Thermal sensing layer for microbolometer and method of making the same

Abstract

The thermal sensing layer for a microbolometer includes a Ge.sub.1-xSn.sub.x film layer, where 0.17x0.25. The Ge.sub.1-xSn.sub.x film layer may be deposited on a substrate layer, such as pure silicon. An additional layer of silicon dioxide may be added, such that the silicon dioxide layer is sandwiched between the silicon substrate and the Ge.sub.1-xSn.sub.x film, In order to make the Ge.sub.1-xSn.sub.x thin film layer, germanium (Ge) and tin (Sn) are simultaneously sputter deposited on the substrate, where the atomic ratio of germanium to tin is between 0.83:0.17 and 0.75:0.25 inclusive. The sputter deposition may occur in an argon atmosphere, with the germanium having a deposition rate of 9.776 nm/min, and with the tin having a deposition rate between 2.885 nm/min and 4.579 nm/min.

Claims

1. A thermal sensing layer for a microbolometer, comprising a thin film of amorphous Ge.sub.1-xSn.sub.x, wherein 0.17x0.25.

2. The thermal sensing layer for a microbolometer as recited in claim 1, further comprising a pure silicon substrate layer, wherein said Ge.sub.1-xSn.sub.x film is deposited on said substrate layer.

3. The thermal sensing layer for a microbolometer as recited in claim 2, further comprising a silicon dioxide layer, the silicon dioxide layer being sandwiched between the silicon layer and the GE.sub.1-xSn.sub.x film.

4. The thermal sensing layer for a microbolometer as recited in claim 1, wherein the Ge.sub.1-xSn.sub.x film comprises Ge.sub.0.83Sn.sub.0.17.

5. The thermal sensing layer for a microbolometer as recited in claim 1, wherein the Ge.sub.1-xSn.sub.x film comprises Ge.sub.0.78Sn.sub.0.22.

6. The thermal sensing layer for a microbolometer as recited in claim 1, wherein the Ge.sub.1-xSn.sub.x film comprises Ge.sub.0.75Sn.sub.0.25.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a side view in section of a thermal sensing layer for a microbolometer.

(2) FIG. 2 is a graph showing electron dispersive X-ray (EDX) spectroscopy results for samples of the thin film layer with differing germanium to tin ratios, and for a control sample of pure germanium.

(3) FIG. 3 is a plot of sheet resistance as a function of temperature for the samples of the thin film layer with differing germanium to tin ratios,

(4) FIG. 4 is a plot of sheet resistance as a function of temperature for the control sample of pure germanium.

(5) FIG. 5 is a plot showing temperature coefficient of resistance (TCR) and resistivity as a function of tin concentration for the samples of the thin film layer with differing germanium to tin ratios, and for the control sample of pure germanium.

(6) FIG. 6A is an atomic force micrograph of a control sample of pure germanium.

(7) FIG. 6B is an atomic force micrograph of a sample of the thin film for a microbolometer with an atomic ratio of germanium to tin of 0.83:0.17.

(8) FIG. 6C is an atomic force micrograph of a sample of the thin film for a microbolometer with an atomic ratio of germanium to tin of 0.78:0.22.

(9) FIG. 6D is an atomic force micrograph of a sample of the thin film for a microbolometer with an atomic ratio of germanium to tin of 0.75:0.25.

(10) Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) A thermal sensing layer for a microbolometer includes a semi-conducting thin film layer including amorphous germanium tin (GeSn). The thin film layer can have a thickness of about 200 nm. The GeSn alloy can be Ge.sub.1-xSn.sub.x, where 0.17x0.25. As shown in FIG. 1, the Ge.sub.1-xSn.sub.x film layer 10 may be deposited on a substrate layer 14, such as pure silicon. An additional layer of silicon dioxide 12 may he added, such that the silicon dioxide layer 12 is sandwiched between the silicon layer 14 and the Ge.sub.1-xSn.sub.x film 10. As a non-limiting example, the Ge.sub.1-xSn.sub.x film layer 10 may have a thickness of approximately 200 nm, the silicon dioxide (SiO.sub.2) layer 12 may have a thickness of approximately 300 nm, and the silicon (Si) layer 14 may have a thickness of approximately 385 m.

(12) In order to make the Ge.sub.1-xSn.sub.x thin film layer, germanium (Ge) and tin (Sn) are simultaneously sputter deposited on the substrate, where the atomic ratio of germanium to tin is between 0.83:0.17 and 0.75:0.25 inclusive. The sputter deposition may occur in an argon atmosphere, with the germanium having a deposition rate of 9.776 nm/min, and with the tin having a deposition rate between 2.885 nm/min and 4.579 nm/min.

(13) In experiments, Ge.sub.1-xSn.sub.x thin films were deposited on silicon substrates topped with 300 nm of thermally grown silicon dioxide (SiO.sub.2), which is provided as electrical insulation. The thickness of each deposited Ge.sub.1-xSn.sub.x thin film was targeted to be 200 nm. The Ge.sub.1-xSn.sub.x thin films were synthesized by simultaneous sputter deposition from a 99.999% pure Ge target and a 99.99% pure Sn target. All depositions were made at room temperature at an argon pressure of 5 mTorr and at a chamber base pressure of 1.810.sup.6 Torr. Germanium was sputter deposited using 280 W of RF power at a deposition rate of 9.776 nm/min. Tin was sputter deposited at three different DC powers: 10 W, 15 W and 20 W, respectively corresponding to deposition rates of 2.885, 3,932 and 4.579 nm/min. In this manner, three different Ge.sub.1-xSn.sub.x thin films samples were prepared, each having a different Sn concentration. In addition, one reference Ge thin film sample, having a thickness of 200 nm, was also prepared.

(14) Electron dispersive X-ray (EDX) spectroscopy was used to determine the elemental composition of the Ge and Ge.sub.1-xSn.sub.x thin films. The measured EDX spectra for the synthesized thin films are shown in FIG. 2. The measured spectra show the presence of Ge L.sub.. Ge L.sub., Ge K.sub., and Ge K.sub. peaks, in addition to Sn L.sub. and Sn L.sub. peaks. The Sn L.sub. and Sn L.sub. peaks were absent, as expected, in the Ge thin film. All measured spectra showed (O) K.sub. peaks, which are due to the underlying SiO.sub.2 layer. The spectra also showed the typical artificial weak (C) K.sub. peaks. It should be noted that the EDX spectra shown are intentionally broken to hide the Si peak due its high intensity, ensuring display clarity for the Ge and Sn peaks. Further, the EDX analysis revealed Sn atomic concentrations of 17%, 22%, and 25% in the Ge.sub.1-xSn.sub.x thin films prepared at Sn deposition rates of 2.885, 3.932, and 4.579 nm/min, respectively, Additionally, the Sn atomic concentration was measured to be 0% in the Ge reference sample.

(15) Atomic force microscopy (AFM) analysis was also performed to examine the surface morphology of the prepared Ge and Ge.sub.1-xSn.sub.x thin films. In general, a low surface roughness is desirable, as it leads to suppressing surface effects, such as dangling bonds at material interfaces which result in lower flicker noise. AFM measurements were made in 1 m1 m scanning areas. The measured rms surface roughnesses, R.sub.q, were 0.56 nm, 0.55 nm, 0.465 nm, and 0.327 for the Ge (shown in FIG. 6A), Ge.sub.0.83Sn.sub.0.17 (shown in FIG. 6B), Ge.sub.0.78Sn.sub.0.22 (shown in FIG. 6C), and Ge.sub.0.75Sn.sub.0.25 (shown in FIG. 6D) samples, respectively. The average surface roughnesses, R.sub.a, were measured to be 0.755 nm, 0.435 nm, 0.369 nm, and 0.257 nm for the Ge, Ge.sub.0.83Sn.sub.0.17, Ge.sub.0.78Sn.sub.0.22, and Ge.sub.0.75Sn.sub.0.25 samples, respectively. It was observed that the decrease in Ge concentration causes a decrease in surface roughness. In general, the measured surface roughnesses of the studied samples are considered to be low and would qualify the thin films to be used in making thermometer layers in microbolometers.

(16) Sheet resistance versus temperature measurements were performed in order to evaluate the thermal sensing properties of the synthesized Ge and Ge.sub.1-xSn.sub.x alloy thin films. The thin film samples were placed on a hot plate, which allowed the temperature to be varied from 293 K to 345 K in 2 K steps, The sheet resistance was measured using a four-point probe tool. The sheet resistance versus temperature measurements for the Ge.sub.1-xSn.sub.x thin films and the Ge reference thin film sample are plotted in FIGS. 3 and 4, respectively. The measured sheet resistances of the Ge.sub.1-xSn.sub.x thin films were found to be inversely proportional to the thin films' temperature, indicating the existence of a semiconducting behavior in all samples. The plotted sheet resistance versus temperature curves were fitted with an exponential decay fit function, confirming that the semiconducting behavior of the thin films follows the Arrhenius relationship: R(T)=R.sub.0e.sup.E/T, where R and T are the resistance and temperature of the semiconducting material, respectively, R.sub.0 is a constant, E is the activation energy, and is Boltzmann's constant.

(17) It can be seen that the Ge.sub.1-xSn.sub.x alloy's sheet resistance values decrease as the Sn concentration increases, which can be attributed to the increase in the metallic Sn content in the thin film. Room temperature (299 K) sheet resistance values varied from 24.36, 8.23, 3.457, and 2.273 M/sq for the Ge, Ge.sub.0.83Sn.sub.0.17, Ge.sub.0.78Sn.sub.0.22, and Ge.sub.0.75Sn.sub.0.25 samples, respectively. This corresponds to room temperature resistivities of 487.2, 164.6, 69.14 and 45.46 .Math.cm for the Ge, Ge.sub.0.83Sn.sub.0.17, Ge.sub.0.78Sn.sub.0.22, and Ge.sub.0.75Sn.sub.0.25 samples, respectively. Further, the activation energies were extracted from the measured resistance versus temperature data, where they represent the slopes of the Arrhenius plots of In(R). versus 1/T curves. The extracted activation energies E were found to be 0.342 eV, 0.312 eV, 0.28 eV and 0.253 eV for the Ge, Ge.sub.0.83Sn.sub.0.17, Ge.sub.0.78Sn.sub.0.22, and Ge.sub.0.75Sn.sub.0.25 samples, respectively. The TCRs were then calculated as

(18) $\mathrm{TCR}=\frac{1}{R}.Math.\frac{\mathrm{dR}}{\mathrm{dT}}=-\frac{E}{{\mathrm{kT}}^2}.$

(19) Accordingly, the room temperature TCRs were found to be 4.45, 3.96, 3.63 and 3.79%/K for the Ge, Ge.sub.0.83Sn.sub.0.17, GE.sub.0.78Sn.sub.0.22, and Ge.sub.0.75Sn.sub.0.25 samples, respectively. The room temperature TCRs were found to decrease as the Sn content in the thin film increases. FIG. 5 depicts the calculated TCR and the measured resistivity versus Sn concentration in the Ge.sub.1-xSn.sub.x alloy thin films.

(20) It is to be understood that the thin film for a microbolometer and method of making the same is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.