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Low-Loss Dielectric Materials Characterization

Circuit designers need dielectric property data for materials at millimeter-wave (mmWave) frequencies to optimize device performance of new hardware and for quality assurance. However, there are no standard traceable reference materials or even agreed upon characterization test methods for materials at mmWave frequencies. Further, at higher frequencies, smaller test samples are required, placing more stringent limits on the allowable dimensional variations in those samples. Without reliable mmWave materials data, manufacturers are forced to extrapolate materials data from low frequencies to high frequencies, which can lead to mistakes that have potentially devastating costs.

Technical Needs, Gaps and Solutions

The technology issues surrounding materials characterization, the associated needs, technology status of those needs, as well as gaps and challenges to overcome, are summarized below. The time period considered is from 2023 to 2033.

Technology Status Legend

For each need, the status of today’s technology is indicated by label and color as follows:

In-table color + label key

Description of Technology Status

Solutions not known

Solutions not known at this time

Solutions need optimization

Current solutions need optimization

Solutions deployed or known

Solutions deployed or known today

Not determined

TBD

Table 1. mmWave Material Characterization Needs, Gaps, and Today’s Technology Status with Respect to Current and Future Needs

 

ROADMAP TIMEFRAME

TECHNOLOGY ISSUE

TODAY

(2023)

3 YEARS

(2026)

5 YEARS

(2028)

10 YEARS

(2033)

CHARACTERIZATION FREQUENCY RANGE

NEED

Tools needed for 28-110 GHz

Tools needed at D-band (110-170 GHz)

Tools needed G-band (220-350 GHz)

Tools needed for >500 GHz

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions need optimization

Solutions not known

GAP

Limited tool availability for high frequencies

Few tool options

Robustness and availability

CHALLENGE

High frequencies place burden on mechanical precision of equipment

Methods still in academic space

CHALLENGE

-

Supporting equipment is expensive (i.e., 100 GHz VNA)

Expensive supporting equipment

CHALLENGE

High equipment cost

ANISOTROPIC MATERIAL CHARACTERIZATION

NEED

Average in-plane or out-of-plane characterizations dominant, little capability for separating in-plane electromagnetic (EM) components

Separate in-plane and out-of-plane EM components (Ex,Ey,Ez) using the same test sample, but different tools; or same tool/different samples

Separate in-plane and out-of-plane EM components (Ex,Ey,Ez) using the same test sample and same tool set

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions need optimization

Solutions not known

GAP

None

Few new fundamentally different methods are in development

CHALLENGE

High frequencies limit flexibility of sample dimensions with known methods

 

Sample geometry incompatibilities across equipment sets

VARIATION IN SAMPLE THICKNESS

NEED

Results generally have 1:1 error with thickness variation

Results less sensitive to thickness variation

Sources of uncertainty not dominated by thickness variation

Sources of uncertainty with only minimal dependence on thickness variation

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions not known

Solutions not known

GAP

Good, uniformly thick samples are not typically available for materials of interest

CHALLENGE

Difficult to get good characterizations if samples are not uniformly thick

Methods with less sensitivity to sample thickness uniformity not known for samples in thin sheet format

No known methods that are good for low-loss materials without sensitivity to sample thickness uniformity

TOOL COMPATIBILITY WITH SAMPLE THICKNESS LIMITATIONS (FOR INDUSTRIAL MATERIAL)

NEED

 (50 um-150 um for Er<5, Freq<60GHz)

Expanded capability thickness range – up to 500 um, 100 GHz

Expanded capability thickness range 1 mm or more, and down to 10 um, up to 100 GHz

Expanded capability thickness range above 1 mm and below 10 um

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions not known

Solutions not known

GAP

None

Techniques in use rely on and assume specific mode structures that limit thickness of samples at higher frequencies

CHALLENGE

Some industries have samples that are not available in thin sheets

Evolutionary methods may be limited to thin samples, may need new methods

Input materials will have significant thickness variation

SAMPLE THICKNESS MEASUREMENT ACCURACY

NEED

Single point thickness +- 3 um reproducibility using micrometers or drop gauges

Accurate non-contact optical methods + 3D point cloud measurements

Easy, accurate, non-contact methods

Easy, inexpensive accurate, non-contact methods

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions need optimization

Solutions not known

GAP

Thickness assessment directly impacts final results, errors in thickness result in significant errors in final result

CHALLENGE

Samples may have spatial thickness variation that needs to be comprehended

Optical thickness systems are uncommon and expensive

 CHALLENGE

Samples may be compliant and compress under micrometer use

 CHALLENGE

Optical methods may be sensitive to sample appearance

THERMAL CHARACTERIZATION

NEED

Characterization at room temperature

Easy characterization over temperature

Increased number of techniques capable of temperature characterization

Extended characterization range <100mK, >300C for space and other applications

CURRENT TECHNOLOGY STATUS

Solutions deployed or known

Solutions need optimization

GAP

None

Few methods/equipment sets compatible with elevated temperature or environmental conditions. 

CHALLENGE

Operator interaction with levers and equipment fixtures at elevated temperatures not compatible with safety requirements

Instrumentation compatibility with extreme temperatures

 

CHALLENGE

Only fast techniques are able to measure humidity soaked samples as thin samples can dry out in several minutes

HUMIDITY CHARACTERIZATION

NEED

Measurement of humidity-soaked samples possible but limited to few equipment types

Humidity characterization capability more routine

Tools widely compatible with humidity characterization

No change in need

CURRENT TECHNOLOGY STATUS

Solutions need optimization

GAP

Humidity characterization is difficult

CHALLENGE

Thin samples can absorb and release environmental moisture quickly - over the span of a few minutes

STANDARD REFERENCE MATERIAL

NEED

No standard

Below 110 GHz discrete frequencies in-plane

Below 110 GHz in-plane and out-of-plane

 

CURRENT TECHNOLOGY STATUS

Solutions not known

 

GAP

No traceable standards exist

 

CHALLENGE

Industry needs to consume SRMs (standard reference materials) to be viable for national labs to support funding

 

OUT OF PLANE QUALITY CONTROL MEASUREMENTS

NEED

Limited accuracy or availability of methods fast enough for quality control.

Faster, QC compatible methods that retain accuracy

Equipment sets designed for fast, accurate QC testing

CURRENT TECHNOLOGY STATUS

Solutions need optimization

GAP

Many QC measurements need out of plane characterizations

Many QC measurements need real-time, out of plane characterizations

CHALLENGE

QC requires rapid, easy and accurate results, ideally non-destructive to the test sample

Approaches to address Needs, Gaps and Challenges

Table 2 considers approaches to address the above needs and challenges. The evolution of these is projected out over a 10-year timeframe using technology readiness levels.

Table 3 presents a range of material characterization techniques relevant for mmWave frequencies. For further information please see the iNEMI project reports [1], [2].

In-table color key

Range of technology readiness levels

Description

2

TRL: 1 to 4

Levels involving research

6

TRL: 5 to 7

Levels involving development

9

TRL: 8 to 9

Levels involving deployment

Table 2. mmWave Materials Characterization Solutions

 

 

EXPECTED TRL LEVEL

TECHNOLOGY ISSUE

POTENTIAL SOLUTIONS

TODAY

(2023)

3 YEARS

(2026)

5 YEARS

(2028)

10 YEARS

(2032)

ANISOTROPIC MATERIAL CHARACTERIZATION

Develop new and disruptive methods for material characterization

3

4

5

9

Converge on common sample geometry

3

5

7

9

SAMPLE THICKNESS VARIATION

Hand-picked samples

9

9

9

9

Use of mechanical methods to modify existing samples to improve thickness uniformity

4

4

4

4

Develop new methods with less sensitivity to thickness variation

1

2

3

5

TOOL COMPATIBILITY WITH SAMPLE THICKNESS LIMITATIONS

Use of mechanical thinning of samples

4

4

4

4

Improve mathematical modeling to enable thicker samples

1

2

3

3

Develop new cavity types or methods that inherently support thicker samples

1

3

5

8

SAMPLE THICKNESS MEASUREMENT ACCURACY

Map of points across samples

5

7

8

9

Use of cost-effective optical methods

1

4

5

7

Use of available traceable standards of different material types
(with differing optical properties)

5

6

6

6

THERMAL CHARACTERIZATION

Design fixturing with thermal considerations and operator safety in mind (120OC)

5

6

7

7

Improve processing and measurement speed

6

8

9

9

HUMIDITY CHARACTERIZATION

Utilize fast measurement techniques to allow humidity soaked samples to be rapidly measured

4

7

8

9

CHARACTERIZATION FREQUENCY RANGE FOR 60-300GHz

Develop new tools and techniques for 60-300 GHz

2

3

4

8

Use of advanced manufacturing techniques

5

5

5

5

Use of customized equipment rather than utilizing an overly capable VNA - Example Q meter

2

5

7

8

STANDARD REFERENCE MATERIAL

Encourage equipment suppliers to supply traceable standards as tool references

4

8

9

9

OUT OF PLANE QUALITY CONTROL MEASUREMENTS

Design custom methods to meet specific QC needs

2

4

5

8

 

 

Table 3. Comparison of common material measurement techniques

Technique [1],[2]

Frequency

Features

2023 TRL

2028 TRL

Comments

Split-post dielectric resonator (SPDR)

Discrete frequency points from 1 GHz up to 15 GHz

  • High measurement precision

  • Single frequency points

  • Easy to use

  • Insensitive to many user errors

  • Typically in-plane component of permittivity

  • Typically extrapolated to 5G mmWaves

  • Typical sample thicknesses less than 1 mm

  • IEC 61189-2-721:2015 [3]

  • Some compatible with thermal chambers

  • Fast measurement time allows sample characterization of moisture absorbed samples

  • Cannot separate in plane X vs Y permittivity

NA

NA

Current (2023) TRL is 8, but limited to below 20 GHz.  Therefore, not applicable for mmWave.

Split-cylinder resonator (SCR)

Discrete frequency points from 10 GHz up to 80 GHz

  • High measurement precision

  • Single frequency points

  • Insensitive to many user errors

  • Typically interpolated to 5G mmWaves

  • Typically averages in-plane components of permittivity

  • Typical sample thicknesses around 100 um

  • Not currently compatible with thermal characterization

  • Fast measurement time allows sample characterization of moisture absorbed samples

  • Cannot separate in plane X vs Y permittivity

  • Simple analytical theory, which is advantageous for error propagation

  • Traceability path only requires physical dimensioning the diameter and length of the cavity halves

  • IPC-TM-650 2.5.5.13 [4]

8

9

Identified as a first candidate method for SRM (standard reference material) development below 110 GHz

Balanced-type circular disk resonator (BCDR)

Multiple discrete frequency points from 10 GHz up to 120 GHz

  • High measurement precision

  • Multiple frequency points

  • Requires full 2-port calibration (mechanical to 110 GHz or electrical to 67 GHz)

  • Typically average out-of-plane component of permittivity

  • Not practical with thermal or environmental characterizations

  • Typical sample thicknesses less than 1 mm

  • Requires two nominally identical samples with identical form factors and thicknesses

  • Complex analytical theory compared to SCR, which makes analytical error propagation challenging

  • Traceability path only requires physical dimensioning of both samples, and all cavity components 

  • IEC 63185 [6]

6

8

 

Fabry-Perot open resonator (FPOR)

Multiple discrete frequencies between 20 GHz up to 120 GHz

  • High measurement precision

  • Multiple frequency points

  • Can be sensitive to user errors

  • Typically in-plane component of permittivity, with potential for separation of in plane X vs Y components

  • Not compatible with thermal or environmental characterizations

  • Complex analytical theory compared to SCR and BCDR, which makes analytical error propagation challenging

  • JIS R1660-2 [6]

7

9

 

Co-planar waveguide (CPW)

Continuous frequencies between 1 kHz up to 1000 GHz

  • Moderate measurement precision

  • User-selected frequency points

  • Can be hard to use

  • Can be sensitive to user errors

  • Requires microfabrication and multiple measurements

  • Requires on-wafer calibration to a known reference impedance

  • Typically a convolution of in-plane and out-of-plane components of permittivity for materials whose thicknesses are comparable to the ground to ground spacing

  • Not practical for development of a traceability path

4

7

 

Microstrip line charaterization

 

  • Moderate measurement precision

  • User-selected frequency points

  • Can be hard to use

  • Can be sensitive to user errors

  • Requires microfabrication and multiple measurements

  • Requires on-wafer calibration to a known reference impedance

  • Requires vias

  • Typically the out-of-plane components of permittivity for materials whose thicknesses are comparable to the ground to ground spacing

4

6

 

Bereskin stripline transmission line method

Continuous frequencies between 1 GHz up to 20 GHz

  • Qualitative measurement precision

  • User-selected frequency points at continuous frequencies between 1 kHz up to 20 GHz

  • Recommended for quality assurance rather than permittivity measurements

NA

NA

Current (2023) TRL is 7, but lacks the necessary precision. 

On-chip resonators and microstrip-ring resonators (MRR)

Discrete frequencies between 1 GHz up to 50 GHz

  • Moderate measurement precision.

  • Can be hard to use

  • Can be sensitive to user errors

  • Requires microfabrication and multiple measurements

  • Requires on-wafer calibration to a known reference impedance

3

5

 

Time-domain materials characterization techniques

15 GHz - 150 GHz    

  • Limited measurement precision

  • User-selected frequency points within antenna measurement bands

  • Sensitive to many user errors

  • Requires flat samples

  • Compatible with thermal characterization.

  • Fast measurement time allows sample characterization of moisture absorbed samples

  • Can separate in plane X vs Y permittivity

  • Complex analytical theory, which makes analytical error propagation challenging

3

5

A few companies advertise THz systems – further assessment needed.

Free-space materials characterization techniques

Up to 1.1 THz

  • Limited measurement precision

  • User-selected frequency points within antenna measurement bands

  • Sensitive to many user errors

  • Requires surface flatness to be much less than the wavelength at the highest measurement frequency

  • Compatible with thermal characterization.

  • Fast measurement time allows sample characterization of moisture absorbed samples

  • Can separate in plane X vs Y permittivity

  • Complex analytical theory, which makes analytical error propagation challenging

3

4

 

(*) TRL = technology readiness level, a readiness metric between 1 and 9


References

  1. iNEMI, “iNEMI 5G Project Report 1: Benchmark Current Industry Best Practices for Low Loss Measurements”, Nov. 2020.

  2. iNEMI, “iNEMI 5G Project Report 2: Benchmark Emerging Industry Best Practices for Low Loss Measurements”, Nov. 2020.

  3. IEC TC 91 - Electronics assembly technology, “Test methods for electrical materials, printed boards and other interconnection structures and assemblies - Part 2-721: Test methods for materials for interconnection structures - Measurement of relative permittivity and loss tangent for copper clad laminate at microwave frequency using a split post dielectric resonator,” IEC 61189-2-721:2015, 29 April 2015.

  4. IPC High Frequency Resonator Test Method Task Group (D-24c), “Relative Permittivity and Loss Tangent Using a Split-Cylinder Resonator“, IPC-TM-650 2.5.5.13, January 2017.

  5. IEC TC 46/SC 46F - RF and microwave passive components, “Measurement of the complex permittivity for low-loss dielectric substrates balanced-type circular disk resonator method,” IEC 63185:2020, 8 December 2020.

  6. JIS, “Measurement method for dielectric properties of fine ceramics in millimeter wave frequency range - Part 2: Open resonator method”, JIS R 1660-2-2004 (R 2008)(R 2013), 20 March 2004.


As part of the 5G/6G MAESTRO project, work on this page is supported by the Office of Advanced Manufacturing in the National Institute of Standards and Technology (NIST), under the Federal Award ID Number 70NANB22H050

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