Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2002-07-15
2004-07-06
Fourson, George (Department: 2823)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S240000, C438S393000
Reexamination Certificate
active
06759250
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention is generally related to the fabrication of integrated circuits (ICs) and, more specifically, to the fabrication of a lead germanate ferroelectric structure having an electrode of layered noble metals.
Platinum (Pt) and other noble metals are used in IC ferroelectric capacitors. The use of noble metals is motivated by their inherent chemical resistance. This property is especially desirable under high temperature oxygen annealing conditions, such as those seen in the fabrication of ferroelectric capacitors. In addition, chemical interaction between noble metals and ferroelectric materials such as perovskite metal oxides, is negligible.
The above-mentioned noble metals are used as conductive electrode pairs separated by a ferroelectric material. One, or both of the electrodes are often connected to transistor electrodes, or to electrically conductive traces in the IC. As is well known, these ferroelectric devices can be polarized in accordance with the voltage applied to the electrode, with the relationship between charge and voltage expressed in a hysteresis loop. When used in memory devices, the polarized ferroelectric device can be used to represent a “1” or a “0”. These memory devices are often called ferro-RAM, or FeRAM. Ferroelectric devices are nonvolatile. That is, the device remains polarized even after power is removed from the IC in which the ferroelectric is imbedded.
There are problems in the use of metal, even noble metal electrodes. Pt, perhaps the widely used noble metal, permits the diffusion of oxygen, especially during high temperature annealing processes. The diffusion of oxygen through Pt results in the oxidation of the neighboring barrier and substrate material. Typically, the neighboring substrate material is silicon or silicon dioxide. Oxidation can result in poor adhesion between the Pt and neighboring layer. Oxidation can also interfere with the conductivity between neighboring substrate layers. Silicon substrates are especially susceptible to problems occurring as a result of oxygen diffusion. The end result may be a ferroelectric device with degraded memory properties. Alternately, the temperature of the IC annealing process must be limited to prevent the degradation of the ferroelectric device.
Various strategies have been attempted to improve the interdiffusion, adhesion, and conductivity problems associated with the use of noble metals as a conductive film in IC fabrication. Titanium (Ti), titanium oxide (TiO
2
), and titanium nitride (TiN) layers have been interposed between a noble metal and silicon (Si) substrates to suppress the interdiffusion of oxygen. However, Ti layers are generally only effective below annealing temperatures of 600 degrees C. After a 600 degree C. annealing, Pt diffuses through the Ti layer to react with silicon, forming a silicide product. Further, the Pt cannot stop the oxygen diffusion. After a high temperature annealing, a thin layer of silicon oxide may be formed on the silicon surface, which insulates contact between silicon and the electrode.
Other problems associated with the annealing of a Pt metal film are peeling and hillock formation. Both these problems are related to the differences in thermal expansion and stress of Pt with neighboring IC layers during high temperature annealing. A layer of Ti overlying the Pt film is known to reduce stress of the Pt film, suppressing hillock formation.
Ir has also been used in attempts to solve the oxygen interdiffusion problem. Ir is chemically stable, having a high melting temperature. Compared to Pt, Ir is more resistant to oxygen diffusion. Further, even when oxidized, iridium oxide remains conductive. When layered next to Ti, the Ir/Ti barrier is very impervious to oxygen interdiffusion. However, Ir can diffuse through Ti. Like Pt, Ir is very reactive with silicon or silicon dioxide. Therefore, a bilayered Ir/Ti or Ir/TiN barrier is not an ideal barrier metal.
Pb
5
Ge
3
O
11
is a promising ferroelectric material candidate for nonvolatile memory, such as one-transistor (1T) applications, because of its moderate polarization and relative low dielectric constant. But, this material is a low symmetry ferroelectric material, and it is widely believed that spontaneous polarization exists only along the c-axis. Furthermore, in the PbO—GeO
2
system, the stability range for the Pb
5
Ge
3
O
11
phase is very limited. Even a relatively small deviation in composition, or in growth temperature, can lead to the formation of other lead germanate (PGO) compounds, or phases. Therefore, successful use of this ferroelectric material is dependent upon deposition processes and the adjoining electrode material.
A suitable bottom electrode is required for depositing useful lead germanate (Pb
5
Ge
3
O
11
) ferroelectric thin films. The bottom electrode must have good adhesion with the substrate, good electrical conductivity, and good barrier properties with respect to oxygen and lead. A good bottom electrode must also improve the fatigue degradation, and reduce the leakage current, of the lead-based ferroelectric thin film. From the processing point of view, the bottom electrode should provide a preferred nucleation and growth surface for the c-axis oriented lead germanate (Pb
5
Ge
3
O
11
) thin film at a relative low MOCVD temperature.
Single-layer platinum is widely used as the bottom electrode in PZT and SBT based nonvolatile ferroelectric memories. However, oxygen and elements such as lead can easily diffuse through Pt and react with barrier layers (Ti, TiN) and/or the substrate (Si or SiO
2
), all of which are susceptible to oxidation. Deleterious oxidation leads to bad adhesion with the substrate, a poor interface between platinum and ferroelectric layers, and a poor interface between platinum and the substrate layers.
Further, severe fatigue is an inherent problem associated with ferroelectric thin films, which is not improved by using a single-layer platinum electrode. Fatigue degradation is thought to be due to the domain pinning by space charge, caused by oxygen vacancy entrapment at the interface between the ferroelectric film and the electrode. It is believed that fatigue properties can be significantly improved by using a conducting oxide electrode to prevent space charge formation at the interface. Finally, hillocks are typically found on the surface of single-layer platinum electrodes during depositing of ferroelectric thin film. The hillocks are caused by stress internal to the platinum film. Hillocks result in increased leakage currents, or even device shorts.
Single-layer iridium bottom electrodes are also used in PZT and SBT based devices. Iridium bottom electrodes have superior barrier properties with respect to lead and oxygen, as well as good adhesion with the substrate. However, higher process temperatures are required for MOCVD c-axis oriented lead germinate (Pb
5
Ge
3
O
11
) thin films deposition processes, comparing to the platinum electrode. The MOCVD temperature range to deposit c-axis oriented lead germanate (Pb
5
Ge
3
O
11
) thin film on the Ir electrode is 500-600° C. Further, a lower nucleation site density causes a very rough lead germanate (Pb
5
Ge
3
O
11
) thin film surface, as compared to the film deposited on platinum electrode. Finally, hillock formation on the surface of the iridium electrode is still observed during MOCVD processing.
Metal oxide electrodes such as RuO
2
are used for PZT based nonvolatile memory to improve the fatigue degradation. But RuO
2
electrodes cause an increase in the leakage current. An oxide/platinum double-layer electrode reduces the leakage current, and improves the barrier properties and hillock problem. However, the leakage current is still larger than that of using a single-layer platinum electrode. Further, a thicker oxide layer is formed, resulting in a higher sheet resistance.
Several other oxygen barrier layers exist including a conductive exotic-nitride layer underneath a platinum layer (Ti—Al—N), a noble-metal-insulator-alloy barrier layer (pd—Si—N),
Hsu Sheng Teng
Li Tingkai
Zhang Fengyan
Fourson George
Rabdau Matthew D.
Ripma David C.
Sharp Laboratories of America Inc.
Toledo Fernando L.
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