HfxAlyOz ALD used to fabricate MIM capacitors
05/01/2004
Alternative dielectric materials that would replace silicon dioxide for applications such as gate-dielectric transistor and integrated passive metal-insulator-metal (MIM) capacitors have to pass the stringent and complex requirements of each application. Creating alloying films using ALD and alloying at the interfaces of the mixture films allows the manufacturing of amorphous films up to 1000°C with minimum leakage current. From an electrical and material point of view, the alloying films are one promising candidate.
With the fast development of wireless and nanoscale devices, there is a strong need for integrated decoupling and filtering capacitors within the back-end-of-line for IC manufacturing. It is necessary, however, to develop high-density capacitors for different purposes, such as the impedance adjustment and suppression of crosstalk effects across interconnects. Currently, Si3N4 or SiO2 materials are used for integrated passive devices (IPD). IPDs require the electrode area — before reaching the capacitor — to take up to 70% of the last metal surface. New high-k dielectric and barrier layers for the first interface within a MIM device are therefore selected; the choice depends on the capacitor dielectric material, frequency, capacitive impedance, and process integration [1].
The bottom and top electrode plates need to be a fully integrated replacement solution with copper damascene in a low-k material. Versatile applications, however, require different operating voltages from 0.8–5.5V; therefore, several specifications that limit the usage of high-k dielectric materials are defined for each application. The most preferred high-k material is Al2O3, which has properties such as a high breakdown field and a large band gap, and tends to be used as the first high-k material in capacitors for IPD and dynamic random-access memory [2].
Development efforts have been undertaken to manufacture integrated passive MIM capacitors with a minimum capacitance density of 5fF/µm2, a leakage current density <10-7A/cm2 at the highest operating voltage of the IC, and a maximum breakdown field of 8MV/cm. With known high-k
materials and nanolaminates, it is a challenge to achieve this level of performance. For instance, charge trapping and dipole effects are the main reliability concerns for integrated passive capacitors. A nanolaminate is a binary composition of monolayers (ML) such as hafnium oxide (HfO2) and aluminum oxide (Al2O3).
Film alloy process
MEMSCAP has developed a scheme of alloying films based on atomic layer deposition (ALD). Different material properties are used for alloying the interfaces [3]. (HO is used as a symbol to be differentiated from a HfO2 ML, due to its minimum stoichiometry film-deposition capabilities by the self-saturated reaction sequence of the ALD technique.) A specific ALD process allows the deposition of a mixture film with a fixed ratio between an AO and a HO ML, and thereby a film composition of HfxAlyOz is formed. An example of this process sequence is shown in Fig. 1; the alloying ALD process does not produce large nucleation.
Above an activated surface, a film of defined composition is deposited by a sequence of self-saturated reactions of a ligand precursor pulse and chemisorptions: purge–reactant, or second ligand precursor pulse, and chemisorptions–purge. Based on the ALD technique, one-fourth of an AO ML could be deposited (Fig. 1). For example, a mixture is a fixed ratio of AO:HO for forming HfxAlyOz. With this ALD process, a complete stoichiometry of AO ML has a thickness of 0.35nm.
Using the atomic alloying nanolaminate (AANL) scheme, a new material engineering method is defined. For the present paper, each film is a mixture of HfxAlyOz with a fixed ratio/film that varies from film to film and also with the thickness. As a result, the stoichiometry is modified for each film of the stack. The stoichiometry and the alloying scheme between each laminated film of the stack are the keys to accomplishing AANL. The variation of stoichiometry is the main criterion for limiting the inelastic losses of nanolaminate materials as well as the electron band-to-band transition [4].
Known quantum effects could explain the absence of inelastic losses using the AANL scheme. Achieving a balance between large band gap and high permittivity is not trivial research. The high dielectric constant is the result of a dipole with electronic, molecular, and ionic contributions. This polarizability is accompanied by soft optical phonons, which are sometimes the source of discrepancy between the predicted and measured dielectric constant values. Although the mole fraction of cations in Al2O3 is higher compared to HfO2, Hf has more ionic cations than Al; the increase in the permittivity is naturally more dependent on the element with higher atomic number (e.g., Hf with larger ionic radius and greater numbers of electrons, which is partially the origin of the electronic dipole at high frequency).
Nevertheless, the inclusion of Al into Hf metal oxide tends to displace the electronic response and therefore increases the polarization in the dielectric material. This double effect tends to reduce the frequency response from low to high frequency when an electric field is applied. The displacement of Hf ions or Al ions within each mixture film (HfxAlyOz) adds to the effective charge by the ionic contribution and thereby increases the permittivity. Also, the motion of atoms that have rich bonding orbital electrons (transition metal groups 3, 4, 5, 6, or those with high atomic number, such as Hf) contributes to the generation of soft phonons in the vibrational mode. Soft phonons are created when Hf and Al atoms resonate in different modes and at different frequencies. This phenomenon is why the permittivity can achieve an acceptable value with a large band gap and a high barrier height, by creating the mixture HfxAlyOz, as well as by alloying the interfaces between two mixture films.
A Hf atom — like other transition metal elements — has partially filled d orbital electrons as well as other nonbonding p orbital electrons, and therefore available states for electron occupancy in the d orbital, which lies within the gap between bonding and antibonding orbital energy levels. An Al atom, or other element with an equivalent electronic structure, however, does not have electrons in the d orbital. Al2O3 has sp2 hybrid orbitals, and thus s and p orbitals, which are filled by the bonding; they define the highest energy of the valence band. Therefore, there is a larger difference between the bonding energy orbital (valence band) and the antibonding energy orbital (conduction band) that produces a larger band gap of Al2O3 compared to HfO2.
By creating mixture films and alloying interfaces between mixtures, a large band-gap mixture HfxAlyOz is achieved with the filling of the d orbital of Hf and with a new order of bonding. This effect creates a scattering phenomenon within the nanomaterial, producing another scattering due to the polarization charge. In a standard nanolaminate material, the addition of the scattering phenomenon has the effect of adding capacitance in series and reduces the dielectric's capacitance, whereas in an alloying film it does not [5]. Furthermore, using the AANL scheme, the alloying interface between two different mixtures (and to each metal electrode barrier) limits the formation of a noncontrolled state at the interface involving carrier traps and dangling bonds, such as the formation of TiOAl or TiOHf at the interface of a TiN columnar structure.
By using the AANL scheme, the primary objective is focused on interface engineering. For the most part, interfaces within the MIM structure require less attention in the manufacturing process than required for an MOS transistor [6]. (Drive current decay in an MOS transistor caused by a decrease in carrier mobility from channel carrier scattering is ultimately a result of field penetration into the channel.)
The control of interfaces with the metal barrier is crucial for ensuring the requisite electron transport to achieve high-density capacitance within the MIM structure, as well as acceptable reliability. Additionally, the alloying process of interfaces between mixtures allows for a smooth transition and a new bonding order within the complex polarized structure of a high-k dielectric material. The interface quality is improved, enhancing the reliability of the MIM capacitor device.
 Figure 2. Comparison of leakage current density at 3.3V and the capacitance enhancement factor vs. EOT of each material and to Al2O3 and HfO2 films. |
The composition and thickness of the high-k materials were varied to find the optimum density of each atomic composition. Different experiments were performed to assess whether a laminated stack of different mixture films could reach the targeted performance and fit the industrial requirement of an integrated passive capacitor.
Experiments on integrated MIM capacitors
Experiments were conducted using ASM International's POLYGON mainframe with PULSAR-2000 modules that used ASM Microchemistry's ALCVD technique on blank 200mm Si wafers. The assessment of different materials allowed the comparison of each material composition on p-type Czochralski-grown Si (100) prime substrate of 15Ωcm, without treatment both before and after ALCVD processing steps. Other details of the experiment include:
- A WNC metal layer used as a bottom electrode and barrier over the silicon surface with a thickness of 35nm.
- AANL samples with thicknesses ranging from 10–13.2nm, using HfCl4, TMA, and H2O at temperatures from 280–320°C.
- For reference purposes, a single film of the high-k dielectric materials (i.e., 6.5nm Al2O3 and 10.5nm HfO2) was also deposited on a WNC layer (thickness = 35nm) by ALCVD using HfCl4, TMA, and H2O at temperatures from 280–320°C.
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Tests were carried out in part by Material Development Corp. (MDC). Some tests, such as those at high frequencies or frequency-dependent parameters, were made in the RF lab at MEMSCAP and are based on the technique of using MDC mercury probes in a noncontrolled environment of moisture, particles, and temperature. High-k materials used in the run are listed in the table.
Results
A nanolaminate has the potential to lower the leakage through interface scattering of electron wave functions in the nanolaminate structure. An alloy film will provide another scattering mechanism to lower the leakage and adds linearly with the total amount of mixture present. In a standard nanolaminate, the scattering mechanism is linearly proportional to the total amount of mixture present. By this AANL alloying scheme, the wave functions are superposed with the scattering effect of standard nanolaminates. The effect of the ratio between HO and AO, density of oxygen, stack order of films, interface type between each film, smoothing interface transition, and bonding interfaces are the parameters of this material engineering run.
 Figure 3. Dissipation factor and comparison of losses between studied dielectric material samples. |
It appears from Fig. 2 that the leakage current takes off at 3.3V for HfO2. This single film as a basic high-k dielectric material gives the highest capacitance enhancement — 49% — with respect to Al2O3. Al2O3 has a capacitance density of 5.5fF/µm2. The stoichiometry of each film and order of laminated stack are likely the influencing parameters of the capacitance enhancement factor and the EOT value. Usually, the thickness and higher AO molar fraction affect leakage and capacitance enhancement.
 Figure 4. Time-to-zero breakdown voltage (TZBD) bench tests with a negative bias of 15V and 0.5V/sec as voltage ramp upon AANL sample C. |
For the purpose of evaluating the losses of the AANL samples, the quality factor Q(f) is measured at 2GHz, using a protocol based on the following equation:
Q(f) = 1/(2πRf)
Here, R and f are the capacitor resistance and the frequency, respectively. Passive capacitors using high-k dielectrics suffer from low quality factors due to high resistance plates and capacitive losses in the 2–4GHz range. The losses are evaluated using the loss tangent parameter (tan δ) and dispersion (Fig. 3). According to the measured quality factor and capacitance density of each AANL sample (see table), the interfaces and interface transition of AANL samples have an impact on the quality factor and, thus, on the losses. This feature is different from basic dielectrics and nanolaminates, for which the Al mole fraction and dielectric thickness are the main frequency-dependent parameters.
By the TZBD tests (Fig. 4), AANL sample C shows the main failures in the high-breakdown field regime (>8MV/cm) and for the most uniform failure mode. This behavior seems to be characteristic for alloying films using the AANL scheme. The wave function of electrons is strongly dependent on the conduction band and on the alloying interfaces of stoichiometries of different mixtures. The HRTEM micrograph (Fig. 5) shows the stack of mixture films.
Conclusion
Using ALD, amorphous alloying films are formed with enhanced capacitor properties that are apparent from a comparison run. One AANL arrangement — sample C — has reached the best performance with the lowest leakage, highest capacitance, and best reliability. The AANL scheme gives the highest quality factor at 2GHz, a lower dissipation factor, low losses (tan δ), the highest breakdown voltage, and the best reliability by TZBD assessment for high voltage within the range of 3.3–4.25V. The AANL scheme represents an alternative for materials engineering for integrated passive MIM capacitors and other applications. Furthermore, alloying films stacks used as a dielectric layer in integrated passive MIM capacitors have a cost of only $0.01/mm2.
Acknowledgments
The author would like to extend special thanks to Suvi Haukka at ASM Microchemistry and Tanja Classen at ASMI for contributions to the ALD.processing.
References
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- M. Ritala, M. Leskelä, in Handbook of Thin Film Materials, Vol. 1, Chapter 2, p. 103, H.S. Nalwa (Ed.), Academic Press, San Diego, 2001.
- J.Y. Lee, B.C. Lai, in Handbook of Thin Film Materials, Vol. 3, Chapter 1, p. 50, H.S. Nalwa (Ed.), Academic Press, San Diego, 2001.
- H.Y. Yu, M.F. Li, B.J. Cho, C.C. Yeo, M.S. Joo, et al., Appl. Phys. Lett., 81, 376, 2002.
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Lionel Girardie is chief scientist and inventor of MEMSCAP proprietary technology, such as high-k technology and integration solutions for CMOS, finFET, MIM, and nanosystems. He is also author and co-author of 55 patents. Contact Lionel Girardie at MEMSCAP SA, Parc des fontaines, ZI Bernin, F38926 Crolles France; ph 33/476-928-500, e-mail [email protected].