NASA CASE NO.

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LAR 15112-1-CU

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The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 at seq.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns. Although NASA encourages

nonexclusive licensing to promote competition and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address inquiries and all applications for license(s) for this invention to the

Technology Applications Team, NASA Langley Research Center, Code 200, Hampton,

Virginia 23681-0001.

Serial No.: 08/361,601

Filed: 11/24/94 LaRC

(NASA-Case-LAR-15112-1-CU)

MICRO-SENSOR THIN-FILM ANEMOMETER

Patent App|ication (NASA. Langley

Research Center) 14 p

N95-22906

Unclas

G3/34 0042565
NASA Case No. 151 12-1-CU 

MICRO-SENSOR THIN-FILM ANEMOMETER 

AWARDS ABSTRACT 

Turbulence measurements in high-speed flows have historically been obtained by hot-wire 

anemometry. However, high stagnation temperatures, high dynamic pressures and flow contaminants 

severely limit the life of hot-wire elements in hypersonic flow. An alternative to conventional hot-wire 

anemometry is hot-film memometry. However, the frequency response of existing hot-film probes is 

inadequate to resolve the full turbulent spectrum for hypersonic flows. 

An anemometer is provided having a micro-sensor thin-film probe. The probe is a half-wedge formed 

from a single crystal of sapphire and is configured to prevent generation of a deta&ed shoclc through 

much of the hypersonic boundary layer. The tip of the probe is rounded to minimize flow disturbance 

and contains an iridium sensor formed on the rounded tip of the probe along the stagnation line. The 

sensor is deposited using a unique microphotolithography technique. 

Novel aspects of the present invention include the configuration of the probe, the use and formation 

of the iridium sensor and the placement of the sensor along the stagnation line of the rounded tip of 

the probe. 

Inventors: Marlc Sheplalc Catherine B. McGinley 

Address: 7 1 86 White Plains Way 1 108 North Green Drive 

, FL 32656 Newport News, VA 23602 

ss: 

Employer: Student NASA LaRC 

Inventors: Eric F. Spina Ralph M. Stephens 

Address: 206 Edmount Drive 530 Pembrolce Avenue 

Syracuse, NY 13203 Norfollc, VA 23507 

SS : ss: 

Employer: Syracuse Univ. NASA LaRC 

Inventors: Pumell Hopson, Jr. Vincent B. Cruz 

407 Cheadle Loop Road Route 4, Box 1365 

Seaford, VA 23696 Haves, VA 23072 

ss: 

Employer: NASA LaRC 

1niGal Evaluator: Michael J. Walsh 

ss: 

NASA LaRC 

SerialNo.: 08/361,701 

Filed: November 2 1, 1 994 
LAR 15112-1 -1- PATENT APPLICATION

MICRO-SENSOR THIN-FILM ANEMOMETER

Origin of the Invention

5 The invention described herein was made jointly in the performance of

work under NASA Grant No. NAG-l-1400 with Syracuse University and a

graduate student, and employees of the United States Government. In

accordance with 35 U.S.C. 202, the grantee elected not to retain title.

]0 Back,qround of the Invention

1. Field of the Invention

]5

The present invention relates generally to a device and method for

measuring turbulence in high-speed flows, and more particularly to a microsensor thin-film probe capable of measuring turbulence in hypersonic flows.

2. Description of the Related Art

20 Turbulence measurements in high-speed flows have historically been

obtained by hot-wire anemometry. However, high stagnation temperatures, high

dynamic pressures, and flow contaminants severely limit the life of hot-wire

elements in hypersonic flow. Nonintrusive measurement techniques such as

laser-Doppler velocimetry and particle-image velocimetry also are limited when

25 applied to hypersonic flow. In particular, data-rate limitations and difficulties in

flow seeding present the most significant obstacles to their accurate application

in high-speed flows. An alternative to conventional hot-wire anemometry is

hot-film anemometry, with a thin metallic film deposited along the stagnation line

of a rigid, dielectric substrate, thus increasing mechanical strength.

30
LAR 15112-1 -2- PATENT APPLICATION

Hot-film probes incorporating various combinations of materials and

construction techniques have displayed excellent durability and moderate

frequency response characteristics in the few high-speed and high temperature

flows in which they have been tested.

5 Ling and Hubbard introduced the thin-film probe as a

resistance-temperature transducer to measure turbulent fluctuations in flowfields

in which hot-wires could not survive (Ling, S.C. and Hubbard, P.G. 1956 "The

Hot-Film Anemometer: A New Device for Fluid Mechanics Research", J.

Aeronaut. Sci. 23, 890). This probe consisted of a thin layer of platinum fused

10 to a glass substrate. The main body of the probe consisted of a 4.0 mm

diameter Pyrex rod with two 32-gauge platinum lead wires (2.0 mm apart)

embedded within the core. The rod was ground down into a 8 ° wedge, tipped

by a 30 ° wedge. Fused on the wedge tip was a thin platinum film sensor (1.0

mm x 0.2 mm) with a nominal cold resistance of 200. The ends of the sensor

]5 were attached to the exposed lead wires by thick platinum overplatings. It was

tested at high temperatures (1100°F) without detectable changes in

thermoelectric properties. Experiments indicated a frequency response of 100

kHz at a flow velocity of 1000 ft/sec.

Later, Seiner used commercial hot-film probes to investigate high-speed,

20 cold jet flows (Mach 0.5-2.0), (Seiner, J.M. 1983 "The Wedge Hot-Film

Anemometer in Supersonic Flow", NASA Technical Paper 2134). The probe

consisted of a thin film of nickel sputtered on a 4 ° semivetrex wedge of quartz

substrate. A protective coating of quartz (0.5-2.0 IJm) was sputtered over the

nickel (1.0 mm x 0.2 mm) for electrical isolation and protection from particles.

25 A maximum frequency response of 130 kHz for the 1:1 balanced CTA bridge

was realized via the square-wave injection test, and was found to be inversely

proportional to the thickness of the protective coating.

More recently, Demetriades and Anders presented a report on the

ongoing development of a constant-current probe for use in

30 high-temperature/hypervelocity flows (Demetriades, A. and Anders, S. G. 1990
LAR 15112-1 -3- PATENT APPLICATION

"Characteristics of Hot-Film Anemometers for Use in Hypersonic Flows", AIAA

Jouma128,2003). The design consists of a platinum sensor (0.5mm x 1.8mm)

painted along the stagnation line of a wedge-shaped glaze bead positioned at

the tip of a twin-bore alumina tube (10.0 cm x 0.25 cm). Results from

5 temperature endurance testing (temperature cycling to 1400°F) demonstrated

the excellentelectrical stability characteristics of the probe. These probes were

also run for hours in high-temperature, high dynamic-pressure environments

(Mach8.0, 800°F and 20.7 kPa)without failure, further confirming the durability

characteristics. Presently, there is no experimental data available from

]0 Demetriadesdocumentingthe frequency response characteristics of this probe.

However, painted sensors contain cross-sectional non-uniformities which lead

to "hot-spots" and sensor failures. In addition, the large sensor surface area

used by Seiner and Demetriades limits the spatial resolution of the

measurement technique.

15 The frequency responsecharacteristicsof the existing hot-film probes are

inadequate to resolve the full turbulent spectrum for hypersonic flows. The "dual

swept-surface" wedge designs used by Seiner are a poor approximation of

stagnation-line heat-transfer, since the sensor extends 0.1 mm away from the

tip on both sides of the wedge. In addition to this fundamental problem, Seiner

20 found that the probe displayed poor directional behavior and was Mach-number

sensitive. Seiner hypothesized that these problems were associated with the

40 ° wedge geometry and recommended examination of a larger semi-vertex

angle or a wedge fitted with a rounded nose. Finally, this design is

unacceptable for measurements in wallbounded shear flows, as shocks

25 emanating from the bottom of the probe would disturb the flow.
LAR 15112-1

Summary of the Invention

-4- PATENT APPLICATION

It is a primary object of the present invention to provide a device and

method for measuring turbulence in high-speed flows such as hypersonic

5 flows.

A further object of the present invention is to measure turbulence in

high-speed flows with a device and method having fast response and

durability in the severe environment of hypersonic airflows.

The foregoing objects are achieved by providing an anemometer

]0 having a micro-sensor thin-film probe. The probe is a half-wedge formed

from a single crystal of aluminum oxide (i.e., sapphire) and is configured to

prevent the wedge from generating a detached shock through much of a

hypersonic boundary layer. The tip of the sapphire probe is rounded to

minimize flow disturbance and contains an iridium sensor formed on the

[5 rounded tip of the probe along the stagnation line.

The iridium sensor is formed by first depositing a layer of copper over

the sapphire substrate in the area where the sensor is to be located. A

layer of photoresist is then deposited over the copper and dried. A contact

print of the sensor shape is then made into the photoresist by exposing the

20 photoresist to ultraviolet light. The photoresist is then developed leaving an

opening to the copper layer corresponding to the sensor shape. The

exposed copper is etched to produce an opening through the copper to the

sapphire substrate corresponding to the sensor shape. The photoresist is

removed and niobium is deposited on the exposed sapphire substrate by

25 electron beam vapor deposition. Without breaking the vacuum, iridium is

deposited by electron beam vapor deposition onto the niobium layer. The

copper is then removed with an etchant and the probe containing the sensor

is annealed in a hard vacuum at approximately 1000°C to stabilize the

resistance of the sensor. Once the sensors are formed, they are connected
LAR 15112-1 -5-

to leads which connect to a coaxial cable.

the driving circuit of the anemometer.

Sensors formed according to this

PATENT APPLICATION

The coaxial cable connects to

method show a significant

improvement in frequency response due to the lower thermal inertia of the

sensor compared to conventional 5.0 #m diameter hot wire and existing hot

films.

B[ief Description of the Drawings

]0

FIG.

FIG. 1A.

FIG.

FIG. 1A is a side view of the thin-film probe of the present invention.

1B is a side view of the tip of the thin-film probe depicted in

1C is an illustration of the angle of a portion of the probe

depicted in FIGS. 1A and lB.

]5 FIG. 2 is a drawing of the sensor region of the thin-film probe of the

present invention.

FIG. 3A is a drawing of the sensor pattern formed through the

photoresist and copper layers.

FIG. 3B is a drawing of the niobium and iridium layers of the sensor

20 deposited through the copper layer.

FIG. 3C is a drawing of the sensor formed on the sapphire substrate.

Description of the Preferred Embodiments

25 Referring to FIG. 1A, the probe 10 geometry consists of a straight

portion leading to an approximately 14 ° half-wedge, 1/2 inch long x 1/8

inch wide, diamond tooled out of a single crystal aluminum oxide (i.e.,

sapphire). The use of sapphire for the substrate material enables the probe

to withstand severe temperatures without mechanical failure or a significant
LAR 15112-1 -6- PATENT APPLICATION

change in dielectric properties. The capability of sapphire to be machined

and polished to a finish less than one micro-inch (r.m.s.) coupled with the

microphotolithographic technique allows for a significant reduction in sensor

size over existing hot-films, thus improving the spatial resolution of the

5 probe. In addition, the substrate material was selected because it has a

low thermal conductivity and a coefficient of thermal expansion that closely

matches the iridium and niobium layers, thus reducing the risk of sensor

detachment during thermal expansion. The 14 ° half-angle represents e=i t

for an air flow with a freestream Mach number slightly below 1.6, thus

]0 preventing the wedge from generating a detached shock through much of

a hypersonic boundary layer. This angle can be modified for application to

specific airflow conditions. The half-wedge geometry is preferred in order

to minimize flow disturbance and allow for near-wall measurements in

boundary layers. Referring to FIG. 1B, the bottom portion of the wedge is

]5 angled upward at 3 ° in an effort to relieve shock induced disturbances

created by the finite curvature of the nose. This will lessen any

boundary-layer disturbance that can propagate upstream, making for more

accurate measurements. This angle of tilt can be modified to suit the

application and different wedge tip radii. The wedge tip 20 is rounded to

20 a radius of curvature of approximately 0.2 ram, and is then diamond

polished to a r.m.s, surface finish of 1 microinch or less to prepare the

surface for deposition of the sensor 30. The radius of curvature of the

wedge tip 20 should be as small as possible to minimize flow disturbance

while still accommodating the sensor. The rounded-nose, wedge tip

25 geometry offers benefits over the "dual swept-surface" probes. Since the

sensor is very narrow, "true" stagnation point heat-transfer will occur:

existing heat transfer data for heated cylinders in turbulent cross-flow

shows a negligible decrease in local Nusselt number for angles less than 5 °

from the stagnation line. It is thus possible to approximate the rounded
LAR 15112-1 -7- PATENT APPLICATION

nose as a cylinder in cross-flow, for which a considerable amount of data

exists in all Mach number regimes. The onset of Mach number

independence may also be reduced, as a normal shock will occur locally in

front of the stagnation line, rather than a weaker oblique shock in front of

5 a wedge.

A "dog-bone" shaped micro-sensor (approx. 2000 A x 12.5 pm x

0.25 mm) 30 of iridium is deposited along the stagnation line 40 of the

substrate using a microphotolithography technique, as described below.

The "dog-bone" shape of the sensor results in reducing failures at the

]0 junction of the lead wire with the sensor because the "dog-bone" is wider

at the ends so the actual junction doesn't become too hot. Additional

turbulence measurements may be made from additional sensors located off

the stagnation line.

To form the sensor 30, an approximately 4000-5000 A thick layer of

]5 copper 50 is deposited by sputtering or vapor deposition onto the sensor

area of the substrate 60. Although copper is preferred, other materials such

as Cr, Ag and Ni can be used which can be selectively etched. A 2.0 pm

thick layer of positive-phase photoresist 70 is then spun onto the copper

layer 50 and baked. A negative of the "dog-bone" shaped sensor pattern

20 is contact printed into the photoresist layer 70 by exposing the photoresist

to ultraviolet light, preferably a highly collimated mercury-vapor light source.

The photoresist is developed, leaving an opening 80 through to the copper

layer 50 with a shape corresponding to the shape of the sensor. The

copper is then etched with a suitable etchant such as ammonium

25 phersulphate to produce an opening in the copper layer shaped like the

sensor pattern. This opening extends to the sapphire substrate 60. The

photoresist 70 is removed and a 150-200 A thick layer of niobium 90 is then

sputter deposited or vapor deposited onto the sapphire substrate 60 through

the opening in the copper layer 50. Although niobium is preferred, other
LAR 15112-1 -8- PATENT APPLICATION

materials such as chromium can be used as long as the material selected is

stable at high temperatures and is essentially nonreactive with adjacent

layers. Deposition by electron beam vapor deposition is preferred because

the vapor flux tends to be directional. The thickness of the niobium layer

5 90 can vary, but must be thick enough to provide an adhesive base and to

ensure that the niobium is contiguous throughout the layer. Without

breaking vacuum, an approximately 2000 A layer of iridium 100 is

deposited on the niobium layer 90 by sputtering or vapor deposition,

although electron bean vapor deposition is preferred. The thickness of the

]0 iridium can be varied to produce a sensor having a desired resistance.

Although iridium is preferred, other materials such as platinum can be used

as long as the material selected is stable at high temperatures, has a

reasonable thermal coefficient of resistance in the temperature range of

interest, and is essentially nonreactive with the underlying layer. The

]5 copper layer 50 is removed with an etchant such as ammonium

phersulphate and the substrate 60 containing the sensor 30 is annealed in

a hard vacuum at approximately 1000°C to stabilize the resistance of the

sensor. The hard vacuum is required because iridium slowly forms volatile

oxides at temperatures above approximately 900°C.

20 The sensor 30 is then connected at each end to a corresponding lead

110 which is electrically connected to the driving circuit of the anemometer

(not shown). Any suitable leads can be used as long as the leads do not

protrude above the surface of the sapphire substrate enough to affect the

airflow. For example, organometallic leads could be painted on, or the leads

25 could be deposited by sputtering or vapor deposition in channels formed in

the substrate. The leads can be any electrically conductive material having

suitable resistance and stability at the operating temperature of the

anemometer.
]0

LAR 15112-1 -9- PATENT APPLICATION

The microphotolithographic technique allows for a significant

reduction in sensor size over previous designs, thus improving the spatial

resolution of the probe. The thermal inertia of the sensor is two orders of

magnitude smaller than that of a conventional 5.0 pm diameter hot wire and

existing hot films. Therefore, a significant improvement in frequency

response is expected. Preliminary results indicate a frequency response of

800 kHz via square-wave injection.

Although the present invention has been described in detail with

respect to certain preferred embodiments thereof, it is understood by those

of skill in the art that variations and modifications in this detail may be made

without any departure from the spirit and scope of the present invention, as

defined in the hereto-appended claims.

]5

What is claimed is:
/0

LAR 15112-1 -1_-- PATENT APPLICATION

MICRO-SENSOR THIN-FILM ANEMOMETER

Abstract

5 A device for measuring turbulence in high-speed flows is provided

which includes a micro-sensor thin-film probe. The probe is formed from a

single crystal of aluminum oxide having a 14 ° half-wedge shaped portion.

The tip of the half-wedge is rounded and has a thin-film sensor attached

along the stagnation line. The bottom surface of the half-wedge is tilted

]0 upward to relieve shock induced disturbances created by the curved tip of

the half-wedge. The sensor is applied using a microphotolithography

technique.
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