Structuring method

Details Structuring method Structuring method

1998-08-03

2002-09-24

Gulakowski, Randy (Department: 1746)

Etching a substrate: processes

Mechanically shaping, deforming, or abrading of substrate

Nongaseous phase etching

C216S057000, C134S001200, C134S033000, C510S176000, C438S959000, C438S963000

Reexamination Certificate

active

06454956

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a structuring method, in particular a method for structuring layers. Such layers including layers of noble metals, ferroelectric materials, and dielectric materials with high relative dielectric constants, that can be etched only with difficulty if at all by plasma or by dry chemical etching.
In the development of large scale integrated memory components, such as DRAMs and FRAMS, the cell capacity should be preserved or even further improved while miniaturization increases. To achieve this goal, thinner and thinner dielectric layers and convoluted capacitor electrodes (trench cell, stack cell) are used. Recently, instead of conventional silicon oxide, new materials are being used, especially paraelectrics and ferroelectrics, between the capacitor electrodes of a memory cell. For example, barium strontium titanate (BST, (Ba,Sr)TiO
3
), lead zirconate titanate (PZT, Pb(Zr,Ti)O
3
), or lanthanum-doped lead zirconate titanate or strontium bismuth tantalate (SBT, SrBi
2
Ta
2
O
9
) have been used in the capacitors of memory cells in DRAMs and FRAMs.
The materials are typically deposited onto already existing electrodes (bottom electrodes). The processing is done at high temperatures, so that the materials that normally form the capacitor electrodes, such as doped polysilicon, easily become oxidized and lose their electrically conductive properties, which would cause the memory cell to fail.
Because of their good oxidation resistance and/or the development of electrically conductive oxides, 4d and 5d transition metals, in particular platinum metals (Ru, Rh, Pd, Os, Ir, Pt) and in particular platinum itself, as well as rhenium are considered promising candidates that might replace doped polysilicon as electrode material in the aforementioned memory cells.
Progressive miniaturization of components also requires the need for substitute materials for aluminum currently used for the conductor tracks. The substitute material should have a lower specific resistance and a lesser electromigration than aluminum. Copper is considered to be the most promising candidate.
The development of magnetic random access memories (MRAMs) also requires the integration of magnetic layers (such as Fe, Co, Ni, or Permalloy) in microelectronic circuits.
If an integrated circuit is to be constructed from the aforementioned materials, which so far are not widely used in semiconductor technology, then thin films of these materials will have to be structured.
Structuring the materials used until now has been done as a rule by the so-called plasma reinforced anisotropic etching methods. Typically physical-chemical processes are employed, in which gas mixtures of one or more reactive gases, such as oxygen, chlorine, bromine, hydrogen chloride, hydrogen bromide or halogenated hydrocarbons, and noble gases (such as Ar, He) are used. The gas mixtures are as a rule excited in an electromagnetic alternating field at low pressures.
The basic operating mode of a plasma etching chamber is known for example from a parallel-plate reactor. A gas mixture, such as Ar and Cl
2
, is delivered to the reactor chamber via a gas inlet and is pumped out again through a gas outlet. A lower plate of the parallel plate reactor communicates via a capacitor with a high-frequency source and acts as a substrate holder. By applying a high-frequency electrical alternating field to an upper plate and the lower plate of the parallel plate reactor, the gas mixture is converted into a plasma. Since the mobility of the electrons is greater than that of the gas cations, the upper and lower plates become negatively charged capacitively compared with the plasma. The two plates therefore exert a strong attraction force on the positively charged gas cations, so that they are exposed to a permanent bombardment with the ions, such as Ar
+
. Since the gas pressure is also kept low, typically 0.1 to 10 Pa, only a slight scattering of the ions among one another and with the neutral particles occurs. The ions meet the surface of a substrate, which is held on the lower plate of the parallel plate reactor, virtually at a right angle. This allows good duplication of a mask on the underlying layer, which is to be etched, of the substrate.
Typically, photoresists are used as mask materials, because they are relatively easy to structure by a light exposure step and a development step.
The physical component of the etching is accomplished by impetus and kinematic energy of the arriving ions (such as Cl
2
+
, Ar
+
). In addition, chemical reactions between the substrate and the reactive gas particles (ions, molecules, atoms, radicals) are initiated or reinforced thereby (the chemical component of the etching), forming volatile reaction products. The chemical reactions between the substrate particles and the gas particles are responsible for high etching selectivities of the etching process.
Unfortunately, it has been found that the aforementioned materials newly used in integrated circuits are among those materials that can be etched chemically only with difficulty if at all. The amount of removal by etching, even if “reactive” gases are used, is predominantly or virtually exclusively due to the physical component of the etching. Because the chemical component of the etching is only slight or is absent, the amount of removal by etching from the layer to be structured is on the same order of magnitude as the amount of removal by etching of the mask or the underlay (etch stop layer). In other words the etching selectivity with respect to the etching mask or underlay is generally low (between about 0.3 and 3.0). Consequently, because of the erosion of masks with sloping flanks and because of the unavoidable faceting (beveling, tapering) on the masks, only slight dimensional stability of the structuring can be assured. The faceting thus limits both the smallest feature sizes that can be attained in the structuring and the attainable steepness of the profile edges in the layers to be structured.
The faceting on the masks, and thus the faceting of the layers to be structured, is greater, the greater the proportion of reactive gases (in particular chlorine) in the gas mixture that is used during the plasma-chemical etching process. Correspondingly, with gas mixtures that have no reactive gas component, examples being pure argon plasmas, the steepest profile edges of the layers to be structured can be created.
In addition to the aforementioned faceting of the layers to be structured, undesired redepositions of the material of layer to be structured can also occur in the structuring process. The redepositions occur for instance on the side walls of the resist mask, and often they can be removed in ensuing process steps only at major effort and expense. Unfortunately, the redepositions occur all the more frequently, the lower the proportion of reactive gases in the gas mixture used during the plasma-chemical etching process. Hence until now, process control was usually limited to assuring low proportions of argon, for instance in a chlorine-argon plasma. Yet increasing the proportion of chlorine in the etching gas mixture leads to increased faceting of the masks.
In the case of platinum etching using a resist mask, the consequence of using reactive gases such as chlorine or hydrogen bromide was that intermediate redepositions form, which disappear again in the further course of the etching. The structures again lead to CD-propagation and to flat platinum edges. By now, they are considered to be the greatest disadvantage of the process that employs both chlorine and a resist mask.
If instead of the resist mask a so-called hard mask is used to structure the layers to be structured, then many of the above difficulties can be markedly reduced. However, structuring a hard mask requires additional process steps, which make the overall process more expensive.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a structuring method which overcomes the above-m

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