Mitigating moisture driven degradation of silicon doped chalcogenides

A method for mitigating moisture driven degradation of silicon doped chalcogenides includes placing a silicon doped chalcogenide composition in a process chamber, passivating dangling silicon bonds of the silicon doped chalcogenide composition by flooding the process chamber with forming gas or with hydrogen plasma, purging the forming gas or the hydrogen plasma from the process chamber, and removing the passivated silicon doped chalcogenide composition from the process chamber.

BACKGROUND

The present invention relates to the electrical, electronic, and computer arts, and more specifically, to methods of preparing materials for use in cross-point memory.

For years, scientists have considered the use of cross-point memory and, more recently, such memories have been practically implemented using Phase Change Materials (PCM). Phase-change-memory has been proven to be a good candidate for storage class memories such as cross-point memory.

SUMMARY

Principles of the invention provide techniques for mitigating moisture driven degradation of silicon doped, arsenic doped chalcogenides. In one aspect, an exemplary method includes placing a silicon doped chalcogenide composition in a process chamber, passivating dangling silicon bonds of the silicon doped chalcogenide composition by flooding the process chamber with forming gas, purging the forming gas from the process chamber, and removing the passivated silicon doped chalcogenide composition from the process chamber.

In another aspect, an exemplary method includes placing a silicon doped chalcogenide composition in a process chamber, passivating dangling silicon bonds of the silicon doped chalcogenide composition by flooding the process chamber with hydrogen plasma, purging the hydrogen plasma from the process chamber, and removing the passivated silicon doped chalcogenide composition from the process chamber.

In another aspect, a passivated silicon doped chalcogenide composition has hydrogen atoms occupying silicon bonds that are not attached to chalcogens.

In view of the foregoing, techniques of the present invention can provide substantial beneficial technical effects. For example, one or more embodiments provide one or more of:

Practical implementation of silicon doped chalcogenides for phase change memory in cross-point arrays, without undesirable off-gassing due to moisture exposure.

Practical implementation of silicon doped chalcogenides for phase change memory in cross-point arrays, without undesirable changes to electrical properties due to moisture exposure.

Principles of the invention provide techniques for not only mitigating moisture driven degradation of silicon doped chalcogenides, but also prevent the materials from outgassing of toxic hydride gases to the environment. This is also beneficial for the safety of environment and people.

DETAILED DESCRIPTION

Principles of the invention provide techniques for mitigating moisture driven degradation of silicon doped chalcogenides.

FIG.1depicts a cross-point memory array100that has phase change memory (PCM) memory cells, e.g.,102, column elements, e.g.,104, and row elements, e.g.,106. In the particular case of cross-point memory, each memory cell102incorporates a chalcogenide as a phase change memory unit114(shown inFIG.8) and incorporates another chalcogenide as an OTS (Ovonic Threshold Switch) selector110(also shown inFIG.8) that enables or disables read/write operations to the phase change memory unit114. Chalcogenides are materials that include one or more chalcogens (e.g., S, Se, Te) as a substantial constituent in their composition.

The chalcogenide of the phase change memory unit114is a PCM that can have amorphous and crystalline phases. A phase change in the PCM is accomplished by supplying a current through a resistive element (e.g., TiN) to heat the PCM briefly and rapidly to switch to the amorphous phase, or slowly and for a longer time to switch to the crystalline phase. The PCM has a relatively high resistance in the amorphous phase, and a relatively low resistance in the crystalline phase.

The chalcogenide of the OTS selector110(seeFIG.8) is a material that transitions between an off-state (relatively resistive) and on-state (relatively conductive), according to an applied voltage. That is, when the voltage applied to the OTS selector110exceeds a threshold voltage Vth, the chalcogenide material experiences a sharp drop in resistivity, enabling a current flow. When the voltage is removed, the chalcogenide material recovers a highly-resistive state.

According to some embodiments, the PCM and OTS material are both chalcogenide materials, where the PCM crystallizes when heated, while the OTS material remains in an amorphous phase when heated.

Referring again toFIG.1, the PCM memory cells102include a passivated silicon doped chalcogenide prepared according to one or more embodiments of the present invention. For example, a passivated silicon doped chalcogenide can be used as an OTS selector110(shown inFIG.8). In other applications, a similarly passivated silicon doped chalcogenide can be used as a PCM of a phase change memory unit114(also shown inFIG.8).

According to some embodiments, the passivated silicon doped chalcogenide exhibits improved electrical properties. Doping the chalcogenides with silicon improves the thermal stability, threshold voltage, leakage current, etc., of the material. According to at least one embodiment, passivating the chalcogenide material stabilizes its characteristics against moisture-driven degradation.

A chalcogenide is a chemical compound including at least one chalcogen anion and at least one more electropositive element. An exemplary chemical formula for an arsenic-doped chalcogenide suitable for one or more embodiments of the present invention is SiAsSe. Other exemplary materials include SiGeAsSe, SiGeAsSeTe, SiGeAsSbSe, and SiGeAsSbSeTe.

FIG.2depicts a chemical structure200for a silicon doped, arsenic-doped chalcogenide (SiSe(As)). Note the standard chemical notation: silicon (Si), selenium (Se), arsenic (As), etc. The ovals, e.g.,202, represent free or dangling bonds. A dangling bond is an unsatisfied valence on an immobilized atom. These dangling bonds are reactive, and can lead to defects and/or impurities in the material.

Although conventional arsenic-chalcogenide glass fibers generally have high stability to atmospheric moisture, we have discovered that exposing silicon-doped chalcogenides to moisture results in chemical degradation of the composition. For example, in a case where a silicon doped chalcogenide is exposed to moisture after thin film deposition or processing, this degradation produces compounds such as hydrogen selenide or (when the chalcogenide is arsenic-doped) arsine gas with its distinctive garlic odor, as well as adversely affecting the desirable electrical properties of the composition.FIG.3depicts interaction of moisture302with the chemical structure200ofFIG.2, according to:

As shown inFIG.3, the oxygen atoms in the moisture302react at the dangling bond202of the arsenic bonded silicon, extracting the silicon from the structure200, and forming silicon dioxide (SiO2)306because SiO2has a significantly lower free energy than water (H2O); in fact, the Gibbs free energy of SiO2is on the order of −856 kJ/mol (kilojoule per mole) whereas the free energy of water is only about −229 kJ/mol in the gas phase. Once the oxygen has reacted with the silicon, this creates dangling bonds from the chalcogens and/or arsenic that previously were attached to the oxygen. The freed ionic hydrogen atoms from the moisture302are then available to attach to the dangling bonds of the chalcogen, e.g., selenium, and/or the arsenic, forming hydrogen selenide (H2Se)308and/or arsine (AsH3)310.

Thus, moisture driven degradation of silicon doped chalcogenides occurs because in the natural state of such a composition the silicon atoms in the composition have dangling bonds that preferentially react with the oxygen atoms of water molecules to produce silicon dioxide and release the ionic hydrogen from the water molecules. The ionic hydrogen then reacts with the chalcogen (e.g., selenium), or with other constituents (e.g., arsenic) to produce gases (e.g., hydrogen selenide, arsine), which can be toxic.

According to some embodiments, dangling bonds of the arsenic bonded silicon atoms and the selenium atoms of a silicon doped chalcogenide composition can be passivated by exposing the composition to a hydrogen atmosphere. For a forming gas passivation process according to one or more embodiments of the present invention, this process can be accomplished with materials under a pressure of about 14 psi (pound per square inch) (1 atm (atmospheres)) to 280 psi (20 atm) and a temperature between about 200° C. (Celsius) to 400° C. For a hydrogen plasma treatment, this process can be accomplished with materials under a pressure of about 1 to 100 torrs (Torr) and a temperature between about room temperature to 100° C.

FIG.4depicts a hydrogen passivated chemical structure400according to some embodiments of the present invention. The hydrogen passivated chemical structure400does not include dangling bonds.

FIG.5depicts the non-interaction of moisture502with the passivated chemical structure400ofFIG.4. Note that the oxygen504of the moisture502does not react with the hydrogen passivated chemical structure400, which includes hydrogen atoms, e.g.,506, attached to the silicon and selenium of the doped composition. More particularly, the oxygen of the moisture502is not reactive with the hydrogen passivated chemical structure400.

FIG.6depicts in a flowchart a method600for hydrogen passivating a silicon doped chalcogenide composition, according to an exemplary embodiment. At602the composition is placed in a process chamber. At604, the process chamber is flooded with forming gas (e.g., dissociated ammonia) containing at least 5 mol % molecular hydrogen with the remainder nitrogen. At606, the process chamber is held at process conditions, e.g., between 200° C. and 400° C. at between 14 psi and 280 psi for at least 60 seconds. At608, the forming gas is purged from the process chamber. At610, the passivated composition is removed from the process chamber.

FIG.7depicts in a flowchart a method700for hydrogen passivating a silicon doped chalcogenide composition, according to an exemplary embodiment. At702the composition is placed in a process chamber. At704, the process chamber is flooded with a hydrogen plasma, e.g., at least 90% by mass hydrogen. At706, the process chamber is held at process conditions, e.g., between room temperature and 100° C. at between 1 and 100 Torr for at least 60 seconds. At708, the hydrogen plasma is purged from the process chamber. At710, the passivated composition is removed from the process chamber.

According to the methods ofFIG.6andFIG.7, there is no oxygen in the forming gas or plasma. Further, the silicon doped chalcogenide preferentially reacts with the hydrogen in the forming gas or plasma, satisfying the dangling bonds202. The hydrogen in the forming gas or plasma reacts with the chalcogens to passivate the dangling bonds202, without removing the chalcogens from the chemical structure.

According to some embodiments, the passivation is a treatment performed after implementation of the composition into, for example, of a PCM and/or an OTS selector of a PCM memory device. According to some embodiments, the passivation is performed before fully encapsulating the memory devices.FIG.8depicts a PCM memory cell102according to an exemplary embodiment. The PCM memory cell102includes a memory stack802that is electrically connected between column metal104and row metal106. The memory stack802includes a top electrode108, an OTS selector110, a middle electrode112, a phase change memory unit114, and a bottom electrode116. The OTS selector110and/or the phase change memory unit114each incorporate a hydrogen-passivated silicon doped chalcogenide having generally the chemical structure400shown inFIG.4.

In operation of the PCM memory cell102, a column voltage is supplied to the column metal104and a row voltage is supplied to the row metal106. When a difference between the row and column voltages exceeds the threshold voltage Vthof the OTS selector110, current flows through the memory stack802.

For a WRITE cycle, the row and column voltages are set so that the flowing current exceeds a threshold current of the phase change memory unit114, and the phase change memory unit114is heated above its phase transition temperature. Once the phase change memory unit114exceeds its phase transition temperature, there are two options: 1) remove the row and column voltages, in which case the current stops flowing and the phase change memory rapidly cools to an amorphous (high resistance) phase; or 2) maintain the row and column voltages for a certain period of time, in which case the current continues flowing and the phase change memory stabilizes in a crystalline (lower resistance) phase.

For a READ cycle, the row and column voltages are set so that the flowing current does not exceed the threshold current; then the current, which varies according to the resistance of the phase change memory unit114, is read as the value of the memory cell. Because the resistance of the phase change memory unit114is analog (it varies according to how long the phase change memory unit114is held above the phase transition temperature), PCM cells enable denser data storage than is achievable with older types of memory.

Given the discussion thus far, it will be appreciated that, in general terms, an exemplary method600, according to an aspect of the invention, includes at602placing a silicon doped chalcogenide composition200in a process chamber, at604passivating dangling silicon bonds of the silicon doped chalcogenide composition by flooding the process chamber with forming gas, at608purging the forming gas from the process chamber, and at610removing the passivated silicon doped chalcogenide composition400from the process chamber. In one or more embodiments, the forming gas contains at least 5 mol % hydrogen. In one or more embodiments, the method further comprises at606holding the silicon doped chalcogenide composition in the flooded process chamber at 200° C.-400° C. for at least 1 minute. In one or more embodiments, the method further comprises at606holding the silicon doped chalcogenide composition in the flooded process chamber at 14-280 psi for at least 1 minute.

According to another aspect, an exemplary method700includes at702placing a silicon doped chalcogenide composition200in a process chamber, at704passivating dangling silicon bonds of the silicon doped chalcogenide composition by flooding the process chamber with a hydrogen plasma, at708purging the hydrogen plasma from the process chamber, and at710removing the passivated silicon doped chalcogenide composition400from the process chamber. In one or more embodiments, the plasma contains at least 90% by mass hydrogen. In one or more embodiments, the method further comprises at706holding the silicon doped chalcogenide composition in the flooded process chamber at room temperature for at least 1 minute. In one or more embodiments, the method further comprises at706holding the silicon doped chalcogenide composition in the flooded process chamber at 1-100 Torr for at least 1 minute. In one or more embodiments, the method further comprises at706holding the silicon doped chalcogenide composition in the flooded process chamber at 100° C. for at least 1 minute.

According to another aspect, a passivated silicon doped chalcogenide composition400has hydrogen atoms occupying silicon bonds that are not attached to chalcogens. In one or more embodiments, a chemical formula of the composition comprises HSiAsSe. In one or more embodiments, a chemical formula of the composition comprises HSiGeAsSe. In one or more embodiments, a chemical formula of the composition comprises HSiGeAsSeTe. In one or more embodiments, a chemical formula of the composition comprises HSiGeAsSbSe. In one or more embodiments, a chemical formula of the composition comprises HSiGeAsSbSeTe.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.