Using I-PVD for copper-based interconnects
10/01/2002
overviewTo date, electrochemical filling with the assist of a PVD seed layer is by far the most widely accepted technique in fabricating Cu interconnects. However, the use of conventional low-pressure physical vapor deposition techniques, such as collimated PVD, has becomelimited. This article describes a method of accomplishing ionized PVD using a point-cusp magnetic field applied RF plasma source that increases theionization fraction of sputtered metal atoms to almost 100%.
The technology in integrated devices has been changing from conventional Al to Cu metallization. The reduction of cross-sectional dimensions of interconnects to the sub-quarter-micron region has led the industry to explore Cu-based interconnects owing to their lower resistance (1.72μωcm), higher current density threshold (5×106A/cm2), and better electromigration (E-M) properties.
Damascene technology became a standard for the fabrication of Cu interconnects because conventional RIE-metal (reactive ion etch) poses many technical difficulties. For example, it is difficult to find a suitable gas chemistry that forms volatile Cu gas molecules for etching. In damascene technology, vias and trenches are etched into planar dielectric film. Then metal films, (e.g., Cu), are deposited in vias and trenches and can be achieved by several techniques, using chemical vapor deposition (CVD), physical vapor deposition (PVD), and electrochemical deposition. In some techniques, the filling is carried out by a single process, such as plasma-assisted or unassisted CVD. In others, a seed layer is deposited first by CVD or PVD, and then filling is carried out by electrochemical deposition. To date, electrochemical filling with the assist of a PVD seed layer is by far the most widely accepted technique in fabricating Cu interconnects.
The use of conventional low-pressure PVD techniques such as collimated PVD, where the deposition is carried out by sputtered neutral atoms, has become limited because the side and bottom coverage of vias and trenches, particularly those with high aspect ratios (A/R), becomes insufficient for the later electroplating process. To increase the side and bottom coverage of vias and trenches, ionized PVD (I-PVD) evolved as a new technology [1]. In I-PVD, sputtered atoms are first ionized within the gas phase and then pulled onto the wafer by applying a negative bias to the wafer. This causes (ionized) atom-bombardment with an angle that is almost normal to the substrate resulting in an increase of side and bottom coverage.
The ionization of sputtered metal atoms in the plasma occurs by electron excitation and penning ionization. (In penning ionization, Cu atoms are ionized by the collisions with excited Ar atoms.) Therefore, increases of plasma density and pressure cause an increase of ionization fraction of sputtered metal atoms. Usually, the plasma density of conventional planar DC sputtering systems is inadequate to yield a higher ionization fraction of sputtered atoms. The ion fraction of the depositing films using planar DC or hollow-cathode configurations are reported to be well below 50% [2]. Therefore, a greater fraction of deposited films are carried out by neutral atoms, causing overhangs at the openings of vias and trenches. The generation of overhangs limits the use of DC plasmas to vias and trenches with feature sizes <0.13μm because it results in void formation during electroplating. Low-pressure DC plasmas also have other disadvantages when the feature size decreases. The sidewall and bottom coverage become asymmetric in vias and trenches close to the wafer edge because ions and neutral atom flux coming to the wafer surface become asymmetric toward the edge.
Developing a PCM-PVD System
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To increase the plasma density and thereby the ionization fraction in DC sputtering systems, some kind of hardware modifications are needed. A secondary plasma source can be added to increase the plasma density, for example. Use of complicated hardware to increase the plasma density and confine the plasma, however, increases equipment cost and complicates maintenance. Thus, a capacitively-coupled RF plasma source was developed with a planar target for I-PVD processing of 200mm and 300mm wafers. A 60 MHz RF current and cluster of point-cusp magnetic fields were employed to increase the plasma density. This is called a point-cusp magnetron (PCM) PVD system. In designing this PCM-PVD system, simpler hardware configuration and ease-of-maintenance were given prime consideration. Some of the process data obtained with this PCM-PVD system is presented in the following sections.
 Figure 1. a) A cross-sectional view of PCM-PVD system, and b) the top view of the magnet arrangement. |
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Experiment
A cross-sectional diagram of a developed plasma processing system is shown in Fig. 1. This system comprises a planar target (Cu or Ta) with a magnet arrangement to generate a cluster of point-cusp magnetic field below the target surface, a lower electrode where a wafer is placed and a cylindrical shield (sidewalls). The target is supplied a 60MHz RF current via a matching circuit.
On the upper surface of the target a large number of small magnets are arranged with equal distance and alternate polarity (Fig. 1b). This configuration generates point-cusp magnetic fields below the target surface and has two main features. First, it provides a strong magnetic field at the lower surface of the target and a magnetic field-free environment on the wafer surface. Owing to the curved magnetic field lines, the magnetic field strength decays exponentially towards the lower electrode. At the lower surface of the target, which is facing the plasma, the field is around 400–500Gauss. Second, this magnet arrangement does not cause a drift of plasma to one side of the reactor due to ExB drift [3]. Accordingly, this configuration of magnets can be used to obtain a high-density plasma at the vicinity of the target and a radially uniform plasma on the wafer surface. Furthermore, since there is no magnetic field on the wafer surface, the lower electrode can also be given RF power when necessary without disturbing the radial uniformity of the plasma over the wafer surface.
The target is simply a planar circular disk. It is cooled by a passive mechanism such that the coolant is not in direct contact with the target. This has two advantages:
- Target changing becomes easy and takes less time.
- During target changing, the plasma processing reactor does not get contaminated by the coolant.
 Figure 2. Plasma density as a function of VHF power at 14Pa. |
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The latter significantly reduces the time to get to a base pressure of 10-8torr after the target change, reducing the preparation time to run new wafers. The distance between target and substrate (T/S) is changeable. The Cu and Ta/TaN depositions are carried out with T/S = 90mm.
Two different PCM-PVD systems to deposit Cu seed and TaN/Ta barrier films are available on the Anelva 1080 platform. In addition, a pre-heat chamber with wafer aligner and a pre-cleaning chamber are also attached to the 1080 platform. Before the film depositions, each wafer is degassed by heating at 350°C and then subjected to pre-cleaning. The pre-cleaning process may be a plasma-assisted sputter-etching process, or a nonplasma chemical reaction based on H2 gas. If plasma-assisted sputter-etching is employed, a layer <10nm thick is etched from the wafer surface. The depositions of Ta/TaN barrier and Cu seed layers are carried out at 25°C and -30°C, respectively.
 Figure 3. Ionization fraction of Cu atoms as a function of pressure. |
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Plasma properties
Once the plasma is generated by a capacitively-coupled mechanism below the target, electrons at the vicinity of the target get confined by the point-cusp magnetic fields. The electrons undergo cyclotron rotation causing an increase in their path length and the number of collisions with gas atoms. This results in an increase of plasma density. The sputtering process is carried out at pressures ≥18 Pa. At these higher pressures, the secondary electrons emitted from the target also contribute to the increase in plasma density. This was measured by using a Langmuir probe at 60mm from the target as a function of RF power (Fig. 2). The plasma density is more than 3×1011 cm-3 at the level of the wafer. The average electron energy was observed to be ∼2.2eV. The ionization fraction of Cu atoms at 70mm from the target was estimated by using a retarding energy grid as shown in Fig. 3. The results indicate that almost all the Cu atoms get ionized before they reach the wafer surface. The higher ionization rate is attributed to the higher plasma density and the lower ionization potential ofCu (7.72eV).
The plasma potential (Vp) and floating potential (Vf) at 4kW of RF power and at 18Pa, the condition used for Cu seed layer deposition, are about 60V and 45V, respectively. The plasma potential monitored for this system is considerably lower compared to those of conventional RF plasmas where a 13.56MHz RF current is used. The lowering of plasma potential is attributed to the higher frequency (60MHz) of RF current employed. During the film depositions, the lower electrode was placed in a floating state that resulted in the ions in the plasma being accelerated towards the wafer only by the potential difference of Vp-Vf = 15V at the sheath region. When the wafer is in a floating state, the potential difference between the plasma and the wafer (DV) is minimized. Existence of smaller DV (<20V) during the film deposition, particularly at the beginning, is very important to prevent the sputtering of underlying film (SiO2 or low-k material). If these sputtered atoms of dielectric material mix with the depositing barrier film, they would result in an increase of film resistivity.
The self-bias voltage (Vdc) of the target at 4kW of 60MHz power and at 14Pa is ∼90V. This Vdc is much smaller than that of conventional plasma systems and is also attributed to the higher frequency of RF current. The sputter yield lies at a lower level (∼0.2) when the target Vdc ∼90V. However, owing to the higher ion flux to the target, this low sputter yield is large enough to give sufficient deposition rates. Moreover, since the sputter yield is low, the sputtered atom density is considerably lower than the plasma density. This increases the opportunity that each sputtered atom gets ionized via gas phase collisions before it reaches the wafer.
 Figure 4. Deposition rate and bottom coverage of Cu films in 0.25μm A/R=5 via holes. |
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Cu and Ta/TaN film properties
The Cu and Ta/TaN depositions were carried out with 4kW and 2.5kW RF powers at 18Pa and 22Pa, respectively. Both Cu and Ta/TaN films show good film uniformity. The nonuniformity estimated is <3.5% (1s) over 200mm wafers. The deposition rate and bottom coverage of Cu films are shown in Fig. 4. The film deposition rate decreases with an increase of pressure because scattering of sputtered atoms by the gas phase atoms reduces sputtered-atom flux onto the wafer. However, the bottom coverage in vias and trenches is observed to increase with pressure. At higher pressures more ionized atoms reach the bottoms of viasand trenches resulting in an increase of bottom coverage.
As shown in Fig. 5, the bottom coverage in 0.25μm A/R=5 via holes can be as high as 65% at 18Pa. The bottom coverage obtained at the same pressure for a 0.25μm A/R=7 via hole is around 35%. Figure 5 also shows that there is no overhang of Cu film at the opening of via holes. A similar observation has been made for PCM-PVD of Ta/TaN films as well. The formation of overhang essentially occurs if the deposition is carried out with more neutral metal-atoms. Because only a very small fraction of neutral atoms are incident at an angle normal to the wafer surface, all other neutral atoms are incident at different angles. It has been observed that the greater the neutral metal-atom density, the larger the overhang at the opening of via holes. In a PCM-PVD system, almost all the Cu atoms coming to the wafer are ionized. Thus, the film deposition occurs only by Cu ions. These ions are accelerated by the sheath potential over the wafer surface and bombard at a 90° angle to the wafer surface. This prevents the formation of overhang at the openings of via holes and trenches and results in an improvement of sidewall and bottom coverage.
The deposited Cu films show a <111> crystal orientation. The bulk resistivity of Cu films is ∼2μωcm. Ta deposition can be carried out to obtain either high resistive b-Ta (150–220μω) or low resistive a-Ta (15–30μω). The b-Ta film shows <002> oriented crystals with a bulk resistivity of ∼180μωcm. Depending on the deposition parameters, the resistivity of TaN films can be controlled such that they lie between 50–210μω. During the deposition of a TaN/Ta barrier film, the deposition mode is changed from TaN to Ta by shutting off the N2 flow fed into the plasma-processing reactor. All the films show good adhesion properties with the underlying film. Even after electroplating and chemical mechanical polishing (CMP), no peeling could be observed.
The thicknesses of barrier and seed layers can be as low as 20nm and 100nm, respectively, and are sufficient to yield good electroplating results. Figures 6a and 6b show micrographs of 0.25 and 0.09μm vias after electroplating. Further, electromigration tests with 100k via-chains proved that there are no voids or film discontinuities before, during or after the tests.
 Figure 5. SEM photographs showing side and bottom coverage of Cu seed film in 0.25μm A/R=5 via hole. |
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Conclusion
A point-cusp magnetic field applied RF plasma source was developed for sputter deposition of Cu/Ta/TaN seed and barrier films on sub-0.15μm-featured vias and trenches by an ionized metal deposition technique. The plasma system employs a planar target and operates with a 60MHz RF current. Owing to the higher RF frequency and the employment of a point-cusp magnetic field, a high-density plasma with ne ≈ 1012cm-3 is generated below the target. This increases the ionization fraction of sputtered metal atoms to almost 100%.
The Cu seed and Ta/TaN barrier depositions are carried out at pressures of 18Pa and 22Pa, respectively. High-pressure depositions yield completely symmetric film coverage in every via and trench throughout the wafer. For both films, the film non-uniformity over 200mm wafers is <3.5% (1s). The bottom coverage of Cu and Ta/TaN films in 0.25μm vias with A/R=7 are about 35% and 30%, respectively. Both Cu and Ta/TaN films show good adhesion properties with the under layer. The seed and barrier films deposited with this system can be successfully integrated into an electroplating process to fill vias and trenches as small as 0.10μm with A/R>5.
 Figure 6. a) SEM micrograph of 0.25μm A/R=7 via holes and b) TEM micrograph of 0.09μm A/R=5 via hole after electroplating. |
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The PCM-PVD system developed for ionized PVD process features a simple, easy-to-maintain configuration. This system yields a higher plasma density, a nearly 100% ionization ratio of sputtered atoms and, therefore, better sidewall and bottom coverage. The deposited Cu and Ta/TaN films show favorable crystal orientations for better electrical and physical properties. The electroplating results point out that seed and barrier films deposited with the PCM-PVD system can be filled by an electroplating process without any void generation. The electroplating results indicate that this system can be used to fabricate Cu interconnects with feature sizes ≤0.10μm with an aspect ratio >5.
References
- S. M. Rossnagel, J. Vac. Sci. Technol. B 16, 2585, 1998.
- S.M. Rossnagel, H. Kim, Proc. of the International Interconnect Technology Conference (IITC-2001), San Francisco, CA, p. 3–5, June 4–6, 2001.
- S. Wickramanayaka, Y. Nakagawa, Jpn. J. Appl. Phys. 37, 6193, 1998.
Sunil Wickramanayaka, Hanako Nagahama, Eisaku Watanabe, Makoto Sato, Shigeru Mizuno, Anelva Corp., Tokyo, Japan
Sunil Wickramanayaka received his PhD in optoelectronics from Shizuoka University, Japan, in 1992, then worked as a Research Fellow at the same university. He has been with Anelva Corp. since 1996. Currently, he is involved in the development of new equipment for dry etching and PVD. Anelva Corp., Semiconductor Equipment Division, 3-5-8 Yotsuya, Fuchu, Tokyo 183-8508, Japan; [email protected].
Hanako Nagahama received her BS in engineering from Muroran Institute of Technology, Japan. She has been working as a research engineer in the PVD equipment division of Anelva since 1998.
Eisaku Watanabe has received his BS and MS degrees in physics from Chuo University, Japan. He has been working as a research engineer in the PVD equipment division of Anelva since 1994.
Makoto Sato received his BS in engineering from Shibaura College, Japan in 1985. He has been involved in the development of various wafer processing equipment. Currently, he is supervising the development of PVD equipment.
Shigeru Mizuno received his BS and MS degrees in Engineering from Tohoku University, Japan. He has more than 15 years of semiconductor experience at Anelva, primarily on PVD and CVD equipment. He is a manager in the PVD equipment division.