Abstract:
A method for programming a MLC memory includes (a) programming the bits of the memory having a Vt level lower than the PV level of a targeted programmed state into programmed bits by using a Vd bias BL; (b) ending this method if each bit of the memory has a Vt level not lower than the PV level of the targeted programmed state, otherwise, continuing the step (c); and (c) setting BL=BL+K 1  and repeating the step (a) if each of the programmed bits has a Vt level lower than the PV level, while setting BL=BL−K 2 , and repeating the step (a) if at least one of the programmed bits has a Vt level not lower than the PV level.

Description:
This application is a continual application of application Ser. No. 11/812,033, filed on Jun. 14, 2007, which has been issued as a U.S. Pat. No. 7,580,292, the subject matter of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a method for programming a multi-level cell (MLC) memory, and more particularly to a method for programming a MLC memory which can have a better tightened program distribution in a read operation. 
     2. Description of the Related Art 
       FIGS. 1A˜1D  are respectively schematic diagrams of threshold voltage (Vt) distributions of the programmed bits for a targeted programmed state in a conventional programming process of a MLC memory. As shown in  FIG. 1A , the memory has an erase-state Vt distribution at first and each bit of the memory is to be programmed to a targeted programmed state. The Vt distribution of the targeted programmed state has a program verify (PV) level (a lower boundary). In order to have tightened Vt distribution of the programmed bits, a pre-PV level for the targeted programmed state is set to be lower than the PV level and two steps of program operations are performed as below. 
     In the first rough program operation, after a number of program shots, the bits of the memory are programmed to have a Vt distribution A as shown in  FIG. 1B , some of which have a Vt level not lower than the pre-PV level of the targeted programmed state as shown by a dotted-line region in  FIG. 1B . At the time, the memory records the bits passing (with a Vt level not lower than) the pre-PV level. Then, the bits with a Vt level lower than the pre-PV level in the Vt distribution A are further programmed to pass the pre-PV level to generate a new Vt distribution B as shown in  FIG. 1C . 
     All the bits of the Vt distribution B pass the pre-PV level, and the memory records the bits passing the PV level first as shown by a dotted-line region in  FIG. 1C . Following that, in the second fine program operation, the bits with a Vt level lower than the PV level in the Vt distribution B are further programmed to pass the PV level to generate the target program distribution C as shown in  FIG. 1D  and complete the whole programming process. 
     However, as shown in  FIG. 1C , after the first program operation, some faster bits with a Vt level lower than and close to the PV level (denoted by a dotted-line region F) will be programmed again in the second program operation, which results in a widened Vt distribution for the target programmed state and in turn increases a bit error rate. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method for programming a MLC memory. Each time when at least one of the programmed bits passes the PV level of a targeted programmed state, the Vd bias is reduced by a fixed amount and when none of the programmed bits passes the PV level of a targeted programmed state, the Vd bias is increased by a fixed amount before the next program operation. By determining if at least one new bit passes the PV level, the program distribution can be tightened and the programming speed can be increased. 
     According to the present invention, a method for programming a MLC memory. The MLC memory has a number of bits, and each bit has a number of programmed states. Each programmed state has a PV level. The method comprises (a) programming the bits of the memory having a threshold voltage (Vt) level lower than the PV level of a targeted programmed state into programmed bits by using a Vd bias BL, and BL is larger than 0; (b) ending this method if each bit of the memory has a Vt level not lower than the PV level of the targeted programmed state, otherwise, continuing the step (c); and (c) setting BL=BL+K 1 , K 1  being a fixed positive amount, and repeating the step (a) if each of the programmed bits has a Vt level lower than the PV level, while setting BL=BL−K 2 , K 2  being a fixed positive amount, and repeating the step (a) if at least one of the programmed bits has a Vt level not lower than the PV level. 
     The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A˜1D  are respectively schematic diagrams of Vt distributions of the programmed bits for a targeted programmed state in a conventional programming process of a MLC memory. 
         FIG. 2  is a flow chart of a method for programming a MLC memory according to a preferred embodiment of the invention. 
         FIG. 3  is a schematic diagram of a WL bias operation according to the preferred embodiment of the invention. 
         FIG. 4  is a schematic diagram of a Vd bias operation according to the preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a method for programming a MLC memory. The bits of the memory not passing the PV level of the targeted programmed state are programmed by using a Vd bias. If at least one of the programmed bits passes the PV level, the Vd bias is reduced by a fixed amount while if none of the programmed bits passes the PV level, the Vd bias is increased by a fixed amount. Then the bits not passing the PV level are programmed again by using the new Vd bias. Therefore, the programming speed can be increased and the program distribution can be further tightened to reduce the bit error rate of the memory. 
     Referring to  FIG. 2 , a flow chart of a method for programming a MLC memory according to a preferred embodiment of the invention is shown. For example, the MLC memory is a flash memory which is a charge localized trapped memory with oxide-nitride-oxide (ONO) structure. 
     The MLC memory includes a lot of cells, such as 1024×256 cells, and each cell has a number of bits, such as two bits in the ONO structure. Each bit has a number of programmed states, such as 11, 10, 00 and 01. Each programmed state has a PV level. For example, the PV level of the programmed state 01 is 5.1V, and the PV level of the programmed state 00 is 4.3V, the PV level of the programmed state 10 is 3.5V. The following steps illustrate a process for programming the bits of the memory to a targeted programmed state, such as 01, 00 or 10. 
     First, in step  200 , check if every bit of the memory passes the PV level, that is, has a Vt level not lower than the PV level 5.1V, 4.3V or 3.5V of the targeted programmed state 01, 00 or 10. If yes, the process is ended and if no, the step  210  is performed continuously. In the step  210 , perform a program operation to program the bits of the memory not passing the PV level of the targeted programmed state by using a Vd bias BL larger than 0, such as 2V˜3V. Next, in step  220 , check if all of the programmed bits in the step  210  pass the PV level (5.1V, 4.3V or 3.5V). If yes, the process is ended and if no, the step  230  is performed continuously. In the step  230 , check if at least one of the programmed bits passes the PV level, that is, an event of at least one new bit pass (ALONBP) is triggered. If there is no ALONBP event triggered, perform the step  240  to set BL=BL+K 1 , wherein K 1  is a fixed positive amount, such as 50 mV, and repeat the step  210  by using the new Vd bias BL. This step  240  is used to combat slower bits and save the program time. 
     Referring to  FIG. 3 , a schematic diagram of a WL bias operation according to the preferred embodiment of the invention is shown, such as 9.5V for the state 01, 8.5V for the state 00 and 7.5V for the state 10. From the beginning to the first ALONBP event, the word line of each programmed bit has a constant voltage drop. 
     Referring to  FIG. 4 , a schematic diagram of a Vd bias operation according to the preferred embodiment of the invention is shown. The program operation is started by using an initial Vd bias BL (denoted by BL ini), and may be performed a number of times by adding the Vd bias BL with the fixed amount K 1  each time (not shown in the figure) until the first ALBOP event is triggered. The value BL ini is chosen for the start of programming. For example, the value BL ini is 2V˜3V for every programmed state 01, 00 or 10. Normally, the Vd bias BL (denoted by BL 1 ) used when the first ALBOP event is triggered is smaller than the first preset voltage Vp 1  (6V˜7V). 
     Following that, when the ALONBP event is triggered, perform the step  250  to set BL=BL−K 2 , wherein K 2  is a fixed positive amount, such as 50 mV, and repeat the step  210  by using the new Vd bias BL. This step  250  is to program the faster bits by a lower Vd bias BL and thus to tighten the Vt distribution. As shown in  FIG. 3 , after the first ALONBP event is triggered, the word line of each programmed bit has a constant voltage, such as 5.3V for the state 01, 4.5V for the state 00 and 3.5V for the state 3.5V. 
     As shown in  FIG. 4 , after the first ALONBP event is triggered, the Vd bias BL is reduced by a fixed amount K 2  to a value BL 2  in the step  250 . When the bits of the memory not passing the PV level are programmed by using the Vd bias BL 2  in the step  210 , it is determined that none of the programmed bits passes the PV level in the step  230 . Therefore, the Vd bias BL 2  is increased by a fixed amount K 1  to be a value BL 3  in the step  240  and the bits not passing the PV level are programmed again in the step  210  to generate the second ALONBP event. Then, the Vd bias BL 3  is reduced by the fixed amount K 2  to be a value BL 4  in the step  250 . When the bits not passing the PV level after the second ALONBP event are programmed by using the Vd bias BL 4  in the step  210 , it is determined that none of the programmed bits passes the PV level in the step  230 . Therefore, the Vd bias BL 4  is increased by a fixed amount K 1  to be a value BL 5  in the step  240  to generate a third ALONBP event. 
     After the third ALONBP event is triggered, the Vd bias BL 5  is reduced by the fixed amount K 2  to a value BL 6  in the step  250 . However, the value BL 6  is smaller than a second preset voltage Vp 2 , such as 2V˜3V. The second preset voltage Vp 2  is equal to the BL ini in the embodiment. Therefore, the bits not passing the PV level after the third ALONBP event are programmed by using the second preset voltage Vp 2  as the Vd bias BL in the step  250 . After the ALONBP event is triggered for a few times, such as three times in the embodiment, most of the faster bits in the memory are programmed to pass the PV level. The left slower bits are continuously programmed to pass the PV level in the following ALONBP events, such as the fourth and fifth ALONBP events until all the bits of the memory pass the PV level. 
     If the Vd bias BL as added by the fixed amount K 1  is larger than a first preset voltage Vp 1 , such as 6V˜7V, in the step  240 , the first preset voltage Vp 1  is set to be the new Vd bias BL for the next program operation. This upper limit Vp 1  can prevent the left slower bits from being programmed to suddenly have a large Vt level and thus widen the program distribution due to using too large Vd bias. 
     By determining whether the ALONBP event is triggered or not, it can be ensured that the faster bits will be programmed to pass the PV level by using a smaller Vd bias BL and the slower bits will be programmed to pass the PV level by using a larger Vd bias BL. Therefore, the program distribution can be even more tightened and the program speed can be increased. 
     By a suitable program control, it can be verified that the Vt distribution of the programmed bits can be tightened to have a width about 300 mV˜400 mV, and a read margin between the adjacent programmed states 01 and 00, 00 and 10, or 10 and 11 is about 400 mV˜500 mV in the invention. 
     The method for programming a MLC memory disclosed by the above embodiment of the invention has the following advantages: 
     1. Compared to the prior-art method, the slower bits of the memory are programmed by using a larger Vd bias. Therefore, the programming speed of the program operation in the invention can be improved. 
     2. Owing that the faster bits of the memory are programmed by using a smaller Vd bias. Therefore, the Vt distribution of programmed bits can be further tightened to give a larger read margin, and thus the bit error rate of the read operation can be greatly reduced. 
     While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.