**5.2 TPD spectral analyses of sol-gel-derived HfO2 thin films**

The desorption of H2O (*m*/*z* = 18) that evolved from sol-gel-derived HfO2 films on Si was analyzed by TPD (Figure 9). The HfO2 films were fired at 450, 550, and 700 °C for 30 min. The vertical axis indicates the current value of QMS. The small peaks below 200 oC are due to the physisorbed H2O (mere adsorption of H2O) on the surface of the HfO2 films and/or chemisorbed Hf-OH in the bulk at the surface area. Based on experiments, the small peaks are attributed to the adsorption of

H2O immediately after the samples were taken out of the furnace and the amount of desorbed water (i.e., adsorbed water) saturated. The desorption states of physisorbed H2O and/or chemisorbed Hf-OH bonds originate from liquid-like water, water molecules, and Hf-OH bonds at nanopore sites in the HfO2 films.

Regarding the major peaks in the TPD spectra, two types of desorption curves are observed at temperatures higher than 200 °C. One has the form of a symmetrically shaped peak (Lorentzian distribution shoulder as shown by the dashed line) at around 320 °C (fired at 450 °C), which is reaction-controlled (Soraru, 2002) (the second-order reaction) (Nishide et al. 2004). The other consists of nonsymmetrical peaks at approximately 420 °C (fired at 550 °C) and 480 °C (fired at 700 °C) which are diffusion-controlled (the first order reaction) (Nishide et al., 2004). When the curve is symmetrical in shape, the peak at around 320 °C (fired at 450 °C) is not caused by physisorbed H2O from the nanopores at the surface area, but can be ascribed to the associated desorption of chemisorbed water (Hf-OH) from the gel film, resulting in the formation of H2O during firing, which is specified to be the secondorder reaction (Nishide et al., 2004).

For the samples fired at 450 °C, Hf-OH bonds in the HfO2 film bulk convert to HfO2 and/or H2O during heating and the resulting H2O contributes to the major desorption peak in the TPD curve [Figure 10(a) and 10(b)], as a result of the following reaction

$$\equiv \text{Ht-OH-HO-Hf} \equiv \rightarrow \equiv \text{Ht-O-Hf} \equiv \, + \, \text{H2O} . \tag{1}$$

Characterization of Sol-Gel-Derived and Crystallized

**ZrO2 thin films** 

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 325

In contrast, most of the HfO2 films fired at 700 °C crystallized, where Hf-OH bonds in the films are conjectured to be tightly locked between crystallized grains and incorporated H2O needs greater energy to percolate by the diffusion control mechanism through small gaps between grains. Therefore, in the TPD curve, the H2O desorption peak shifts to higher

**5.3 Adsorption mechanism of physisorbed H2O clarified by TPD using sol-gel-derived** 

To clarify whether the small peaks (small protrusions) between 100 and 200 °C in the TPD curves were due to physisorbed H2O (mere adsorption of H2O) on the surface of the ZrO2 thin films and/or chemisorbed Zr-OH in the bulk at the surface area (Figure 11), the following three experiments were carried out (Shimizu et al., 2009). First (process ①), a sample immediately after firing at 350 °C for 30 min was measured by TPD until 350 °C and then (process ②) successively measured by TPD again from room temperature to 350 °C; finally (process ③), the sample was exposed to air for 59 h and then measured by TPD from room temperature to 700 °C. The TPD curve of H2O in process ① was in good agreement with the typical curve of a ZrO2 thin film fired at 350 °C for 30 min. No peaks were observed in process ②, indicating that the small protrusions and major peak vanished during heating

Fig. 11. TPD curves for processes (1), (2), and (3). In process (1), a sample immediately after firing at 350 °C for 30 min was measured by TPD until 350 °C, followed by process (2) in which the sample was successively measured by TPD again from room temperature to 350 °C; finally, process (3) in which the sample was exposed to air for 59 h and then measured by

TPD from room temperature to 700 °C (Shimizu et al., 2009).

temperatures and decreases steadily as the firing temperature increases.

Fig. 9. TPD curves of H2O (*m/z* = 18) that evolved from sol-gel-derived HfO2 films on Si when fired at 450, 550, and 700 °C for 30 min (Shimizu et al., 2007).

Fig. 10. A schematic speculation of H2O desorption from HfO2 films during TPD measurements for both amorphous and crystalline states. (a) Hf-OH bonds in the sol-gel derived HfO2 films and (b) the formation of desorbed H2O as a result of the following reaction: ≡ Hf-OH HO-Hf ≡ → ≡ Hf-O-Hf ≡ + H2O (Shimizu et al., 2007).

Fig. 9. TPD curves of H2O (*m/z* = 18) that evolved from sol-gel-derived HfO2 films on Si

<sup>O</sup> Hf

O

Hf

Hf

O O

O

O

H2O

Hf

O

Hf

 (a) (b) Fig. 10. A schematic speculation of H2O desorption from HfO2 films during TPD

Hf

O

reaction: ≡ Hf-OH HO-Hf ≡ → ≡ Hf-O-Hf ≡ + H2O (Shimizu et al., 2007).

measurements for both amorphous and crystalline states. (a) Hf-OH bonds in the sol-gel derived HfO2 films and (b) the formation of desorbed H2O as a result of the following

when fired at 450, 550, and 700 °C for 30 min (Shimizu et al., 2007).

Hf

OH

Hf

Hf

O O

OH

O

Hf

In contrast, most of the HfO2 films fired at 700 °C crystallized, where Hf-OH bonds in the films are conjectured to be tightly locked between crystallized grains and incorporated H2O needs greater energy to percolate by the diffusion control mechanism through small gaps between grains. Therefore, in the TPD curve, the H2O desorption peak shifts to higher temperatures and decreases steadily as the firing temperature increases.

## **5.3 Adsorption mechanism of physisorbed H2O clarified by TPD using sol-gel-derived ZrO2 thin films**

To clarify whether the small peaks (small protrusions) between 100 and 200 °C in the TPD curves were due to physisorbed H2O (mere adsorption of H2O) on the surface of the ZrO2 thin films and/or chemisorbed Zr-OH in the bulk at the surface area (Figure 11), the following three experiments were carried out (Shimizu et al., 2009). First (process ①), a sample immediately after firing at 350 °C for 30 min was measured by TPD until 350 °C and then (process ②) successively measured by TPD again from room temperature to 350 °C; finally (process ③), the sample was exposed to air for 59 h and then measured by TPD from room temperature to 700 °C. The TPD curve of H2O in process ① was in good agreement with the typical curve of a ZrO2 thin film fired at 350 °C for 30 min. No peaks were observed in process ②, indicating that the small protrusions and major peak vanished during heating

Fig. 11. TPD curves for processes (1), (2), and (3). In process (1), a sample immediately after firing at 350 °C for 30 min was measured by TPD until 350 °C, followed by process (2) in which the sample was successively measured by TPD again from room temperature to 350 °C; finally, process (3) in which the sample was exposed to air for 59 h and then measured by TPD from room temperature to 700 °C (Shimizu et al., 2009).

Characterization of Sol-Gel-Derived and Crystallized

(Shimizu et al., 2010).

conventional HfO2 films.

HfO 2

Refractive index

(Shimizu et al., 2010).

350 400 450 500 550 600 650 700

Thickness

Firing Temperature (o

1.2

1.4

1.6

1.8

Refractive index

2

2.2

2.4

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 327

a HNO3 solution as the catalyst, the crystallization temperature was reduced to less than 470 °C compared with 560 °C for the "HCOOH sol" HfO2 films. The lattice interplanar distances calculated using the Bragg equation were 0.319 and 0.286 nm, in contrast to the reported values of 0.314 and 0.288 nm, respectively (Shimizu et al., 2004). These results probably differed from the crystallization temperature for the monoclinic structure (1

(Nishide et al., 2000) because different sol solutions were employed in each case. The "HCOOH sol" HfO2 films remained in the amorphous state up to a higher temperature (560 °C) than the "HNO3 sol" films (crystallized at 470 oC). Based on TPD measurements, HCOOH and HNO3 desorb at temperatures below 350 oC, indicating that an intrinsic amorphous HfO2 film without using a catalyst for either film stably exists above 350 oC

**6.2 Thicknesses and refractive indexes dependent on sol solution of HfO2 thin films**  The thickness of the sol-gel-derived HfO2 films decreased with increasing firing temperature (Figure 13). It is seen that the smallest thickness was 6 nm for the "HCOOH sol" HfO2 film fired at 700 °C, which is about 1 nm thinner than the thinnest "HNO3 sol" HfO2 film. The difference is due to the properties of the catalyst used and this result shows that the "HCOOH sol" HfO2 film may be suitable for use as the gate insulator of highly integrated CMOS devices. However, its electrical performance should be superior to that of

(a) (b)

1.2

1.4

1.6

1.8

Refrective index

2

2.2

2.4

The refractive indexes began to increase at approximately 550 oC for the "HCOOH sol" film and at 450 oC for the "HNO3 sol" film. These temperatures are in good agreement with those at which crystallization occurs, as obtained by XRD analysis [Figures 12(a) and 12(b)]. The maximum refractive indexes obtained were 1.70 for the "HCOOH sol" film and 1.95 for the "HNO3 sol" film, although the reported value for the HfO2 crystal (monoclinic) is 2.19. The

Fig. 13. Thicknesses and refractive indexes of sol-gel-derived HfO2 films based on both (a) "HCOOH sol" and (b) "HNO3 sol" fired at 350, 450, 550, and 700 °C for 30 min in air

C)

HfO2 Thickness (nm)


C)

Refractive index

HfO2 Thickness (nm)

350 400 450 500 550 600 650 700

Firing Temperature (<sup>o</sup>

HfO 2

Thickness

until 350 °C in the first TPD measurement. In contrast, in process ③, the small protrusions between 100 and 200 °C appeared again. This result provides evidence that the small peaks (small protrusions) were caused by adsorption of H2O immediately after the samples were taken out of the furnace and that the amount of desorbed water (i.e., adsorbed water) saturated during the exposure time. Thus, the small protrusions in the TPD curves can be attributed to physisorbed H2O and/or chemisorbed Zr-OH bonds at the surface area.
