US 7557024 B2
More complete charge transfer is achieved in a CMOS or CCD imager by reducing the spacing in the gaps between gates in each pixel cell, and/or by providing a lightly doped region between adjacent gates in each pixel cell, and particularly at least between the charge collecting gate and the gate downstream to the charge collecting gate. To reduce the gaps between gates, an insulator cap with spacers on its sidewalls is formed for each gate over a conductive layer. The gates are then etched from the conductive layer using the insulator caps and spacers as hard masks, enabling the gates to be formed significantly closer together than previously possible, which, in turn increases charge transfer efficiency. By providing a lightly doped region between adjacent gates, a more complete charge transfer is effected from the charge collecting gate.
1. A process for fabricating a semiconductor device comprising:
forming a conductive layer over a substrate;
forming a resist layer directly on the conductive layer;
patterning the resist layer according to a desired arrangement of gates to be formed from the conductive layer, leaving a portion of the conductive layer exposed through the patterned resist layer;
partially etching the conductive layer using the patterned resist layer as masks to form a plurality of conductive structures, each conductive structure having a first width and being separated from each adjacent conductive structure by a first gap width;
removing remnants of the patterned resist layer, such that the plurality of conductive structures consist substantially entirely of conductive material from the conductive layer;
forming spacers on sidewalls of the conductive structures; and
etching the exposed conductive layer, such that the exposed conductive layer other than the plurality of conductive structures is substantially removed.
2. The process according to
the partial etching of the conductive layer reduces the thickness of the conductive layer where exposed through the patterned resist layer to at least approximately one-half of the original thickness thereof.
3. The process according to
doping a region between two adjacent ones of the plurality of conductive structures.
4. The process according to
5. The process according to
6. The process according to
the conductive structures are formed in a top half of the conductive layer.
7. The process according to
the conductive structures have a height equal to the height of the top half of the conductive layer.
8. The process according to
doping a region between two adjacent ones of the plurality of conductive structures.
9. The process according to
10. The process according to
This application is a divisional of U.S. patent application Ser. No. 10/688,974, which was filed Oct. 21, 2003, now U.S Pat. No. 6,998,657 the disclosure of which is hereby incorporated herein by reference in its entirety.
The invention relates generally to semiconductor imaging devices, in particular to silicon imaging devices which can be fabricated using standard CMOS processes, or alternatively, CCD fabrication processes. Particularly, the invention relates to CMOS and CCD imagers and a method of fabricating CMOS and CCD imagers with improved charge transfer between gates, and furthermore with reduced image lag in CMOS imagers.
There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), complementary metal oxide semiconductor devices (CMOS), photodiode arrays, charge injection devices and hybrid focal plane arrays. Among these, CCDs and CMOS imagers are the most commonly used in digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television. Each type of imaging device has advantages and disadvantages relative to the other.
CCDs imagers have a greater sensitivity to light and have better dynamic range than CMOS imagers, and therefore yield superior quality images. CCDs are also capable of large formats with small pixel size, and produce less noise (visual artifacts). As a result of these advantages, CCDs are the preferred technology for high end imaging applications.
However, CCD imagers also suffer from a number of disadvantages. For example, they are susceptible to radiation damage, exhibit destructive read out over time, require good light shielding to avoid image smear, and have high power dissipation for large arrays. Additionally, while offering high performance, CCD arrays are difficult to integrate with CMOS processing due in part to a different processing technology and to their high capacitances, which complicates the integration of on-chip drive and signal processing electronics with the CCD array. Further in this regard, CCDs must be manufactured at one of a small number of specialized fabrication facilities, thus greatly increasing production costs and limiting economies of scale. CCDs also must transfer an image by line charge transfers from pixel to pixel, requiring that the entire array be read out into a memory before individual pixels or groups of pixels can be accessed and processed. CCDs may also suffer from incomplete charge transfer from pixel to pixel during charge transfer, which results in image smear.
On the other hand, CMOS imagers have the advantage of being compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion). On-chip integration of electronics provides the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a compact system size. CMOS imagers also allow random access to the image data, and have low fabrication costs as compared with CCD imagers since standard CMOS processing techniques can be used. Additionally, CMOS imagers have low voltage operation and low power consumption because only one row of pixels at a time needs to be active during readout and there is no charge transfer (and associated switching) from pixel to pixel during image acquisition.
Both CCD and CMOS imagers perform the necessary functions of (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of the accumulated image charge; (4) converting the accumulated image charge to a voltage; and (5) output and amplification of the signal voltage representing the charge from each pixel in the imager. Both CCD and CMOS imagers include an array of pixels, each pixel having a substrate and a photosensitive area formed in or on the substrate and which converts photons from the incident light into charge, either electrons or holes. CCD and CMOS imagers differ, however, in their structure and manner of processing accumulated charges after photon to charge conversion.
The basic structure of a pixel within a CCD imager is shown in
Substrate 10 includes a buried channel 8 formed in the substrate 10 under the electrodes 12 a, 12 b. Typically in a CCD imager, the substrate is doped p-type, whereupon the buried channel is doped n-type. When a voltage is applied to gate electrode 12 b, for example, photons from the incident light are converted to electrical charge in the buried channel 8 under the “activated” gate 12 b, and a well 13 is formed in the substrate in which the charge is accumulated under the activated gate 12 b. Charge is contained in the well by applying appropriate voltages to the gate electrodes 12 a surrounding the activated gate to form zones of higher potential surrounding the well 13, thus confining the accumulated charge in the well 13.
The accumulated charge is transferred out of the pixel by “moving” the well from one gate electrode 12 to another in the pixel by alternating the voltages applied to the different electrodes until the charge is moved out of the pixel. In this manner, the pixel charges are moved through the array 15 row by row (
Depending on the number of gates in each pixel within a particular CCD architecture, a complete charge transfer cycle may be completed for each pixel in four phases, three phases or two phases, in accordance with the clock signal PCLK. For example, a timing diagram for a four phase CCD is shown in
An exemplary CMOS imager is described below with reference to
Like a CCD imager, the CMOS imager includes a focal plane array of pixel cells. As shown in
The photodetector circuit 14 is shown in part as a cross-sectional view of a semiconductor substrate 16 formed typically of a p-type silicon, and having a surface well of p-type material 20. An optional layer 18 of p-type material may be used if desired, but is not required. Substrate 16 may be formed of, for example, Si, SiGe, Ge, and GaAs. Typically the entire substrate 16 is a p-type doped silicon substrate and may contain a surface p-well 20 (with layer 18 omitted), but many other options are possible, such as, for example p on p− substrates, p on p+ substrates, p-wells in n-type substrates, or the like.
An insulating layer 22 of silicon dioxide, silicon nitride or other suitable material is formed on the upper surface of p-well 20. A photogate 24 thin enough to pass radiant energy or of a material which passes radiant energy is formed on the insulating layer 22. The photogate 24 receives an applied control signal PG which causes the initial accumulation of pixel charges underneath the photogate 24 and in n+ region 26. The n+ type region 26, adjacent one side of photogate 24, is formed in the upper surface of p-well 20.
A transfer gate 28 is formed on insulating layer 22 between n+ type region 26 and a second n+ type region 30 formed in p-well 20. The n+ regions 26 and 30 and transfer gate 28 form a charge transfer transistor 29 which is controlled by a transfer signal TX. When a transfer signal TX is applied to the transfer gate 28, the charge accumulated in n+ region 26 is transferred into n+ region 30. The n+ region 30 is typically called a floating diffusion node, and is also a node for passing charge accumulated thereat to the gate of a source follower transistor 36 described below.
A reset gate 32 is also formed on insulating layer 22 adjacent and between n+ type node 30 and another n+ region 34 which is also formed in p-well 20. The reset gate 32 and n+ regions 30 and 34 form a reset transistor 31 which is controlled by a reset signal RST. The n+ type region 34 is coupled to voltage source VDD. The transfer and reset transistors 29, 31 are n-channel transistors as described in this implementation of a CMOS imager circuit in a p-well. It should be understood that it is possible to implement a CMOS imager in an n-well, in which case each of the transistors would be p-channel transistors. It should also be noted that while
Photodetector circuit 14 also includes two additional n-channel transistors, source follower transistor 36 and row select transistor 38. Transistors 36 and 38 are coupled in series, source to drain, with the source of transistor 36 also coupled over lead 40 to voltage source VDD and the drain of transistor 38 coupled to a lead 42. The gate of transistor 36 is coupled over lead 44 to n+ region 30. Charge from the floating diffusion node at the n+ region 30 is typically converted to a pixel output voltage by the source follower output transistor 36. The drain of row select transistor 38 is connected via conductor 42 to the drains of similar row select transistors for other pixels in a given pixel row. A load transistor 39 is also coupled between the drain of transistor 38 and a voltage source VSS. Transistor 39 is kept on by a signal VLN applied to its gate.
The imager includes a readout circuit 60 which includes a signal sample and hold (S/H) circuit including a S/H n-channel field effect transistor 62 and a signal storage capacitor 64 connected to the source follower transistor 36 through row transistor 38. The other side of the capacitor 64 is connected to a source voltage VSS. The upper side of the capacitor 64 is also connected to the gate of a p-channel output transistor 66. The drain of the output transistor 66 is connected through a column select transistor 68 to a signal sample output node VOUTS and through a load transistor 70 to the voltage supply VDD. A sample and hold signal (SHS) briefly turns on the S/H transistor 62 after the charge accumulated beneath the photogate electrode 24 has been transferred to the floating diffusion node 30, and from there, to the source follower transistor 36 and through row select transistor 38 to line 42, so that the capacitor 64 stores a voltage representing the amount of charge previously accumulated beneath the photogate electrode 24.
The readout circuit 60 also includes a reset sample and hold (S/H) circuit including a S/H transistor 72 and a signal storage capacitor 74 connected through the S/H transistor 72 and through the row select transistor 38 to the source of the source follower transistor 36. The bottom side of the capacitor 74 is connected to the source voltage VSS. The upper side of the capacitor 74 is also connected to the gate of a p-channel output transistor 76. The drain of the output transistor 76 is connected through a p-channel column select transistor 78 to a reset sample output node VOUTR and through a load transistor 80 to the supply voltage VDD. A sample and hold reset signal (SHR) briefly turns on the S/H transistor 72 immediately after the reset signal RST has caused reset transistor 31 to turn on and reset the potential of the floating diffusion node 30, so that the capacitor 74 stores the voltage to which the floating diffusion node 30 has been reset.
The readout circuit 60 provides correlated sampling of the potential of the floating diffusion node 30, first of the reset charge applied to node 30 by reset transistor 31 and then of the stored charge from the photogate 24. The two samplings of the diffusion node 30 charges produce respective output voltages VOUTR and VOUTS of the readout circuit 60. These voltages are then subtracted (VOUTS-VOUTR) by subtractor 82 to provide an output signal terminal 81 which is an image signal independent of pixel to pixel variations caused by fabrication variations in the reset voltage transistor 31 which might cause pixel to pixel variations in the output signal.
A transfer gate voltage pulse TX, similar to the reset pulse RST, is then applied to transfer gate 28 of transistor 29 to cause the charge in n+ region 26 to transfer to floating diffusion node 30. It should be understood that for the case of a photogate, the transfer gate voltage TX may be pulsed or held to a fixed DC potential. For the implementation of a photodiode with a transfer gate, the transfer gate voltage TX must be pulsed. The new output voltage on line 42 generated by source follower transistor 36 current is then sampled onto capacitor 64 by enabling the sample and hold switch 62 with signal SHS. The column select signal is next applied to transistors 68 and 70 and the respective charges stored in capacitors 64 and 74 are subtracted in subtractor 82 to provide a pixel output signal at terminal 81. It should also be understood that CMOS imagers may dispense with the transistor gate 28 and associated transistor 29, or retain these structures while biasing the transfer transistor gate 28 to an always “on” state.
Both CMOS and CCD imagers are susceptible to inefficient charge transfer between gates. In the CMOS imager shown in
In order to improve the charge transfer between gates in both CMOS and CCD imagers, the gates must be spaced as close together as possible. The gates are formed by depositing a single layer of polysilicon (or other suitable conductive material) on the substrate surface (over the insulating layer such as silicon dioxide, silicon nitride, etc.). The individual gates are then patterned from the blanket deposited layer by applying a layer of photoresist over the polysilicon (or other conductive) material, and exposing the photoresist through a reticle to develop the portions of the photoresist where the gates are to be formed. The undeveloped portions of the photoresist are then removed. Once the shaped photoresist layer has been obtained on the blanket deposited layer of conductive material, the gates are shaped by etching the layer of conductive material around the patterned photoresist layer.
The smallest distance between semiconductor structures using known patterning methods such as that mentioned above is subject to the physical limitations of how thin a distinguishable line or gap can be formed in the photoresist layer by patterning with the reticle. Recent advances in technology enable lines and spaces between semiconductor structures to be 0.13 micrometers apart, i.e., about 1300 Angstroms. Even with these measurements, however, the resulting gaps between polysilicon gates still yield incomplete charge transfer.
The present invention addresses the problem of incomplete and inefficient charge transfer between gates formed on a semiconductor substrate in a CMOS or CCD imager. In particular, the present invention provides a method of fabricating a plurality of single layer gates on a CMOS or CCD imager which significantly reduces the gaps between gates, to thereby reduce or eliminate the problem of incomplete charge transfer.
The method includes blanket depositing the conductive material from which the gates will ultimately be formed, as is standard practice in the art, and then blanket depositing a layer of insulator material, such as an oxide or nitride material, and patterning the insulator material in a manner similar to that in which the conductive layers are patterned in the prior art to form the CMOS or CCD pixel gates. The patterned insulator structures are referred to as “caps.” Next, spacers are deposited on the sides of the patterned insulator material to decrease the width of the gaps between caps. Using the spacer-reduced gaps between the insulator caps on top of the conductive layer, the conductive layer is etched, resulting in gate structures which are approximately 300 Angstroms apart.
A variation of this method includes blanket depositing a layer of the conductive material from which the gates are to be formed, and then depositing a layer of resist over the conductive material layer. The resist is patterned according to the desired gate arrangement, and the conductive layer is partially etched to form gate-like structures of the conductive material protruding above the remaining thickness of the conductive material layer. Next, spacers are formed along the sidewalls of the gate-like structures, and the remaining thickness of the conductive layer around the gate-like structures is etched away. During this second etch process, the portion of the conductive material between the spacers is also etched, leaving the resulting gate structures which spaced apart by approximately the distance between the spacers formed on the sidewalls of adjacent gates.
A second aspect of the present invention may be used during the fabrication of CMOS imagers either separately or in conjunction with the method briefly described above, and includes providing a lightly doped region n− between the photogate and an adjacent gate, instead of standard heavily doped region n+.
Additional advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.
The terms “wafer” and “substrate” used in the description includes any semiconductor-based structure having an exposed surface on which to form the circuit structure used in the invention. “Wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions and/or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but may be based on silicon-germanium, germanium, or gallium arsenide.
To provide a foundation for understanding the present invention, an example of a prior art process for forming the transistor gates for an image sensor is described below with reference to
A layer of photoresist 208 is then applied over the conductive layer 206, and the photoresist is developed by exposure to a light through a reticle to produce the desired pattern of the transistor gates. Upon removal of the undeveloped portions of the photoresist, the developed photoresist portions 208 a remain on the conductive layer 206, as shown in
Individual transistor gates 206 a are then formed by etching the conductive layer 206 through to the insulating layer 204. Conductive layer 206 may be directionally etched by a suitable process such as reactive ion etching, or any other method readily known in the art, including immersion or spray-type wet etching, and plasma, or ion milling. Subsequent to the formation of the transistor gates 206 a, the patterned photoresist is then removed by wet etch or dry etch methods such as exposing the wafer to an oxygen-containing plasma, to obtain the structure shown in
The distance between transistor gates 206 a is denoted in
To address the problem of incomplete charge transfer, the transistor gates may be formed as double polysilicon structures, such as that shown in
Referring back to
Although no doped region is required to couple the photogate 230 with the transfer gate 236, a doped region 244 may be formed under the photogate 230 to provide a well in which charges generated at photogate 230 can accumulate until transferred to the floating diffusion region 246. The double polysilicon structure therefore requires careful alignment when performing the implanting of the doped region 244 to ensure that the doped region 244 does not extend across the area to be occupied by transfer gate 236 in a later processing step.
This double polysilicon process also suffers from the fact that all transistors formed by the first polysilicon deposition, including the photogate 230 and the reset gate 232, cannot be silicided gates, which would improve circuit speed and performance, for at least two reasons: (1) the top silicide layer cannot be oxidized to provide a reliable insulating oxide between the photogate 230 and the transfer gate 236, and (2) a silicide layer on top of the photogate would block signal light from passing through the photogate into the signal storage region 244 below the photogate.
The invention discussed below also addresses the problem of incomplete charge transfer but without any of the disadvantages discussed heretofore.
As shown in
Thus far, the process is similar to the prior art process illustrated in
Next, a resist layer 110 is deposited on the insulator layer 108 and then patterned, whereby the undeveloped resist is removed to leave behind developed portions 110 a, as shown in
Exposed portions of the insulator layer 108 are then etched away using a directional etch method such as reactive ion etching, or other suitable removal process such as immersion or spray-type wet etching, and plasma or ion milling, and the remaining resist portions 110 a are removed by wet or dry etch methods to thereby form insulator caps 114 on the surface of conductive layer 106, as seen in
Referring now to
After forming the spacers 116 on the sidewalls of the insulator caps 114, another etch process is performed to etch through the conductive layer 106, using the insulator caps 114 and spacers 116 as hard masks, to yield the gate structures 118 as illustrated in
Using the process of the present invention, the distance between the conductive gate structures 118 is much smaller than previously achieved using a mask and resist alone. In the example described herein, the smallest achievable distance “z” between insulator caps 114 in
An alternative method for forming gate structures in accordance with this aspect of the invention is shown in and described with reference to
Next, as can be seen in
The conductive layer 126 is then partially etched, preferably to approximately half the thickness of the originally deposited conductive layer 126 in the regions not covered by the resist portions 128 a. The resist is removed, leaving the structure shown in
Referring now to
After forming the spacers 130, the conductive layer 126 is etched again. This time, the regions thinned in the previous etch process are removed completely, and the thickness of the gate-like portions 126 a between the spacers 130 is thinned. As seen in
In addition to the processes described above with reference to
In the conventional CMOS imager illustrated in
A second aspect of the present invention addresses this problem, and is applicable in connection with imagers having 3T, 4T, 5T, 6T or 7T structure, such as the imager having a photogate, a transfer gate and a reset gate as described above with reference to
According to this aspect of the invention, gates 150, 152 and 154 are formed over a substrate 164, as shown in
An n− doped region has a lesser affinity for holding onto electrons than an n+ doped region, resulting in more complete charge transfer out of the n− doped region. Thus, although the region 162 between gates 152 and 154 may be n+ doped as in the prior art CMOS imagers, it is preferably also n− doped. Similarly, the region between any two adjacent transistor gates in a CMOS imager may be lightly doped according to the present invention, wherein such gates may include the photogate, the transfer gate, the reset gate, the source follower gate, the row select gate, and/or the storage gate.
This concept may also be implemented in a CCD imager by providing a lightly doped region between two transistor gates along the charge transfer path of a readout cycle. Preferably, a lightly doped region is formed between each pair of adjacent gates in the charge transfer path of a readout cycle.
The depth and concentration density of the dopant ions implanted into each region 156, 162 is determined by the implant range and diffusion in the substrate, which in turn is impacted by the temperature during the implantation process and the time duration at that temperature. Generally, however, an n+ doped region has a concentration of about 5·1014 ions/cm2 to about 1·1016 ions/cm2, with 1·1015 ions/cm2 to about 3·1015 ions/cm2 being typical. In the present invention, the n− doped region 156 has a concentration of about 3·1011 ions/cm2 to about 1·1014 ions/cm2, with 1·1012 ions/cm2 to about being 1·1013 ions/cm2 being preferred. For a doped region having a depth of about 1 μ (10−4 cm) and using a concentration of 1·1012 ions/cm2, therefore, the n− doped region 156 has a concentration density of ρ=(1·1012 ions/cm2)/(10−4 cm)=1 ·1016 ions/cu.cm.
Any suitable doping process may be used to form the n− doped region 156 and the n+ doped region 162. For example, the regions 156 and 162 may be formed by ion implantation, and may be performed in an ion implanter device by implanting appropriate n-type ions (e.g., arsenic, antimony, phosphorous, etc.) at an energy level of about 10 KeV to about 200 KeV into the substrate 164 to a depth of approximately 200-1000 Angstroms. A resist and mask may be used to shield areas of the substrate which are not to be doped. Since the gates 150 and 152 define the boundary along two sides of region 156, and the gates 152 and 154 define the boundary along two sides of region 162, the resist and mask need only define the boundaries of the regions to be doped along the sides not constrained by the gates. Optionally, the n− region 156 may be formed by blanket doping the exposed surfaces of the substrate.
It should be noted that in many transistors, the source and drain are essentially interchangeable, and interconnections specified herein should not be interpreted as solely limited to those described. In addition, while the transistors have been described as n-type or n-channel, it is recognized by those skilled in the art that a p-type or p-channel transistor may also be used if the structures are uniformly oppositely doped from that described. For example, gates 150, 152 and 154 in
Each pixel in the imaging array 15 of
The imagers of
The processing system 500 illustrated in
The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the objects, features, and advantages of the present invention. However, it is not intended that the present invention be strictly limited to the above-described and illustrated embodiments. Any modifications, though presently unforeseeable, of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.
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