Optical device having strained buried channel

Provided is an optical device having a strained buried channel area. The optical device includes: a semiconductor substrate of a first conductive type; a gate insulating layer formed on the semiconductor substrate; a gate of a second conductive type opposite to the first conductive type, formed on the gate insulating layer; a high density dopant diffusion area formed in the semiconductor substrate under the gate and doped with a first conductive type dopant having a higher density than the semiconductor substrate; a strained buried channel area formed of a semiconductor material having a different lattice parameter from a material of which the semiconductor substrate is formed and extending between the gate insulating layer and the semiconductor substrate to contact the high density dopant diffusion area; and a semiconductor cap layer formed between the gate insulating layer and the strained buried channel area.

TECHNICAL FIELD

The present invention relates to a silicon photonics, and more particularly, to an optical device of a metal-insulator-semiconductor (MIS) structure.

BACKGROUND ART

Conventional data transmission using metal lines has limited transmission speed due to delay of a resistor-capacitor of a transmission line. In particular, the communication speed between a central processing unit (CPU) and a graphics processing unit (GPU) required for games has gradually increased. Thus, the demand for higher communication speeds has also increased. Silicon photonics has been suggested to satisfy this demand and can provide high integration. In particular, a monolithic device for optical communications can be manufactured to reliably transmit high-speed data generated inside a chip to an external chip. Also, an optical device using silicon photonics does not need to be additionally packaged, as opposed to using another device outside a chip. Thus, packaging cost can be reduced, and the operation speed is not limited by the packaging. As a result, high-quality data can be transmitted to an external chip.

However, elements such as a light source, an optical modulator, an optical light receiving device, a low noise amplifier, a limiting amplifier, an optical coupler, a multiplexer, a demultiplexer, an optical filter, etc. must be improved to use silicon photonics. The optical modulator is the core element.

An optical modulator has a maximum extinction ratio and a rapid switching structure. Also, the process of manufacturing the optical modulator is simple and may be compatible with a silicon process. For example, in a Mach-Zehnder structure obtaining an extinction ratio using an optical phase difference, an optical phase must be changed using a more efficient method to obtain a maximum extinction ratio.

An optical modulator having a metal-oxide-silicon (MOS) capacitor structure has been suggested for conventional silicon photonics.

FIG. 1is a cross-sectional view showing the important parts of an optical modulator having a MOS capacitor type structure according to the prior art. Referring toFIG. 1, an optical modulator10having a MOS capacitor structure includes a silicon on insulator (SOI) substrate including a silicon substrate12, a buried oxide layer (BOX)14, and an n-type silicon substrate16. A pair of n+-type doping areas22used as electron sources are formed on both sides of a p-type polysilicon layer20, which is an optical beam path formed on the n-type silicon substrate16. The n+-type doping areas22are grounded away from an optical condensing area24around the optical beam path.

Electrons and holes accumulate above and below a gate insulating layer22formed between the n-type silicon substrate16and the p-type polysilicon layer20.

Metal contacts32are formed on dopant areas34connected to the p-type polysilicon layer20to apply a positive driving voltage VDto the dopant areas34. An insulating layer36is formed between the metal contacts32and the optical condensing area24.

In the optical modulator shown inFIG. 1, the optical condensing area24is formed mainly under the gate insulating layer22, and a phase is delayed by the electrons accumulated under the gate insulating layer22.

FIG. 2shows an optical mode distribution of light condensed in the optical condensing area24. Referring toFIG. 2, reference character A denotes a normalized optical profile obtained when the gate insulating layer22is relatively thin, and reference character B denotes a normalized optical profile obtained when the gate insulating layer22is thicker than in the normalized optical profile A.

As shown inFIG. 2, the maximum peak of the condensed light is positioned under the gate insulating layer22.

DISCLOSURE OF INVENTION

Technical Problem

However, the n+-type doping areas22must be as far away as possible from the optical condensing area24to prevent optical attenuation in the optical condensing area24. If a distance between the n+-type doping areas22and the optical condensing area24is too great, an optical modulation speed becomes slow. Thus, the switching speed of an optical device is reduced.

Technical Solution

The present invention provides an optical device capable of preventing optical attenuation in an optical condensing area and increasing optical modulation efficiency to secure a desired switching speed and an improved operation speed.

According to an aspect of the present invention, there is provided an optical device including: a semiconductor substrate of a first conductive type; a gate insulating layer formed on the semiconductor substrate; a gate of a second conductive type opposite to the first conductive type, formed on the gate insulating layer; a high density dopant diffusion area formed in the semiconductor substrate under the gate and doped with a first conductive type dopant having a higher density than the semiconductor substrate; a strained buried channel area formed of a semiconductor material having a different lattice parameter from a material of which the semiconductor substrate is formed and extending between the gate insulating layer and the semiconductor substrate to contact the high density dopant diffusion area; and a semiconductor cap layer formed between the gate insulating layer and the strained buried channel area.

If the first conductive type is a p-type, the strained buried channel area may be formed of a material having compressive stress. For example, the strained buried channel area may be formed of a SiGe layer. The strained buried channel area may have a retrograded doping profile.

If the first conductive type is an n-type, the strained buried channel area may be formed of a material having tensile stress. For example, the strained buried channel area may be formed of a SiC layer. Alternatively, the strained buried channel area may be formed of a strained Si layer having tensile stress. In this case, the optical device may further include a SiGe buffer layer formed in the semiconductor substrate under the strained buried channel area in the semiconductor substrate.

The high density dopant diffusion area may include first and second high density dopant diffusion areas contacting the strained buried channel area. In this case, the first and second high density dopant diffusion areas may be spaced apart from each other in the semiconductor substrate with the gate disposed therebetween. At least one of the first and second high density dopant diffusion areas may be grounded.

The high density dopant diffusion area may include a first high density dopant diffusion area which contacts the strained buried channel area and a second high density dopant diffusion area which does not contact the strained buried channel area and is formed in the semiconductor substrate. In this case, the first and second high density dopant diffusion areas may be spaced apart from each other in the semi-conductor substrate with the gate disposed therebetween. At least one of the first and second high density dopant diffusion areas may be grounded.

The optical device may further include a current blocking insulating layer formed on the semiconductor cap layer to cover sidewalls of the gate and sidewalls of the gate insulating layer.

The semiconductor cap layer may include a protrusion extending from an upper surface of the strained buried channel area to the gate insulating layer. A sidewall of the protrusion of the semiconductor cap layer may be covered with the current blocking insulating layer. The semiconductor cap layer may be a silicon epitaxial layer.

The optical device may further include: a conductive layer formed on an insulating layer and connected to the gate so as to apply a driving voltage to the gate; and a metal line connected to the conductive layer through a via contact. The via contact may be horizontally spaced apart from the gate. The conductive layer may be formed of doped polysilicon.

The optical device may include a Mach-Zehnder interferometer optical modulator adopting the optical device as an optical phase shifter.

The optical device may include a Michelson interferometer optical modulator adopting the optical device as an optical phase shifter.

The optical device may include a ring-resonator interferometer optical modulator adopting the optical device as an optical phase shifter.

The optical device may include a multi-channel light intensity equalizer adopting the optical device as an optical attenuator.

The optical device may include an optical switch adopting the optical device as an optical phase shifter.

The optical device may include a variable optical filter adopting the optical device as the optical phase shifter.

Advantageous Effects

According to the present invention, in an optical device having a strained buried channel according to the present invention, a strained buried channel area can be formed of a semiconductor material between a pair of high density dopant diffusion areas in a semiconductor substrate. The semiconductor material can have a different lattice parameter from the material of the semiconductor substrate.

The strained buried channel area can be formed in a position in which optical condensing is maximum, to manufacture a silicon-based MIS optical device. Thus, the same optical phase can be changed with a low capacitance and a low resistance. In particular, an optical modulator having a silicon-based CMOS capacitor type structure can adopt a strained buried channel area using a carrier modulation effect. Thus, resistance can be lowered. Also, charge can accumulated due to the low capacitance. In addition, charge mobility can be improved using a strained channel structure, and an optical mode confinement factor of a charged layer can be increased in the strained buried channel area, to reduce a modulation distance. Thus, the operation speed of the optical device can be increased. Moreover, the strained channel structure can be compatible with a relatively simple MIS structure. Thus, the optical device can be manufactured to have a large area. As a result, manufacturing cost can be lowered and optical attenuation can be reduced, to obtain improved optical phase shift characteristics. Furthermore, an oxide layer having high interface characteristics can be used. Thus, a trap density can be lowered to minimize deterioration of performance during high-speed operation.

BEST MODE

FIG. 3is a cross-sectional view showing the essential parts of an optical device according to an embodiment of the present invention.

In the present embodiment, the optical device100is constructed on a silicon-oxide-insulator (SOI) substrate110, in which a first silicon substrate112, a buried oxide layer (BOX)114, and a second silicon substrate116are sequentially stacked. However, the present invention is not limited to this. For instance, the optical device100according to the present embodiment may use a bulk silicon substrate instead of the second silicon substrate116shown inFIG. 1. In this case, the optical device100may use a structure in which a material layer having a lower refractive index than a silicon substrate covers the backside of the silicon substrate.

The second silicon substrate116may be doped with p-type or n-type dopant.

The optical device100according to the present embodiment includes a gate insulating layer120formed on the second silicon substrate116and a gate122formed on the gate insulating layer120. The gate insulating layer120may be a silicon oxide layer. The gate122may be formed of a semiconductor layer doped with the opposite conductive type dopant to a conductive type of the second silicon substrate116. For example, if the second silicon substrate116is formed of a p-type silicon layer, the gate122may be formed of n-type polysilicon.

A pair of high density dopant diffusion areas132and134are formed in the second substrate116on both sides of the gate122. The high density dopant diffusion areas132and134are doped with a dopant of the same conductive type as the second silicon substrate116but to a higher doping density. The high density dopant diffusion areas132and134operate as ohmic layers for an ohmic contact, and may be separately grounded. It has been described with reference toFIG. 3that the pair of high density dopant diffusion areas132and134are formed on both sides of the gate122. Alternatively, only one of the pair of high density dopant diffusion areas132and134may be formed.

A strained buried channel area140extends between the gate insulating layer120and the second silicon substrate116under the gate122. Also, a semiconductor cap layer142is formed between the gate insulating layer120and the strained buried channel area140. The semiconductor cap layer142may be a silicon epitaxial layer.

A current blocking insulating layer150is formed on both sides of the gate122on the semiconductor cap layer142. Sidewalls of the gate insulating layer and sidewalls of the gate122are covered by the current blocking insulating layer150. Thus, the length LGof the gate122is restricted by the current blocking insulating layer150. For example, the current blocking insulating layer150may be a silicon oxide layer.

A conductive layer172is formed on the current blocking insulating layer150. The conductive layer172is connected between a metal line layer174and the gate122to apply a driving voltage to the gate122. The conductive layer172may be formed of doped polysilicon. Also, the metal line layer174may be formed of aluminum (Al). A via contact176may be formed at a distance d from the gate122in a horizontal direction to electrically connect the conductive layer172to the metal line layer174. Thus, a resistor-capacitor (RC) time constant of the optical device can be reduced. Also, the conductive layer172can be formed of polysilicon doped to a high density to improve electrical coupling of the via contact176with the gate122and further reduce the RC time constant of the optical device100.

Referring toFIG. 3, reference numeral180denotes a current blocking insulating layer. For example, the current blocking insulating layer180may be a silicon oxide layer.

The semiconductor cap layer142includes a protrusion142aof the upper surface of the strained buried channel area140which extends to the gate insulating layer120. A sidewall of the protrusion142aof the semiconductor cap layer142is covered by the current blocking insulating layer150.

The gate122may be formed in a higher position due to the protrusion142aof the semiconductor cap layer142, and thus a buried channel may be positioned more deeply. Thus, optical attenuation in an optical condensing area160caused by the gate122can be reduced. However, the present invention is not limited to the structure ofFIG. 3in which the protrusion142ais formed in the semiconductor cap layer142. The semiconductor cap layer142may be formed on the upper surface of the strained buried channel area140but may not include the protrusion142a. It must be understood that this structure is included in the scope of the present invention.

The strained buried channel area140may be formed of a semiconductor material having a different lattice parameter from the material of the second silicon substrate116. For example, if the second silicon substrate116and the high density dopant diffusion areas132and134are of a p-type, i.e. a phase is to be changed using holes, the strained buried channel area140may be formed a SiGe layer having a larger lattice parameter than the second silicon substrate116and the semiconductor cap layer142. The SiGe layer epitaxially grown on the second silicon layer116has the same lattice network as a silicon layer and has a compressive stress due to a lattice parameter difference in the second silicon layer116when the SiGe layer is epitaxially grown on the second silicon layer116. The content of Ge may be adjusted in the strained buried channel area140formed of the SiGe layer, to adjust the compressive stress in the strained buried channel area140to the desired level. Thus, the content of Ge in the strained buried channel area140formed of the SiGe layer may vary. Also, the strained buried channel area140formed of the SiGe layer may have a retrograde doping profile.

Mode for Invention

FIG. 4shows the energy distribution around the strained buried channel area140of the optical device100of theFIG. 3formed of the SiGe layer.

As shown inFIG. 4, if a negative voltage is applied to the gate122, a hole channel is formed in the semiconductor cap layer142. However, the hole channel is also formed around the strained buried channel area140due to an offset of a balance band. Thus, a trap density can be lowered, and a sufficient hole density can vary under a rapid switching condition.

As described above, if a phase is to be changed using holes, an optical phase modulation ratio of holes is generally approximately double that of electrons. Thus, if the second silicon layer116is of p-type and the high density dopant diffusion areas132and134are of p+to change the phase using the holes, the strained buried channel area140may be formed of the SiGe layer to improve the mobility of the holes. Thus, the resistance of the second silicon substrate116may be reduced, and the operation speed of the optical device100may be improved.

As another example, if the second silicon substrate116and the high density dopant diffusion areas132and134are of n-type, i.e. the phase is to be changed using electrons, the strained buried channel area140may be formed of a SiC layer. The strained buried channel area140formed of the SiC layer has a tensile stress. The content of C may be adjusted in the strained buried channel area140formed of the SiC layer to adjust the tensile stress in the strained buried channel area140to the desired level. Thus, the content of C in the strained buried channel area140formed of the SiC layer may vary.

As another example, although not shown, the strained buried channel area140may be formed of a Si layer having a tensile stress. In this case, an additional SiGe buffer layer (not shown) may be formed under the strained buried channel area140to apply the tensile stress to the strained buried channel area140.

As shown inFIG. 3, the strained buried channel area140extends between the pair of high density dopant diffusion areas132and134so that both sides of the strained buried channel area140contact the pair of high density dopant diffusion areas132and134. However, the present invention is not limited to this. In the present invention, the strained buried channel area140may contact only one of the pair of high density dopant diffusion areas132and134. Here, the one of the high density dopant diffusion areas132and134which does not contact the strained buried channel area140may constitute a contact for stabilizing the potential of the second silicon substrate116.

The strained buried channel area140may be formed in a position corresponding to a peak light position in the optical condensing area160. For this purpose, various optimal methods may be adopted. For example, a channel of the strained buried channel area140may be formed of a multiple quantum well, and the thicknesses of the gate insulating layer120and the semiconductor cap layer142may be adjusted to form the strained buried channel area140in the desired position. As another example, although not shown, the strained buried channel area140may be positioned between the gate122and the current blocking insulating layer150.

As described above, the strained buried channel area140including a buried channel may be formed of a material having a different lattice parameter from the second silicon layer116to improve the mobility of electrons in the buried channel. Thus, although the high density dopant diffusion areas132and134are located far from the optical condensing area160, a modulation length may be substantially reduced. The high density dopant diffusion areas132and134may be located at a distance from the optical condensing area160without increasing the modulation length. Thus, optical attenuation in the optical condensing area160may be reduced.

In particular, if the strained buried channel area140is formed of the SiGe layer, the SiGe layer has a higher refractive index than a pure Si layer and thus is more beneficial to light confinement to a channel area. Thus, the modulation length may be further reduced, which lowers an equivalent capacitance and further improves the operation speed.

Also, the buried channel may be formed by the strained buried channel area140to reduce a metal-oxide-semiconductor (MOS) capacitance of the buried channel. The two characteristics described above contribute to increasing the operation speed and optical delay ratio of the optical device100.

FIG. 5shows the important parts of an optical device200according to another embodiment of the present invention.

The optical device200illustrated inFIG. 5includes a Mach-Zehnder interferometer optical modulator adopting the optical device100ofFIG. 3as an optical phase shifter210.

The Mach-Zehnder interferometer optical modulator includes a passive waveguide206, a Y-light intensity demultiplexer208, the optical phase shifter210, and a Y-light intensity multiplexer218. The optical phase shifter210may be the optical device100having the structure illustrated inFIG. 3.

A continuously incident beam having a continuous light intensity is input to the passive waveguide206through a nonreflective layer202. The Y-light intensity de-multiplexer208demultiplexes the continuously incident beam to two arms212and214of the Mach-Zehnder interferometer optical modulator. InFIG. 5, the optical phase shifter210is provided at the arm214. However, in the optical device200according to the present embodiment, the optical phase shifter200may be provided at least one of the arms212and214. A phase is modulated by applying a modulated voltage VRFthrough a modulation signal applying port216of the optical phase shifter210. The beam modulated by the optical phase shifter210is offset or reinforcement interfered by the Y-light intensity multiplexer218to output an optical signal having a modulated intensity through a nonreflective layer222.

A reflection occurs from an as-cleaved facet of the passive waveguide206due to a refractive index difference at the air interface. Thus, the nonreflective layers202and222may be disposed at the as-cleaved facet of input and output parts of the passive waveguide206. The unreflective layers202and222may be disposed to incline to the as-cleaved facet of the passive waveguide206to further reduce the reflection from the as-cleaved facet of the passive waveguide206.

FIG. 6shows the important parts of an optical device300according to another embodiment of the present invention.

The optical device300illustrated inFIG. 6includes a Michelson type optical modulator adopting the optical device ofFIG. 3as an optical phase shifter310.

The Michelson type optical modulator includes a passive waveguide306, a Y-light intensity demultiplexer308, and the optical phase shifter310.

The optical phase shifter310may be the optical device100having the structure illustrated inFIG. 3. A continuously incident beam302having a continuous light intensity is input to the passive waveguide306, and the Y-light intensity demultiplexer308demultiplexes the continuously incident beam into two arms312and314of a Michelson interferometer. The optical phase shifter310having the structure of the optical device100ofFIG. 3is provided at least one of the two arms312and314. InFIG. 6, the optical phase shifter310is provided at the arm314. A phase is modulated by applying a modulated voltage VRFthrough a modulation signal applying port316of the optical phase shifter310. The beam modulated by the optical phase shifter310is reflected from an as-cleaved facet or a facet on which a high reflecting layer320is deposited. Thus, a phase is modulated once more. Thereafter, the Y-light intensity de-multiplexer308offsets or reinforcement interferes the continuously incident beam302to output an output beam332having a modulated light intensity. A reflection occurs from an as-cleaved facet of the passive waveguide306due to a refractive index difference at the air interface. Thus, a nonreflective layer304may be deposited on an input part of the passive waveguide306. The nonreflective layer304may be disposed toward input and output parts of the passive waveguide306to incline to the as-cleaved facet of the passive waveguide306so as to further reduce the reflection from the as-cleaved facet. Also, a circulator330may be additionally provided to separate the continuously incident beam302and the modulated output beam332from each other.

The Y-light intensity demultiplexer208, the Y-light intensity multiplexer218, and the Y-light intensity demultiplexer308shown inFIGS. 5 and 6may be replaced with a direction coupler or a multimode interferometer (MMI) coupler.

FIG. 7shows the important parts of an optical device400according to another embodiment of the present invention.

The optical device400illustrated inFIG. 7includes a ring-resonator type optical modulator adopting the optical device100ofFIG. 3as an optical phase shifter410.

The ring-resonator type optical modulator includes a passive waveguide406and the optical phase shifter410provided at a ring resonator420. The optical phase shifter410may be the optical device100having the structure illustrated inFIG. 3. A continuously incident beam is input to the passive waveguide406and then coupled by the ring resonator420. Thus, if a modulated voltage VRFis applied through a modulation signal applying port416of the optical phase shifter410of the ring resonator420, an effective refractive index varies. Thus, an input wavelength λ0is no longer coupled to the ring resonator420but is output to the passive waveguide406. According to this principle, an optical signal is modulated depending on an applied voltage. A reflection occurs from an as-cleaved facet of the passive waveguide406due to a refractive index difference at the air interface. Thus, a nonreflective layer404may be deposited on an input part of the passive waveguide406. Also, nonreflective layers404and424may be disposed toward input and output parts of the passive waveguide406to incline to the as-cleaved facet of the passive waveguide406so as to further reduce the reflection from the as-cleaved facet of the passive waveguide406. The optical device400illustrated inFIG. 7, may be used as a variable optical filter.

FIG. 8is a block diagram showing the important parts of an optical device500according to another embodiment of the present invention.

The optical device500illustrated inFIG. 8adopts the optical device100ofFIG. 3as a variable optical attenuator510.

Referring toFIG. 8, the optical device500includes an optical transmitter502, the variable optical attenuator510, and an optical receiver512. The variable optical attenuator510may be the optical device100having the structure illustrated inFIG. 3. The static or dynamic intensity of an optical signal generated by the optical transmitter502is adjusted by the variable optical attenuator510, and then the optical signal is transmitted to the optical receiver512.

FIG. 9is a block diagram showing the important parts of an optical device600according to another embodiment of the present invention.

The optical device600illustrated inFIG. 9includes a multi-channel light intensity equalizer adopting a plurality of the optical device100ofFIG. 3as variable optical attenuators610. Equivalent output light intensities of multi-channel wavelengths having different light intensities are adjusted using the plurality of variable optical attenuators610.

Referring toFIG. 9, the optical device600includes a plurality of variable optical attenuators610, an optical demultiplexer602connected to input and output units of the variable optical attenuators610, and an optical multiplexer604. Each of the variable optical attenuators610may be the optical device100having the structure illustrated inFIG. 3.

Multi-wavelengths λ1, λ2, . . . , λn-1, λnhaving different light intensities are classified into wavelength channels by the optical demultiplexer602. The optical intensities may be adjusted using the variable optical attenuators610and then transmitted to the optical multiplexer604through an optical fiber or an optical passive waveguide.

FIG. 10shows the important parts of an optical device700according to another embodiment of the present invention.

The optical device700illustrated inFIG. 10includes an optical switch adopting the optical device ofFIG. 3as an optical phase shifter710.

Referring toFIG. 10, the optical switch includes a pair of input passive waveguides702and704, the optical phase shifter710installed at least one of the pair of input passive waveguides702and704, and output passive waveguides712and714. The optical phase shifter710may be the optical device100having the structure illustrated inFIG. 3.

Optical signals incident on the input passive waveguides702and704pass a direction coupler720including the optical phase shifter710and are then output to the output passive waveguides712and714. In the direction coupler720, a coupling ratio of the optical signals from an upper passive waveguide to a lower passive waveguide may be adjusted depending on the variation of an effective refractive index depending on a voltage applied to the optical phase shifter710of the direction coupler720as in the ring-resonator type optical modulator illustrated inFIG. 8.

The optical device700illustrated inFIG. 10may be used in a variable optical filter, an optical modulator, or the like.

Passive waveguides used in the optical devices200,300,400,500,600, and700illustrated inFIGS. 5 through 10may be formed of a Si-based, GaAs-based, InP-based, GaN-based, or ZnO-based semiconductor material, or polymer, lithium niobate, optical fiber, or the like. Also, a passive waveguide, an optical multiplexer, a variable optical attenuator, or an optical phase shifter may be integrated into a monolithic substrate using Si, InP, GaN, or GaAs.

INDUSTRIAL APPLICABILITY

The optical device according to the present invention may be applied to a high-speed optical modulator, a high-speed optical switch, a high-speed variable optical attenuator, a high-speed optical filter, a high-speed multi-channel output light equalizer, or the like.