8-2-1 Thickness Analysis
Optical measurement has its detection limit on precise thin film thickness determination especially in the case of multi-layer structure with similar optical properties for each layer.
The HfO2/interfacial layer stack structure is one of the examples. Cross-sectional TEM (x-TEM) inspection is the most common and precise method to determine thin film thickness.
300 sccm and the pressure is 1.5mbar. The measured thickness of HfO2 layer and interfacial layer are plotted as Fig.8-1(e). The scale is calibrated with the lattice image of the Si substrate.
It is observed that the thickness of HfO2 layer increases after 600℃ PDA and tends to saturate after annealing at higher temperatures. It is postulated that the as-deposited HfO2 film is metal-rich because the Hf-precursor may not decompose completely at 400℃ so that the HfO2 layer grows continuously during PDA. Once all hafnium atoms react with oxygen, the HfO2 film thickness becomes saturated. This postulation will be discussed again in the subsection D.
The thickness of interfacial layer increases with the increase of PDA temperature monotonically. Because HfO2 is a poor diffusion barrier for oxygen, oxygen may diffuse through HfO2 layer and reacts with silicon or hafnium atoms to form SiO2-like or silicate-like interfacial layer during PDA. The increase of HfO2 layer thickness (from 3.2nm to 4.2nm) is larger than the increase of interfacial layer (from 1.85nm to 2.0nnm) after 600℃ PDA, which explains the decrease of CET as seen in Chapter3 results. Furthermore, the large increase of interfacial layer thickness after 1000℃ PDA accounts for the apparent decrease of leakage current.
Electron probe microanalysis (EPMA) with energy dispersion spectrometer (EDS) was employed to identify the composition of interfacial layer. Fig.8-2 shows the high resolution x-TEM micrograph of the sample deposited at 400℃ followed by a PDA at 1000℃ in N2 ambient. The positions at where EPMA were performed are indicated on the micrograph. The EDS spectrum at the lower portion of interfacial layer shows significant Hf signal as shown in Fig.8-2(b). Although the spatial resolution of EPMA is not good enough so that the Hf signal may come from the HfO2 layer, it is still suspected that the interfacial layer is Hf-silicate but not SiO2 because the Hf signal is very strong.
Fig.8-3 shows the X-TEM micrographs of samples deposited at 400℃ on NH3-treated wafer. The O2 flow rate is 500sccm and the pressure is 5mbar. As stated in chapter 2, the thickness of surface layer after NH3 treatment is 1nm measured by ellipsometry method.
Using the Si lattice image as scale, the measured thickness of HfO2 layer and interfacial layer from the TEM micrographs are plotted as Fig.8-3(d). The phenomenon of apparent increase of HfO2 layer after PDA shown in Fig.8-1(e) is not observed in this case. It might be due to the sufficient supply of O2 during deposition and thus the amount of un-reacted Hf is reduced.
similar to that of the 400℃ deposited samples but the leakage current has a difference of several orders of magnitude. This observation concludes that the leakage current reduction of samples deposited at 500℃ can not be explained by the formation of interfacial layer. Another important observation is that the interfacial layer of HfO2 film deposited on NH3-treated wafer is thinner than that on HF-treated wafer. This observation implies that the surface treatments used in this thesis are not efficient enough. Therefore, more efficient surface treatment method must be developed to scale-down the CET to sub-1nm regime.
8-2-2 Thin Film Composition
The compositions of HfO2 layer was analyzed with XPS using the method similar to that used to determine the Si and SiO2 transition region composition [1]. Both pure Hf and pure HfO2 powder were prepared. The electron binding energy of Hf in Hf an HfO2 state can be detected. The electron binding energy of Hf in the deposited HfOx film was analyzed at the same condition. And then the x value can be interpolated. The calculated results of some samples were shown in Fig.8-5. Most of the samples show x values very close to 2. This result indicates that the stoichiometry of the deposited HfO2 film is correct. The film deposited in N2O ambient show lower x value after PDA at higher temperature, this might be due to some Hf-O bonds are replaced by Hf-N bonds so that the content of O in the film is reduced. It is noted here that the electron binding energy of Hf-N is lower than the Hf-O bond.
After 1000℃ PDA, the x-value decrease slightly. It might relate to the growth of interfacial layer. Fig.8-6 shows the measured XPS spectrum can be deconvoluted into two spectra. The spectrum corresponding to lower binding energy is related to Hf-silicate [2]. It seems that the samples deposited in N2O ambient show stronger Hf-silicate than the sample deposited in O2
ambient. This observation might explain the decrease of x value of the N2O ambient deposited film after high temperature annealing.
XPS depth profile was also employed to confirm the formation of silicate layer. Fig.8-7 shows the XPS spectrum of Hf4f7/2 of HfO2 film after different surface sputtering time. Since the sputter rate of as-deposited film is much faster than that of the annealed film, the detection position is closer to the interfacial layer for the as-deposited sample. It is clear that as the detection position becomes closer to the interfacial layer, the signal corresponded to HfO
8-2-3 Surface morphology
Surface morphology of HfO2 films deposited at different temperatures and ambient followed by different post annealing temperatures and ambient were examined by AFM. The thickness of the deposited films are around 6nm~8nm. According to the published literature, rough surface is expected as HfO2 crystallizing after PDA. Smooth surface is desired since roughness will enhance local electric field, which is believed to be detrimental to the gate dielectric integrity. Fig.8-8(a), (b), and (c) show the AFM images of HfO2 films deposited at 345℃ without PDA, after 900 ℃ PDA in N2 ambient, and after 900℃ PDA in O2 ambient, respectively. The other deposition parameters are O2 flow rate=300sccm and pressure=1.5mbar. The corresponded root-mean-square roughness (Rms) values are 0.569nm, 0.519nm, and 0.467nm. The low temperature deposited film show large roughness but PDA can slightly improve.
Fig.8-9 shows the AFM images of HfO2 film deposited at 400℃ with PDA at different temperatures in N2 ambient. During deposition, the O2 flow rate was 300 sccm and the pressure was 1.5 mbar. The Rms values are 0.203nm, 0.163nm, 0.178nm, and 0.159nm for the samples without PDA, after 600℃ PDA, after 750℃ PDA, and after 1000℃ PDA, respectively. Fig.8-10 compares the leakage current density at -1V and the CET and Rms values of these samples. It is observed that the surface roughness is irrelevant to the PDA temperature and the high leakage current of the as-deposited and 600℃ annealed samples can not be attributed to surface roughness.
Fig.8-11 shows the AFM images of HfO2 films deposited at 500℃ in pure O2 deposition ambient and N2O deposition ambient. The flow rates of both gases and pressure are 500 sccm and 5 mbar, respectively. The surface of HfO2 film deposited in N2O ambient is rougher than that deposited in O2 ambient. It has been reported that by adding nitrogen into HfO2 film, the crystallization temperature could be raised [2]. It is thus postulated that the smooth surface of HfO2 film deposited in N2O ambient could be attributed to its amorphous nature. However, as has been observed in chapter 2 that the leakage current of HfO2 film deposited in N2O ambient is much higher that that deposited in O2 ambient. It is concluded that the surface roughness does not affect the leakage current again. Before ending this subsection, it should be noted that the Rrms value of original Si substrate is around 0.2nm, which is similar to and slightly better than that of the surface of HfO2 films deposited at 400℃ and 500℃,
In this subsection, grazing angle X-Ray diffraction (G-XRD), planar view TEM inspection, and TEM diffraction were employed to identify the crystalline and micro-structure of the deposited HfO2 films. Fig.8-12 shows the G-XRD spectra of HfO2 films of 8nm thick deposited at 345℃, 400℃ and 500℃ without PDA. The O2 flow rate is 300 sccm and the pressure is 5mbar. For HfO2 films deposited at 345℃ and 400℃, the intensities of the signals are extremely weak. As a result, it is unlikely to exactly identify the structure of HfO2 thin films with these broad and weak signals. The only thing can be verified is that the films deposited at low temperature show poor crystalline. However, for the 500℃ deposited film, several diffraction signals can be identified to be HfO2 phases. It is supposed that the film becomes polycrystalline as deposited at higher temperature because crystalline state is the most stable state of HfO2 [3].
Fig.8-13(a) and (b) show the XRD spectra of HfO2 films deposited in O2 ambient and N2O ambient, respectively, followed by PDA in N2 ambient at different temperatures. For the HfO2 film deposited in O2 ambient, the as-deposited films show clear HfO2 orthorhombic crystal structure. As the PDA temperature increases, the signal intensity becomes more and more strong. On the other hand, as deposited in N2O ambient, the as-deposited film is amorphous type and the crystallization temperature increases to 750℃ as shown in Fig.8-13(b). Fig.8-14 compares the XPS binding energy of N in HfO2 film. The film deposited in pure O2 ambient shows weak N-signal where the nitrogen content comes from the Hf-precursor. The N2O gas incorporate additional N into the HfO2 film so that the HfO2 film deposited in N2O ambient shows stronger N-signal. These observations indicate that nitrogen incorporation in HfO2 film destroys the HfO2 structure and then increases crystallization temperature.
The detailed micro-structure was investigated by high-resolution plane-view TEM (PV-TEM) inspection. The PV- TEM micrographs of HfO2 thin film deposited at 400℃ followed by different PDA temperatures are shown in Fig.8-15. The O2 flow rate is 300 sccm and the deposition pressure is 1.5mbar. The as-deposited film shows many nano-scale grains (nano-crystals) separated by amorphous state material. The average grain size is 20 nm. After 600℃ PDA, the amorphous region reduces and more nano-crystals are observed. Continuous increase the PDA temperature to 1000℃, nano-crystals merge and the grain size increases to
points. The sample without PDA shows some bright points. As the PDA temperature increases to 600 and 750℃, the density of bright points increases dramatically. Comparing the TEM micrographs in Fig.8-15 and the conductive AFM images in Fig.8-16 with the leakage current in Fig.8-10, it is quite reasonable to conclude that the increase of leakage current after 600℃
PDA arises from the crystallization of the HfO2 film. Because the resolution of AFM image is not high enough, we can not judge whether the leakage path is through the nano-crystals or at the boundary of the nano-crystals. After PDA at 1000℃, the leakage current decreases apparently due to the growth of interfacial layer. The lacking bright points in the conductive AFM image does not mean that the HfO2 film do not leak after 1000℃ PDA, but reflects the fact that the thick interfacial layer effectively blocks the current transport.
Fig.8-17 shows the PV-TEM micrographs of HfO2 films deposited at 500℃ in O2
ambient and N2O ambient. The O2 flow rate is 500 sccm and the pressure is 5 mbar. Not similar to the HfO2 film deposited at 400℃, the film deposited at 500℃ in O2 ambient shows completely poly-crystalline structure. The grain size of the as-deposited film is 20 nm. The grain size increases from 30 nm to 50 nm as the PDA temperature increases from 600℃ to 1000℃. On the contrary, the HfO2 film deposited in N2O ambient shows amorphous nature.
Even if after 1000℃ PDA, only few nano-crystals are observed.
It had been reported and widely accepted that crystallization of HfO2 film will result in high leakage current. And it is assumed that the leakage path is the crystallized grain. Our results on the 400℃ deposited HfO2 film seems support the explanation. However, if it is true, the HfO2 film deposited at 500℃ should exhibit the highest leakage current and the films deposited in N2O ambient should exhibit the lowest leakage current under the same CET.
Unfortunately, the leakage current measured in chapter 3 shows totally different results.
Therefore, the leakage path can not be the nano-crystals. The most possible leakage path becomes the boundary of the nano-crystals.
Fig.8-18 compares the microstructure of the HfO2 films deposited at 400℃ and 500℃. The nano-crystals of the 400℃ deposited film are round shape and are surrounded by amorphous region. On the other hand, the nano-crystals of 500℃ deposited film are polygonal shape and contact to each other tightly. It is suspected that the boundary between the amorphous region and the nano-crystals has different composition or structures. This postulation is possible because the impurities in Hf precursor may not decompose completely
concentration region also stops the growth of nano-crystals. Finally, the nano-crystals are surrounded by an amorphous region and show round shape. Since the major impurity in the film is nitrogen and the compound of HfN is metallic, the boundary of nano-crystals serves as leakage path. At high deposition temperature, for example 500℃ precursor decomposes and , Hf reacts with O completely. Nano-crystals grow until they touch with each other. Therefore, nano-crystals contact tightly. The lack of high impurity region of the high temperature deposited HfO2 film accounts for the low leakage current.
Because the nano-crystals are so small, G-XRD does not provide sufficient resolution to identify the phase and orientation. Fig.8-19 and Fig.8-20 show the TEM diffraction pattern of samples deposited at 400℃ and 500℃, respectively. Although the 400℃ deposited sample exhibit poor crystalline, consistent with G-XRD result, it tends to be polycrystalline after 1000℃ PDA. Classical polycrystalline diffraction pattern is observed in the 500℃ eposited d samples. Plenty crystalline structures are observed while XRD only reflects the strongest one.
The actual structure of nano-crystals is identified from the high resolution plane view TEM image. As indicated in Fig.8-21 and 8-22, two d-spaces of diffraction planes as well as the intersection angle could be measured from the two dimensional lattice images, and then the structure of the grain can be identified. Most of the grains are identified as the orthorhombic structural HfO2. It is worthy to note that XRD signals obey the “selection rule”
so that some d-spaces measured from the TEM image may be not found in XRD data sheet For general comment, amorphous gate dielectric is suitable for device applications due to the lower leakage current, superior heterogeneous interface quality, and better blocking capability against impurity diffusion from poly-Si gate electrode. However, once nano-crystals form in the dielectric film, leakage current increases. Although adding nitrogen into HfO2 increases the crystallization temperature, high leakage current still occurs due to few nano-crystals. Instead, HfO2 film with fully crystallized structure, for example deposited at 500℃ and O2 flow rate 500sccm (i.e. high deposition temperature with sufficient oxygen supply), show the lowest leakage current among the HfO2 films studied in this thesis.
8-3 Summary
HfO2 film results in HfO2 layer increase during PDA. The interfacial layer increases with the increase of PDA temperature. However, its thickness at medium PDA temperature does not increase apparently. Therefore, the leakage current is not dominated by the interfacial layer.
After 1000℃ PDA, the interfacial layer increases to around 3 nm so that the current transport is blocked effectively. The interfacial layer is identified as silicate-like material. It must be pointed out that the surface treatment methods employed in this thesis are not efficient enough to suppress the growth of interfacial layer. To further scales down the CET into sub-1nm regime, advanced surface treatment method must be developed.
The surface roughness of the high temperature (>=400℃) deposited HfO2 films is good enough. It is observed that the surface roughness of HfO2 film depends on the deposition condition but is not affected by PDA conditions apparently. This observation indicates that the surface roughness does not play role on the observed leakage current.
The most stable state of HfO2 is crystalline state so that the HfO2 films tend to crystallize during PDA. Nano-crystals are observed in films deposited at any conditions. For HfO2 films deposited at low temperature or with insufficient O2 supply, the nano-crystals are separated by amorphous region and show round shape. It is postulated that the boundary of these nano-crystals contains high concentration impurities especially nitrogen. The HfN-like boundary layer accounts for the leakage current of these samples. Hf-precursor decomposes completely during high temperature deposition, and therefore, with sufficient O2 supply, the film becomes polycrystalline completely. The lack of high impurity boundary layer results in very low leakage current.
HfO2 with different crystalline structures may have different flat-band voltages and/or oxide charges. This fact may contribute to the kink of the C-V characteristics shown in section 8-2.
References
[1]. International Technology Roadmap for Semiconductors (ITRS), 2003 Edition, Semiconductor Industry Association (SIA), http://public.itrs.net
[2]. C. H. Chien, Private Communication
[3]. M. Koyama, K. Suguro, M. Yoshiki, Y. Kamimuta, M. Koike, M. Ohse, C. Hongo and A.
Nishiyama, “Thermally Stable Ultra-Thin Nitrogen Incorporated ZrO2 Gate Dielectric Prepared by Low Temperature Oxidation of ZrN” IEDM Tech. Dig. sec.20-03 , 2001
0 200 400 600 800 1000
HfO2 layer thickness Interfacial layer thickness various post annealing temperatures in N2 ambient for 30sec: (a) as-dep., (b) N2-600℃, (c) N2-750℃, and (d) N2-1000℃. The measured thickness of
Fig.8-3. X-TEM micrographs of samples deposited at 400℃ on NH3-treatedwafer. The O2
flow rate is 500sccm and the pressure is 5mbar. (a) as-dep. (b) N2-600℃ (c) N2-1000℃ , and (d) HfO2 /interfacial layer thickness and correlative leakage current..
Fig.8-4. X-TEM micrographs of samples deposited at 500℃ on NH3-treatedwafer. The O2
flow rate is 500sccm and the pressure is 5mbar. (a) as-dep. (b) N2-600℃ (c)
30 25 20 15 10
T=500oC,P=5mbarr,O2=500sccm T=400oC,P=5mbarr,O2=500sccm T=345oC,P=5mbarr,O2=500sccm T=500oC,P=5mbarr,N2O=500sccm
O/Hf ratio
T=500oC,P=5mbarr,O2=500sccm T=400oC,P=5mbarr,O2=500sccm T=345oC,P=5mbarr,O2=500sccm T=500oC,P=5mbarr,N2O=500sccm
O/Hf ratio
Annealing temperature(oC)
Fig.8-5. Hf metal and HfO2 XPS spectra for composition analysis and the calculated results of different deposition condition thin film.
(a) Hf spectra of Hf metal and pure HfO2
Fig.8-6. Deconvoluted of HfO2 thin film Hf spectra with N2-1000℃ post deposition annealing. (a) HfO2 deposited at T=500℃, O2=500sccm. (b) HfO2 deposited at
Fig.8-7. XPS spectrum of Hf4f7/2 of HfO2 film after different surface sputtering time. HfO2
was deposited at T=500℃, P=5mbar, and O2=500sccm. (a) as-deposition.
(b)T=1000℃ for 30sec N2 ambient PDA.
Fig.8-8. AFM images of HfO2 deposited at T=345 ℃ O2 ambient with various post
Fig.8-9. AFM images of HfO2 deposited at T=400℃, O2=300sccm, P=1.5mbar with different annealing temperature for 30sec. (a) As deposition, (b) N2-600℃, (c) N2-750℃, and (d) N2-1000℃
0 200 400 600 800 1000
Fig.8-10. Leakage current density, CET and Rms values versus annealing temperature of the samples shown in Fig.8-9.
Fig.8-11. AFM images of HfO2 film deposited at T=500℃ in pure O2 deposition ambient and N2O deposition ambient.
(a) O2 reaction chamber, As deposited samples.
(b) N2O reaction chamber, As deposited samples.
(c) O2 reaction chamber, N2 600℃ 30sec post annealing
20 30 40 50 60
HfO2(200) HfO2(111) HfO2(-110)
HfO2(-111)
T=500oC
T=400oC
T=345oC
Intensity(a.u.)
Different depostion temperature:
O2 flow rate:300sccm P=5mbar
Fig.8-12. G-XRD spectra of HfO2 film deposited at T = 345℃, 400℃, and 500℃ without
Fig.8-12. G-XRD spectra of HfO2 film deposited at T = 345℃, 400℃, and 500℃ without