass

and volume but not its density since it is a homogeneous object.

A simple mass value assignment restores both mass and volume

information to the artifact.

1.1 Desirable Characteristics of Artifact

The restraints placed on the artifact density standard can be delineated into two basic considerations: (1) stability and (2)

transfer of the density unit, that is to make use of the density

knowledge embodied in the artifact. The thrust here " is directed

toward practical things and we discuss what should be avoided to

maintain stability and to optimize transfer of the unit. We

accept the theoretical long term stability of single crystal

silicon.

1.2 Density

Transfer of the density unit from a silicon working standard to

any other suitable object is best accomplished with the hydrostatic weighing technique [1]*. The loss in weight due to the

buoyant force of the liquid is maximized for silicon due to its

relatively low density. Ideally, the force exerted by the

crystal mass should be near the load capacity of the balance used

to measure buoyant loss, thereby minimizing the effects of balance imprecision.

* The numbers in brackets refer to similarly numbered references

at the end of this paper.
1 . 3 Homogeneity

There are subtle connotations relating to the homogeneity of the

silicon artifact. We are Interested here In density but need to

know the artifact volume In order to transfer the knowledge contained therein. If the crystal Is not homogeneous In structure,

all density knowledge would be lost whenever Its mass and volume

were changed such as when accidentally chipped. However, when a

homogeneous sample Is chipped, we need only to redetermine Its

mass to assign a new volume based on previous density

Information.

Since two or more standards are desirable for redundancy In the

trans fe^ij^ens It a simple test for homogeneity Is to determine

the density of two or more crystal artifacts fabricated from the

same boule. If the measured differences do not exceed a few

parts per million In density (a normal range) then the crystals

are probably acceptable for use as artifact standards. X-ray

diffraction techniques provide another method for detecting

defects In the crystal lattice,

2.0 Special Requirements

2.1 Geometry

Most metrology laboratories follow the custom of minimizing

surface area to mass ratio as a desirable practice in mass

measurement. Without proof, this procedure is recommended for

silicon crystals and large surface area to mass ratios should be

avoided. A far more practical reason for minimizing this ratio

is evident during the hydrostatic weighing " process when the

surrounding medium is one to two thousand times more dense than

the air during a mass determination. Large surface areas can

couple the balance beam most effectively to turbulence in the

hydrostatic fluid. When turbulence is present in the liquid,

momentum is Imported to the beam via the large surface area of

the artifact on the balance pan. Damage to the balance beam may

result

.

Ideally, a sphere has the best surface area to mass ratio, but a

good compromise is the right circular cylinder whose height is

equal to its diameter. One should choose a silicon boule

diameter that approximates this condition if possible.

2 . 2 Freedom from Cracks

The one defect that can manifest Itself in the most insidious

manner is an undetected crack within the crystal structure.

Ideally, one would like to perform cutting and grinding

- 2 -
operations in a way that minimizes the production of cracks.

Preventive techniques are discussed in a different section of

this paper. However, in the initial stages of fabrication,

cracks may become apparent to the eye, but in any case should be

looked for in detail during all phases of manufacture of the

artifact.

Cracks generally are not visible until some initial etching has

removed cutting and grinding work damage and the surfaces begin

to appear polished. A low power (lOX) microscope may be helpful

in looking for cracks. The most likely places to look for cracks

are near edges and wherever grinding has been done. If a crystal

has been centerless ground as many boules are, there is a lot of

surface area where cracks may be hidden. Cracks should be cut

away as early as possible in the mass adjustment process if a particular finished mass value is desired.

The finished artifact must be free of cracks or it will be

unreliable as a density standard. The effect usually manifests

itself by producing large dispersions in the density values

assigned by repeated measurement.

2 . 3 Beveled Edges

The finished geometry of the silicon artifact should be free of

sharp edges . There are two very important reasons for this

requirement. They are (1) less chance of chipping and (2) less

risk of a crack occurring if dropped. The reasons cracks must be

eliminated are obvious and have already been discussed.

Chipping, on the other hand, changes the crystal mass. The

volume assignment for a given crystal is directly proportional to

mass and is necessary to the hydrostatic volume ratio comparison

used to assign density values to other objects.

Another consideration that makes a stable crystal mass desirable

is one of man hours of labor. The effort required to prove that

crystal mass has not changed with use is considerably less than

to prove that it really has and to assign a new value to it.

3.0 Method of Fabrication

3.1 General

If a crystal is to be cut and etched to a particular mass value,

the starting mass must be higher than the finished value. The

best procedure is to cut the crystal oversize, grind bevels or

radii on sharp edges, provide for identification and etch away

work damage. At this point, one can judge whether or not further

cutting or grinding is required. If so, the resulting work

damage can be confined to one end of a cylinder or one side of a

rectangular structure.

- 3 -
Most of the cutting and grinding procedures are standard

techniques encountered in the optical grinding and polishing

operations. The hand grinding and etching necessary to obtain a

particular mass value can be performed by the laboratory worker

himself as the required skill is quickly developed.

3.2 Sawing

Sawing is best done by a diamond saw with the specimen bonded to

a support block. Slow, safe feed rates should be used to avoid

breaking the crystal. Free hand sawing on a cutoff wheel is

possible for small boule diameters but not recommended. Allow

about 10% more mass, for samples of 200g or less, than is

required to compensate for etching and unforeseen problems. One

can expect to remove about .005" [2] of depth from a ground

surface before encountering virgin material.

3.3 Grinding

In the author's opinion, most cracks originate in the grinding

process, therefore heavy grinding on machines is to be avoided if

possible. When a crystal boule can be purchased without surface

grinding having been done, the potential user can then proceed

with his special Interest in mind.

Hand grinding of flat surfaces on a stationary plate or a slow

turning grinding lap is the best procedure for removing mass and

should be performed after the edges have been beveled. This

helps to prevent chipping and cracking during the grinding

operation. Ordinary window glass will suffice as a lapping plate

using water and an abrasive as one would on a lap. For gross

removal, a very course grit abrasive such as a synthetic alximinum

oxide of 50-80 ym size is used. To finish, a finer size, such as

20 ym is used before etching.

3.4 Beveling Edges

In place of sharp edges, a beveled edge or radius is preferred.

There are two satisfactory methods for removing sharp edges that

will not cause cracks, and a third that Involves some risk. A

skilled operator can grind free hand against the side of a cutoff

wheel made of molded rubber in which abrasives are Imbedded. The

disc is slightly flexible and contact pressure can be kept low.

Another method is to turn a cylindrical specimen on a lathe and

use a small piece of wood coated with abrasive to grind a

curvature on the edges

.

The cutoff wheel leaves a rough surface that requires a

considerable amount of etching to achieve a satisfactory polish

but is very adaptable to most shapes. The lathe method is

restricted to cylinders and is the slower method, but has less

work damage if the proper grit sizes are used.

_ 4 _
A third method for grinding edges on a cylinder employs a

contoured lens grinding lap that in essence is a spinning cone.

The cylindrical crystal artifact is held so that only the edge

contacts the cone walls where the grinding takes place. Initially, care must be taken with this method to avoid chipping the

sharp edges

.

3.5 Lapping

Lapping highly polished surfaces on crystjal artifacts is not neccessary, since the crystal will still require an acid etch to

remove all work damage. There is a lapping process that uses

acid to remove work damage generated by the lapping process

Itself, producing an optical quality surface. However, optical

quality surfaces on the crystal artifact are unneccessary

.

3.6 Etchants

The metrologlst is probably aware of all of the aforementioned

techniques used to shape a silicon crystal and to adjust the

mass. The methods are standard techniques used on a variety of

glasses. The acid etch may not be a familiar procedure and the

metrologlst will probably do this himself. Extreme care should

be given to personal protection from the acid solutions as their

action is Instant in the destruction of tissue. This is especially true of hydrofluoric acid. Other than these precautions,

the techniques are simple.

There are various etchant solutions used in the semiconductor

industry. Described here are three solutions with which the

author has had experience. They can be characterized as having

slow, moderate and fast etching rates. The formulations by

volume for these etchant solutions are as follows

Etch Ratio

Fast 1 Part

1 Part

1 Part

Moderate 5 Parts

3 Parts

4.5 Parts

Chemical

Slow 1 Part

20 Parts

Hydrofluoric Acid (HF)

Nitric Acid (HNO^)

Acetic Acid (CH^OOH)

HNO3

HF

CH3OOH

HF

HNO,

Strength Grade

48% Reagent ACS

70% Reagent ACS

Glacial Reagent ACS

SAME AS ABOVE

- 5 -
To prepare these acid solutions and to use them as well, a few

precautions should be observed. From a safety standpoint, an

exhaust hood with a source of water is mandatory. Eye protection

and gloves should also be used. The axiom of not pouring water

into acid should be observed when handling acids. All beakers,

bottles, etc. should be cleaned and dried before use to prevent

contamination of the etchant solution. The etchants can be

stored in polyethylene bottles with no harmful effects and with

safer handling. Since hydrofluoric acid etches glass, this

precludes the use of glassware as a storage container. The best

policy is to use all polyethylene beakers and bottles in these

procedures

.

3.7 Etching

Choosing which etchant to use depends on several factors. The

fast etch solution will get very hot during the initial etch when

work damage is being removed and tends to propagate cracks

,

scratches, etc. The moderate solution tends to increase over all

time required but tends to leave a smoother finish and gives

better control over the mass adjustment. The slow etchant

solution is most useful where the final mass value is to be

controlled in the 100 microgram region.

A good compromise is to use the moderate etchant to remove all

work damage. At this point, visually inspect the crystal for

cracks, deep scratches, and pits. Once the surface is

acceptable, i.e., no cracks, scratches, or pits are evident, the

etch rate can be increased using the fast etch. Then the

moderate solution is used when the final mass value is close at

hand. If further surface improvement is desired, continued

moderate etching would be the method of choice.

Etching rates (grams/minute) vary with crystal surface area and

are not reliable until work damage is removed. Then a stop watch

and direct reading balance can be used to keep track of the rate

of mass removal. The etch period must be uniform and a fresh

solution used each time. For the fast etch, a period of Ih

minutes is about the maximum time unless a very large volume of

etchant is used. The weaker etch solutions require somewhat

longer periods of exposure.

The volume of etchant need only be two or three times the crystal

volume but must completely cover the crystal. Smaller volumes of

etchant like these will generate higher temperatures and

accelerate the etch rate; larger volumes will have an opposite

effect.

Before etching, the crystal and all utensils that will be in

contact with the acid should be chemically cleaned. Initial

- 6 -
cleaning can be soap and water, followed, after drying, with one

rinse in trichloroethylene and two in methanol. An' alternate

cleaning after soap and water is one rinse in benzene, one in

acetone and two in methanol. After etching has begun, simply

drying all utensils is sufficient unless contaminated. However,

the crystal should be chemically cleaned each time to avoid oxide

stains.

There are several ways to halt the etching process. The most

reliable is to expeditously remove the crystal and plunge

(quench) it directly into a beaker of distilled water. Then

follow with a tap water dilution until the acid is gone. A

second method is to dilute the etchant with nitric acid, then

distilled water, followed by tap water. All of these steps occur

with the crystal in place. An abbreviated version is to omit the

nitric acid dilution and begin with distilled water. These last

two techniques can be dangerous since water is poured into acid

and are not the recommended techniques. A problem with oxide

formation on larger crystals occurs with the second method. The

first method allows reuse of the etchant after short periods of

etching several times, a convenient feature in adjusting to a

particular mass.

3.8 Removing Cracks and Pits

Cracks are best removed by localized grinding with the cutting

edge of a pliable cutoff wheel turning at its normal cutting

speed. Of course, the work damage must be etched away afterwards

so a visual re-examination can be performed. The cutoff wheel

leaves a rough finish and requires heavier than normal etching to

achieve a highly polished surface. Once the crack appears to

have been removed, further surveillance of the site should be

maintained during future etches. This will ensure that further

localized grinding, if required, will begin while there is still

an excess of crystal mass.

Pits (holes) are most likely to appear on the outer skin of the

boule where grinding has been done and are not harmful in themselves. However, pits sometimes develop in cracks where the etch

rate is likely to be accelerated. The finished product need not

be free of pits but they should be removed when they may conceal

a crack. Under the microscope, harmless pits appear as highly

polished craters of uniform color. Beware of those with color

change (silver to white in appearance) and those which take on

the shape of a line. If a pit is particularly deep and sharp, it

is likely to be a place to trap gas upon immersion in liquid.

Although techniques are used to remove such gas, there is more

assurance if the pit is removed in the same manner as a crack.

- 7 -
3.9 Identification

Avoid vibrating engraving tools as they can damage the crystal so

deeply as to render it worthless. The safest techniques are to

select a s5nnbol that can be shaped from a gold wire about 0.5 to

1 mm in diameter and lay it on the crystal during the initial

etch. The etch rate is accelerated at the point of contact

thereby propagating a faint outline of the symbol. Waxing the

specimen and removing wax in the shape of a symbol is another

technique for etching symbols onto a crystal or the symbol can

be sandblasted on the surface before etching. With extreme care,

a diamond awl can be used to engrave the symbol using a very

light scratch pressure.

**************

REFERENCES

[1] Bowman, H.A., Schoonover, R.M., and Carroll, C.L., A Density

Scale Based on Solid Objects, J. Res., Nat. Bur. Stand. (US)

78A (Phys. and Chem.), No. 1, 13-40 (Jan. -Feb. 1974).

[2] Soper, R.B., Silicon Device Processing, Paper #40, Mechanical Damage, Its Role in Silicon Surface Preparation, Nat. Bu.

Stand. (US), Spec. Publ. 337, pages 412-417.

USCOMM-NBS-DC - 8 -
NBS-IHA (REV. 7-73)

U.S. DEPT. OF COMM.

BiBi mt^PAPHIf* DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1019

2. Gov't Accession

Ino.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Notes on the Preparation of Silicon Density Artifacts

5. Publication Date

23 February 1976

6. Performing Organization Code

7. AUTHOR(S)

R. M. Schoonover

8. Performing Organ. Report No.

NBSIR 76-1019

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

SAME

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most sigfxificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The preparation of single crystal silicon density

standards as normally done at the National Bureau

of Standards is described. This description is

designed to guide other laboratories in construction of these standard artifacts and to facilitate

their intercomparison.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Density; etching; grinding; sawing; silicon; stemdard; weighing

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