L.J. Balk, R.M. Cramer, and G.B.M. Fiege
Thermal Analyses by Means of Scanning Probe Microscopy
Abstract:
The reduction of the lateral dimensions of modern integrated
devices leads to an increase of their power densities. The resulting local
heating could cause malfunctions or the destruction of these devices. Therefore
new techniques for thermal analyses with high spatial and temperature resolution
have to be developed.
Summary:
Since the invention of scanning
probe techniques, several methods have been developed for the characterization
of thermal device properties. All these methods allow a simultaneous detection
of the topography as well as either the thermal distribution or the thermal
diffusivity.
Majumdar et al. created a nanothermocouple junction as a
temperature sensor by using a gold-silicon dioxide-platinum sandwich system
evaporated on an SFM standard cantilever. Based on this physical principle
several techniques such as the measurement of the contact potential between
sample and tip have been developed for SFM (Nonnenmacher et al.). A similar STM
based system (proposed by Stopka et al.) is able to analyze thermal properties
on conductive samples at a constant distance.
However, these techniques only
allow to detect the thermal distribution and not the thermal diffusivity, a
second important thermal property, which can only be performed by local external
heating of the sample. To overcome this problem we are using a bent platinum
wire acting either as a heat source to distinguish the local thermal diffusivity
or as a microscopic thermally resistive tip to measure the temperature
distribution of the sample.
To measure the local thermal diffusivity of a
device, the tip is used as one leg of a Wheatstone bridge and heated up by Joule
heating (Dinwiddie et al.). When the tip is brought into contact with the
sample, the heat flow into the sample will cool down the probe. This cooling
will reduce the tip resistance and unbalance the bridge. A feedback loop will
rise the voltage applied to the bridge in order to rebalance it by heating the
tip. This voltage represents the thermal diffusivity variations across the
surface with nanometer resolution.
For the measurements of the temperature
distribution the applied bridge voltage can be significantly reduced to exclude
self-heating of the tip by using an AC voltage as bridge supply, and lock-in
detection of the bridge output. The probe is scanned in contact mode over the
sample. Due to temperature variations in the sample surface the resistance of
the platinum wire changes and brings the bridge circuit out of balance. The
recorded bridge output voltage is used to produce the two-dimensional
temperature profile.
A higher spatial resolution and a depth profiling can
be performed by modulating the frequency of the heat flux, due to the dependence
of its penetration depth (Balk et al.). So it is possible to distinguish the
three-dimensional temperature distribution and the local thermal diffusivity of
modern devices with high spatial resolution by the resistive probe technique.
These complementary analyses allow to correlate thermal phenomena such as heat
generation and transfer.
Majumdar A. et al., Experimental Heat Transfer 9, p. 83-103 (1996)
Nonnenmacher M. Wickramasinghe H. K., Appl. Phys. Lett. 61, p.168-70 (1992)
Stopka M. et al., Material Science and Engineering B, 24, p. 226-28 (1994)
Dinwiddie R.B. et al., Thermal Conductivity 22, Technomic Publishing Co,
Lancaster, 668-77
Balk L.J. et al., Inst. Phys. Conf. Ser. 146, p. 655-58
Keynote paper at the "IPFA 97, 6th International Symposium
on the Physical and Failure Analysis of Integrated Circuits"
(21.-25.7.1997, Singapore)