Difficulties and TDTR Solutions of Thermal Conductivity Measurement of Diamond Thin Films

Hardest known material, highest thermal conductivity at room temperature, wide light transmission range, hardest material, least compressible, and chemically inert to most substances is enough to make it highly regarded, So it's not all that surprising that diamond is often called the "ultimate engineering material"

Physical properties of some diamonds

Although it turns out that the process is very difficult

Although the standard enthalpy difference between graphite and diamond is only 2.

But finally, in 1992, a growth technique called HPHT (high-pressure high-temperature) emerged and was released by General Electric as the standard technique that has been used to produce industrial diamonds for decades

In this process, graphite is compressed to tens of thousands of atmospheres in a hydraulic press, heated to over 2000 K in the presence of a suitable metal catalyst, until the diamond crystallizes

The disadvantage of the HPHT method is that it can only produce nanoscale to millimeter-scale single crystal diamonds, which limits its application range

Until the advent of chemical vapor deposition (CVD) production methods of diamond and thin films of diamond, this form of diamond could allow more of its superlative properties to be exploited

Chemical vapor deposition, as the name suggests, involves mainly gas-phase chemical reactions that take place above a solid surface, resulting in deposition onto that surface

The following figure shows some of the more common preparation methods

The resulting film is a polycrystalline product with many grain boundaries and defects, and exhibits a columnar structure extending upward from the substrate

However, as the film becomes thicker, the crystal size increases while the number of defects and grain boundaries decreases

When the diamond film is used as a thermal management heat sink as mentioned below, the film is usually separated from its substrate, and the bottom 50-100 um is removed by mechanical polishing

On the one hand, in terms of thermals, diamond has the highest thermal conductivity (1000~2000W/(m·K)) among natural substances known so far, which is 4 times larger than that of silicon carbide (SiC) and larger than that of silicon (Si).

The following figure shows the thermal conductivity parameters of common materials and diamond materials:

On the other hand, the performance of synthetic diamond films is often far below this high level
.

And in terms of day-to-day performance, modern high-power electronic and optoelectronic devices (5G applications, heat dissipation of semiconductor chips, etc.
) face serious cooling problems due to the large amount of heat generated in a small area
.
For fast cooling, some heat sinks/heat-dissipating coatings made of high thermal conductivity materials are often needed on the heating and cooling ends (radiators, fans, heat sinks, etc.
)

CVD diamond has thermal conductivity far superior to copper over a wide temperature range, and it also has the advantage of being electrically insulating
.

As early as 1996 Werner et al.
were able to use large-area CVD diamond plates with thermal conductivity of about 2 W mm-1 K-1 for various thermal management applications
.
These include substrates for integrated circuits (Boudreaux 1995), heat sinks for high-power laser diodes (Troy 1992), and even as substrate materials for multi-chip modules (Lu 1993)
.
This allows the device to run at higher speeds because the device can be packed more tightly without overheating
.
And device reliability is also expected to improve, since for a given device, the confluence will be lower when mounted on diamond
.

Compared with the popular graphene, diamond also has its unique advantages
.

Thermal conductivity characterization of diamond thin films is not a simple problem, especially when the film thickness is very thin

DARPA's Electronic Thermal Management Diamond Thin Film Heat Transport project once brought together researchers from five universities to fully characterize the thermal transport and material properties of CVD diamond thin films for further improvement The heat transfer characteristics show the challenges of processing optimization on the application side
.

 

Among them, femtosecond high-speed thermal reflectometry (FSTR) (also known as femtosecond time domain thermal reflectometry (TDTR) test system) for thermal conductivity measurement of materials with special requirements plays an extremely important role, which is often used in accurate measurement.
The study of thermal conductivity of micrometer-thick anisotropic films with high surface roughness, and in some cases thermal conductivity and thermal boundary improvement of CVD diamond films for thermal management applications in high-power electronic devices According to attractive research played a decisive guiding role
.

Common thermal testing methods for materials include laser flash method, 3-Ω method, steady-state four-probe method, suspended electric heating method, Raman thermal imaging method, time domain thermal reflectometry (TDTR),
etc.

 

For thermal measurements of CVD diamond films, which may require multi-layer analysis, fine spatial resolution, high-precision analysis, and the ability to resolve film properties and interfaces during the process, femtosecond high-speed thermal reflectometry (FSTR) ( Also known as the Femtosecond Time Domain Thermal Reflectometry (TDTR) test system) has become one of the most commonly used thermal conductivity measurement methods in the past decade
.

Femtosecond High Speed ​​Thermal Reflectometry (FSTR), also known as Femtosecond Time Domain Thermal Reflectometry (TDTR), is used to measure thermal conductivity in the range of 0.
1 W/mK to 1000 W/mK and beyond
 

The system is suitable for various sample measurements, such as polymer films, superlattices, graphene interfaces, liquids,
etc.

In summary, femtosecond high-speed thermal reflectometry (FSTR) is a pump-probe photothermal technique that uses an ultrafast laser to heat a sample and then measure its temperature response within a few nanoseconds
.
The pump (heating) pulses are modulated over a range of frequencies, which not only controls the depth of heat penetration into the sample, but also allows the use of lock-in amplifiers to extract surface temperature responses with higher signal-to-noise ratios
.

 

The probe light (temperature sensing) pulse is passed through a mechanical stage that can delay the arrival of the probe relative to the pump pulse in the range of 0.
1 to several ns to obtain a temperature decay curve
.

As mentioned above, typical diamond samples are rough, non-uniform and of varying thickness characteristics due to growth characteristics

This presents some challenges for thermal measurement of CVD diamond thin films by femtosecond high-speed thermal reflectometry (FSTR)
.

 

Specifically, the rough surface will affect the collection of probe light by reflection, and the actual surface shape is non-planar due to the rough surface, which will also introduce additional errors to the theoretical thermal transfer modeling analysis.
The samples were polished to reduce surface roughness, but still had to deal with film inhomogeneities and differences in isotropic properties
.

For anisotropic materials, exact analytical solutions for 2D and 3D anisotropy exist, but this makes the determination of thermal conductivity and thermal boundary resistance more difficult, with additional unknown properties
.

Even though there is always an unknown boundary thermal resistance in the sample and between the conductive layer aluminum molds, usually two unknown properties can be extracted from the sample using a single modulation frequency, which means that in most cases the measurement can extract the layer thermal conductivity
.

 

However, for diamond samples, the longitudinal and transverse thermal conductivities within the sample are different, which means that additional measurements are required to extract these two properties; this can be corrected by changing some system parameters, see description of system parameters (for details contact Please ask Shanghai Haoliang Optoelectronics)
.

Another difficulty is determining the heat capacity of diamond CVD, which varies widely depending on the growth quality and the amount of non-diamond carbon (NDC) present in the sample
.
In this case for diamond films <5 um, the measurement will penetrate completely through the diamond sample, reaching the sample to the underlying substrate material
 

(The TDTR measurement and analysis methods of diamond thin films in different situations in the above picture will be very different)
 

This makes the measurement sensitive to diamond-substrate boundary resistance as well
.
This means that the measurement may have a total of five unknown parameters: 1) aluminum film-diamond boundary thermal resistance, 2) intra-diamond transverse thermal conductivity, 3) intra-diamond longitudinal thermal conductivity, 4) diamond heat capacity, 5) diamond- Boundary thermal resistance between substrate materials
 

Even if combined with certain analysis and processing methods, see the equipment description (please contact Shanghai Haoliang Optoelectronics for details), it is difficult to accurately extract all unknown parameters
.

It is important to confirm the sample size of the measurement relative to the sample size; femtosecond high-speed thermal reflectometry (FSTR) is usually based on standard bulk material heat transfer modeling, while some measurements are now made with increasingly For single crystal semiconductors, the assumptions of heat transfer models based on bulk materials are valid, but for more defective and heterogeneous materials, such as CVD diamond, this assumption is only an approximation
.

Typically, during diamond growth, the grain gradients are very large, which can also lead to very large thermal conductivity gradients
.

In addition, local variations in non-diamond carbon (NDC) content, grain size or surface roughness may also affect local measurements of thermal conductivity
.
In the TDTR measurement, the heating depth can be controlled by controlling the modulation frequency, thereby realizing the sampling depth control (for detailed technical discussion, please contact Shanghai Haoliang Optoelectronics)

For samples with different thermal conductivity and different heating frequencies, the sampling in the measurement film may vary from 1-2 um to 20 um (correspondingly, the film thickness exceeds 300 microns)
 

More challenges and technical details, limited by space, will continue to be discussed in subsequent updates.
If you are interested in communicating about related equipment and technical issues, you can contact Shanghai Haoliang Optoelectronics for more information
.

About Haoliang Optoelectronics:

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, Ltd.
is a professional agent of well-known optoelectronic products in China, and it is also an optoelectronic product agent enterprise that has developed rapidly in recent years
.
In addition to having a group of professional technical sales engineers, we also have a strong technical support team
.
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.
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