Abstract:
A flash memory structure comprises a semiconductor substrate, a source region, a drain region, a first insulating dielectric layer, a floating gate, a second insulating dielectric layer, and a control gate. The semiconductor substrate has a first top surface and a second top surface that is lower than the first top surface. The source region and the drain region are respectively in the second top surface and the first top surface of the semiconductor substrate, and the semiconductor substrate connecting the source region and the drain region is a vertical channel region. The whole channel region is covered by the first insulating dielectric layer, the floating gate, the second insulating dielectric layer, and the control gate in turn.

Description:
RELATED APPLICATIONS 
   The present application is a divisional application of U.S. patent application Ser. No. 10/981,653, filed Nov. 5, 2004, and is based on, and claims priority from, Taiwan Application Serial Number 93124229, filed Aug. 12, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention pertains to flash memory and, in particular, to a flash memory structure that has a vertical channel region with multiple gates and the fabrication method thereof. 
   2. Related Art 
   Flash memory is one kind of non-volatile memory. It is used to store data in memory unit. Not only can it maintain the memory of data without being charged, it further has the feature of multiple writing/rewriting. Therefore, it has rapidly developed to become the new generation memory device in recent years. 
   However, all the current flash memory structures have the problem of no scaling. Particularly when the sizes of electronic products and semiconductor devices become smaller, the fabrication of flash memory faces the bottleneck of being unable to satisfy the requirement of small device sizes and good device properties at the same time. 
   Since the source region, the drain region, and the channel region in the device are located on the same plane, variations in the device size will directly affect the channel length, which in turn forms a restriction in the device size definition. 
   The conventional flash memory structure is stacked gate flash memory, whose structure is similar to electrically erasable and programmable ROM (EEPROM). It has a control gate and a floating gate stacked on the device channel. It achieves the programming purpose by injecting hot carriers from the drain into the floating gate. Although the stacked gate flash memory has the advantage of a simpler structure for minimizing the device size, the hot carrier injection efficiency is very low. As the device size shrinks, not only is the channel length shortened, the area of the floating gate and the control gate stacked on the channel also has to decrease. This reduces the capacitance between the floating gate and the control gate, causing increase in the operating voltage. 
   To increase the hot carrier injection rate, most people add gate units. For example, in one type of source injection flash memory, there are three gate units. In addition to the stacked control gate and the floating gate, an injection gate is further provided above the source to induce the injection of hot carriers from the source to the floating gate. Although this type of flash memory can increase the hot carrier injection efficiency with the installation of the injection gate, the addition injection gate results in a larger device size. 
   There is also a split-gate flash memory structure, which has the same advantage as the source injection flash memory. By increasing the size of the control gate, the coupling ratio between the control gate and the floating gate and the hot carrier injection efficiency can be simultaneously increased, thereby lowering the operating voltage. Nonetheless, it also has a larger size due to the control gate design. 
   Therefore, how to keep the desired flash memory characteristics while at the same time minimizing the device size is an important research trend in the field. Moreover, as the device size becomes smaller, the punch-through or breakdown phenomenon may happen due to the short carrier channel. This will damage the device and reduce the device reliability. 
   SUMMARY OF THE INVENTION 
   An objective of the invention is to provide a flash memory structure and the fabrication method thereof. A device with a vertical channel region is provided to both keep the desired device characteristics and increase the scaling ability of the device. Moreover, the gate structure covers the vertical channel using multiple surfaces, forming a multiple-surface gate. This can increase the operating efficiency of the flash memory. 
   A flash memory structure and the fabrication method thereof are proposed according to the above-mentioned objective. According to an embodiment of the invention, the flash memory structure includes at least a semiconductor substrate, a source region, a drain region, a first insulating dielectric layer, a floating gate, a second insulating dielectric layer, and a control gate. The semiconductor substrate has a first top surface and a second top surface, with the former higher than the latter. The source region and the drain region are located in the second top surface and the first top surface of the semiconductor substrate, respectively. The semiconductor substrate connecting the source region and the drain region is a vertical channel, and the vertical channel further contains an L-shaped three-dimensional structure. 
   The first insulating dielectric layer is formed on the vertical channel of the semiconductor substrate to completely cover it. The floating gate, the second insulating dielectric layer and the control gate cover the first insulating dielectric layer in sequence. Due to the here-dimensional structure of the vertical channel region, the floating gate can surround the vertical channel region in many faces. In a preferred embodiment, the floating gate surrounds three side surfaces of the vertical channel region. In another embodiment, the floating gate further covers the top surface of the vertical channel region. The first insulating dielectric layer is a tunneling oxide layer. The floating gate may be a polysilicon layer or a silicon nitride layer. The control gate is a polysilicon layer. 
   According to another embodiment, the flash memory fabrication method includes the step of patterning a semiconductor substrate to remove part of the semiconductor substrate to a depth, forming island blocks in the semiconductor substrate. A trench is thus formed between each two island blocks. The top surface of the island block is the first top surface of the semiconductor substrate. The trench is the second top surface of the semiconductor substrate. 
   Afterwards, the first insulating dielectric layer is formed on the semiconductor substrate, covering the island blocks and the trenches at the same time. A charge storage pattern made from a charge storage layer is formed on the first insulating dielectric layer. The second insulating dielectric layer and the conductive layer are formed in sequence on the charge storage pattern to cover it. 
   After the second insulating dielectric layer and the conductive layer are formed, they along with the charge storage layer are further patterned to form at least a gate stack unit. The first insulating dielectric layer in part of the trenches and that in part of the island blocks are exposed on both sides of the gate stack unit. In particular, the gate stack unit covers the range of the three side surfaces and/or the top surface of some island blocks, and is positioned on part of the trenches, and the island blocks. 
   Afterwards, ion implantation is performed on the first top surface and the second top surface of the semiconductor substrate, forming the source region and the drain region, respectively. 
   The charge storage layer and the conductive layer mentioned above are used as the floating gate and the control gate, respectively. The semiconductor substrate connecting the source region and the drain region is a vertical channel region, whose three side surfaces and/or top surface are covered by the gate stack unit. In particular, the vertical channel region has an L-shaped three-dimensional structure. 
   From the above description of the disclosed flash memory structure, one sees that variations in the channel length does not have any effect on the device size due to the design of the vertical channel region. That is, the channel length and the device size are independent. When scaling the device, the channel length is not constrained. The device thus made can still keep good properties. Moreover, the contact area between the gate and the channel region is not restricted by the change in the device size. Even when the device size shrinks, an appropriate channel length can be maintained to avoid the punch-through or breakdown phenomenon, increasing the reliability and quality of devices. 
   The disclosed structure covers the vertical channel region in many faces to form a multiple-surface gate configuration. Therefore, not only can it effectively increase the contact area between the control gate, the floating gate and the channel region, but also increases the coupling ratio and the device operating capability. The capacitance between the control gate and the floating gate can be increased to lower the operating voltage. 
   Besides, using the L-shaped three-dimensional structure in the vertical channel region, an L-shaped carrier path is formed to increase the hot carrier injection rate during the programmable operation of the flash memory, increasing the operating speed and performance of the device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the invention will become apparent by reference to the following description and accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention, and wherein: 
       FIGS. 1A to 1D  are schematic cross-sectional views of a method for making a flash memory structure in accordance with embodiments of the invention; and 
       FIG. 2  is a schematic cross-sectional view of a flash memory structure according to a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
   The invention provides a flash memory structure and the fabrication method thereof. Using the design of a vertical channel region, the length of the carrier channel is not affected by variations in the device size. Therefore, one can scale the device without influencing the device characteristics. The disclosed flash memory further has the configuration of multiple-surface gate surrounding the carrier channel. This is especially helpful in increasing the operating characteristics of the flash memory and the coupling ratio between the control gate and the floating gate. In the following, we use specific embodiments to explain the disclosed method. 
     FIGS. 1A to 1D  show schematic cross-sectional views of a fabrication method for a flash memory structure. 
   In  FIG. 1A , a p-type well is formed on a semiconductor substrate  100 . Its formation method includes performing ion implantation and patterning the semiconductor substrate  100 , defining the electrical property of the semiconductor substrate  100  and the positions of trenches  101  and island blocks  102  thereon. One choice of the material of the semiconductor substrate  100  is silicon (Si). 
   In  FIG. 1B , a tunneling oxide layer  104  is first formed on the semiconductor substrate  100 , followed by forming a charge storage layer  106  on the tunneling oxide layer  104 , filling the trenches  101 . The charge storage layer  106  is used as a floating gate. The tunneling oxide layer  104  functions as an insulating dielectric layer between the floating gate and the semiconductor substrate  100 . One choice of the material of the tunneling oxide layer  104  is silicon dioxide (SiO 2 ). The charge storage layer  106  may be a polysilicon layer or a silicon nitride layer. 
   The charge storage layer  106  is then patterned, forming a charge storage circuit pattern parallel to the cross-sectional direction. Afterwards, an inter-layer dielectric layer  108 , a conductive layer  110 , and a nitride covering layer  130  are formed in sequence on the charge storage layer  106 , covering the charge storage circuit pattern at the same time. In particular, the conductive layer  110  serves as a control gate. The inter-layer dielectric layer  108  is an insulating dielectric layer between the control gate and the floating gate. The material of the inter-layer dielectric layer  108  may be silicon dioxide or a stack layer consisting of oxide/nitride/oxide (O/N/O). The conductive layer  110  may be a polysilicon layer. 
   With reference to  FIG. 1C , a conventional photolithography and etching process is employed to pattern the charge storage layer  106 , the inter-layer dielectric layer  108 , the conductive layer  110 , and the nitride covering layer  130 , defining the gate stack unit. The gate stack unit is composed of a control gate unit  112 , an inter-layer dielectric layer  108 , and a floating gate unit  114 . 
   After defining the control gate unit  112  and the floating gate unit  114 , a sidewall spacer  119  is formed on both sides of the gate stack unit to maintain the electrical isolation among the gate units, as shown in  FIG. 1D . The material of the sidewall spacer  119  may be silicon nitride. 
   Afterwards, photo resist and a photolithography process are used to define a source opening and a drain opening. For example, a photo resist layer is first formed on the semiconductor substrate  100 . Then the photolithography process is performed to simultaneously or respectively define the source opening and the drain opening. The photo resist layer inside the openings is removed later. 
   An ion implantation step is performed on the semiconductor substrate  100  in the source opening and the drain opening, forming an n-type source region  120  and an n-type drain region  122 . Finally, the residual photo resist layer is removed. This completes the procedure of making a flash memory structure with a vertical channel region. If subsequent processes for devices further proceed, such as forming an insulating dielectric layer and forming conductive metal in the source opening and the drain opening, one can make a more complete flash memory control unit. In particular, when a carrier moves from the source region  120  to the drain region  122 , a three-dimensional L-shaped channel (indicated by the arrow  126 ) is formed. 
   The three-dimensional flash memory structure formed using the described method is shown in  FIG. 2 . The programming of the flash memory is done with the carriers moving from the source region  220  to the drain region  222  on the semiconductor substrate  200 . At the drain region  222 , the carriers penetrate through the tunneling oxide layer  204  and reach the floating gate  214  in the channel hot carrier injection (CHEI) mode. The path traveled by the carriers from the source region  220  to the drain region  222  is the carrier channel. The control gate  212  and the floating gate  214  are separated by an inter-layer dielectric layer  208 . 
   Since the carrier channel in the invention is a three-dimensional vertical structure, which is different from the horizontal channels in the prior art, the floating gate  214  and the control gate  212  form a multiple-surface surrounded channel region. Thus, a flash memory structure in accordance with the invention has multiple-surface gate controls. In a preferred embodiment of the invention, the floating gate  214  and the control gate  212  surround three side surfaces of the channel region. In another embodiment, the floating gate  214  and the control gate  212  surround three side surfaces and the top surface of the channel region. 
   As the disclosed flash memory has a vertical carrier channel, variations in the length of the channel only affect the height in the longitudinal direction without influencing the device size. That is, the channel length and the device size are independent. As a result, the channel size is not restricted by scaling the device. 
   Besides, due to the three-dimensional structure of the vertical channel, the carriers first move upward from the source region  220  and then turn horizontally toward the drain region  222  at around the drain region  222  when moving from the source region  220  to the drain region  222 . Therefore, a three-dimensional L-shaped channel is formed. In this case, the carrier density at the turning point of the L-shaped channel is higher, rendering a higher hot carrier injection rate. This improves the overall carrier injection efficiency of the device. 
   From the above-mentioned embodiments of the invention, one sees that the disclosed flash memory structure and the fabrication method thereof diminish the restriction in the device size by the channel length due to the vertical channel design. Therefore, the device size can be scaled without sacrificing the desired device characteristics. The contact area between the gate and the channel region is not restricted by the device size. 
   Furthermore, even when the device size shrinks, an appropriate channel length is maintained to prevent the punch-through or breakdown phenomenon. Thus, the device reliability and quality is assured. 
   As a result of the structure design in the invention, the control gate and the floating gate that cover the channel form a multiple-surface gate style. Therefore, the contact area between the control gate, the floating gate and the channel region can be effectively increased without adding more gates. This increases the coupling ratio and the device operating capability. Hence, the capacitance between the control gate and the floating gate is increased to reduce the operating voltage and to enhance the device performance without affecting the device size. 
   In addition, using the structure of a vertical channel region, the L-shaped carrier path speeds up the hot carrier injection rate during the operation of the flash memory, further enhancing the operating speed and performance of the device. 
   Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.