Patent ID: 12261407

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

For a better understanding of the objectives, technical solutions, and advantages of the present application, hereinafter the present application will be described in further detail in connection with the accompanying drawings and some illustrative embodiments. It is to be understood that the specific embodiments described here are intended for the mere purposes of illustrating this application, instead of limiting.

For a more detailed and complete description of the present disclosure, the following contends provide an illustrative description of the implementation and specific embodiments of the present disclosure. This however does not represent the only way to implement or use the specific embodiments according to the present disclosure. The embodiments disclosed herein cover the features of a number of specific embodiments and the method steps and sequences used to construct and operate these specific embodiments. However, other specific embodiments can also be used to achieve the same or equal functions and sequence of steps.

In the application scenario of semiconductor lithography, excimer lasers are based on the burst operating mode of the laser. By the burst operating mode, it means that after the laser generates a sequence of laser pulses, there is a time interval during which operation is paused, which is the burst interval, and then the laser continues to output another sequence of lasers pulses, and so on.FIG.1is an energy distribution diagram of the laser provided by the embodiment of the present disclosure operating in the Burst operating mode, where12represents the laser light pulse sequence,11represents the Burst interval. Due to the existence of the Burst interval11, the energy of the first few pulses of each laser pulse sequence is much higher than that of the subsequent pulses under the condition that the discharge high voltage of the laser remains unchanged, as illustrated inFIG.13. This phenomenon is defined as energy overshoot. Due to the phenomenon of energy overshoot, the stability of the laser output energy is difficult to meet the energy stability requirements of semiconductor lithography when the discharge high voltage remains unchanged.

The energy level of the pulse can be adjusted by controlling the corresponding discharge voltage value of this pulse. In the present field, the discharge voltage value is generally calculated by the PI control algorithm. Therefore, the present disclosure proposes a segmented feedback control method based on the position difference of the pulses in the pulse sequence where the pulses are located, this method specifically includes: first obtaining the measured energy value of the n-th pulse in the m-th pulse sequence: then calculating the difference between the measured energy value and the preset energy value; when n is a positive integer less than z, calculating the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence according to a first mathematical model; when n is an integer greater than (z−1), calculating the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence according to a second mathematical model; and generating the (n+1)th pulse in the m-th pulse sequence according to the calculated discharge voltage value. In the above, z is the sequence number of the pulse that undergoes energy overshoot in the pulse sequence. Regarding the burst intervals of the same excimer laser, the sequence positions of the overshoot pulses are similar in different pulse sequences. Thus, the appropriate z value can be optimized after obtaining a large amount of data from the pulse energy test experiment. According to some embodiments of the disclosure, z may be an integer that lies in the range of 10 to 100. Furthermore, the common z value may lie in the range of 15 to 35. For example, in other embodiments of the disclosure, z is typically selected as the value of 20 for the control, and the first mathematical model is used to calculate the discharge voltage value of the 1st to 20th pulses of each pulse sequence, and the second mathematical model is used to calculate the discharge voltage value of the pulses after the 20th pulse of each pulse sequence.

According to some embodiments of the disclosure, the basis for establishing the first mathematical model refers to the discharge voltage and light output energy of the pulses with the same sequential number in the historical pulse sequences. Therefore, starting from the second sequence, the calculation of the discharge voltage value of the first z pulses of each sequence can all refer to the discharge voltage value and light output energy value of the pulses with the same sequential number in the historical pulse sequences, while the discharge voltage value of the first z pulses of the first sequence can be selected as an appropriate average value based on a large amount of experiments. In particular, let the preset energy value be Eset, the measured energy value of the n-th pulse in the m-th pulse sequence when the laser emits light be Emeasured(m,n), and the difference between the measured energy value and the preset energy value be denoted as Eerror(m,n), then the PI control algorithm is used based on the differences, and the first mathematical model is established as expressed in the following formula (1):

HV⁡(m,n+1)=PKp⁢1(Eerror(m-1,n+1)+PK1PT1⁢∑i=1m-1⁢Eerror(i,n+1))(1)

where Eerror(m−1, n+1) is the difference between the measured energy value and the preset energy value in the (n+1)th pulse in the (m−1)th pulse sequence, Ej=1m-1Eerror(i, n+1) represents the sum of the difference between the measured energy value and the preset energy value of each (n+1)th pulse in the 1st˜(m−1)th pulse sequences, PKp1is the proportional parameter of the PI control algorithm, PK1is the integral parameter of the PI control algorithm, and PT1is the control period parameter of the PI control algorithm. Through this first mathematical model, when the difference between the measured energy value and the preset energy value of the (n+1)th pulse in each of the 1st˜(m−1)th pulse sequences is known, then the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence can be estimated.

In the process of implementing the control algorithm, due to the existence of Ei=1m-1Eerror(i, n+1), it will take up a lot of memory and be prone to integral saturation phenomenon also appears, thus making it difficult to implement. Thus, in the engineering implementation, the incremental form of PI feedback control algorithm is adopted, and the implementation formula is expressed in the following formula (2):

Δ⁢HV⁡(m,n+1)=pKp⁢1(Eerror[m-1,n+1)-Ee⁢r⁢r⁢o⁢r(m-2,n+1))+PKp⁢1⁢P⁢K1P⁢T1⁢Eerror(m-1,n+1)(2)

where ΔHV(m, n+1) represents the amount of change between the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence and the (n+1)th pulse in the (m−1)th pulse sequence, Eerror(m−1, n+1) is the difference between the measured energy value and the preset energy value of the (n+1)th pulse in the (m−1)th pulse sequence, Eerror(m−2, n+1) is the difference between the measured energy value and the preset energy value of the (n+1)th pulse in the (m−2)th pulse sequence, PKp1is still the proportional parameter of the PI control algorithm, PK1is still the integral parameter of the PI control algorithm, and PT1is still the control period parameter of the PI control algorithm. Then the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is expressed in the following formula (3):
HV(m,n+1)=HV(m−1,n+1)+ΔHV(m,n+1)  (3)

Where HV(m−1, n+1) is the discharge voltage value of the (n+1)th pulse in the (m−1)th pulse sequence.

According to other embodiments of the present disclosure, the basis for establishing the second mathematical model only needs to refer to the discharge voltage value and the emitted light energy value of the historical pulses in the same sequence, and does not involve the historical pulse sequences and the positions of the pulses in the sequence. As such, a PI feedback control algorithm is created, and the second mathematical model is established as is expressed in the following formula (4):

HV⁡(m,n+1)=Kp⁢1(Eerror(m,n)+K1T1⁢∑j=2+1n⁢Eerror(m,j))(4)

where Eerror(m,n) is the difference between the measured energy value and the preset energy value of the n-th pulse in the m-th pulse sequence, Σj=z+1nEerror(m, j) represents the sum of the difference between the measured energy value and the preset energy value of each of the (z+1)˜n-th pulses in the m-th pulse sequence, Kp1is the proportional parameter of the PI control algorithm in the model, K1is the integral parameter of the PI control algorithm in the model, and T1is the control period parameter of the PI control algorithm in the model. Through this second mathematical model, when the difference between the measured energy value and the preset energy value of each of the (z+1)˜n-th pulses in the same pulse sequence is known, the discharge voltage value of the (n+1)th pulse in the pulse sequence can be estimated.

Similarly, in the process of implementing the control algorithm, due to the existence of Σj=z+1nEerror(m, j), it will take up a lot of memory and be prone to integral saturation phenomenon also appears, thus making it difficult to implement. Thus, in this project, the incremental form of PI feedback control algorithm is adopted, and the implementation formula is expressed in the following formula (5):

Δ⁢HV⁡(m,n+1)=Kp⁢1(Ee⁢r⁢r⁢o⁢r(m,n)-Eerror(m,n-1))+Kp⁢1⁢K1T1⁢Eerror(m,n)(5)

where ΔHV(m, n+1) represents the amount of change between the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence and the n-th pulse in the m-th pulse sequence, Eerror(m, n−1) is the difference between the measured energy value and the preset energy value of the (n−1)th pulse in the m-th pulse sequence, Eerror(m,n) is the difference between the measured energy value and the preset energy value of the n-th pulse in the m-th pulse sequence, Kp1is still the proportional parameter of the PI control algorithm in the model, K1is still the integral parameter of the PI control algorithm in the model, and T1is still the control period parameter of the PI control algorithm in the model.

It should be noted that when the measured energy value of the z-th pulse in the m-th pulse sequence is obtained, then according to the above definition, z is greater than (z−1), so the discharge voltage value of the (z+1)th pulse in the m-th pulse sequence is estimated according to formula (4), namely the second mathematical model, as expressed in the following formula (6):

HV⁡(m,z+1)=Kp⁢1(Eerror(m,z)+K1T1⁢∑j=z+1z⁢Eerror(m,j))(6)

Otherwise when the measured energy value of the (z−1)th pulse in the m-thpulse sequence is obtained, then according to the above definition, (z−1) is less than z, so the discharge voltage value of the z-th pulse in the m-th pulse sequence can be estimated according to formula (1), namely the first mathematical model, as expressed in the following formula (7):

HV⁡(m,z)=PKp⁢1(Eerror(m-1,z)+PK1PT1⁢∑i=1m-1⁢Eerror(i,z))(7)

The incremental form of the discharge voltage value of the (z+1)th pulse in the m-th pulse sequence can be obtained by the formula (6) minus the formula (7), as expressed in the following formula (8):

Δ⁢HV⁡(m,z-1)=Kp⁢1(Ee⁢r⁢ror(m,z)+K1T1⁢∑j=z+1z⁢Eerror(m,j))-PKp⁢1(Eerror(m-1,z)+PK1PT1⁢∑i=1m-1⁢Eerror(i,z))(8)

Because the formula (8) is not equal to the formula (5), the incremental form of the discharge voltage value of the (z+1)th pulse in the m-th pulse sequence cannot be expressed by the formula (5). Therefore, in the formula (5), n should be an integer greater than z.

Then when n is an integer greater than z, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is as expressed in the following formula (9):
HV(m,n+1)=HV(m,n)+ΔHV(m,n+1)  (9)

Where HV(m,n) is the discharge voltage value of the n-th pulse in the m-th pulse sequence.

In the actual implementation, the discharge voltage value of the laser is limited by the inherent properties of the high-voltage module device, and there are maximum and minimum limits. Therefore, the first voltage threshold and the second voltage threshold that meet the inherent properties of the device are set in the two control algorithm segments. When the calculated HV(m, n+1) is greater than the first voltage threshold, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is controlled to be equal to the first voltage threshold, and the (n+1)th pulse in the m-th pulse sequence is generated according to the first voltage threshold. When the calculated HV(m, n+1) is less than the second voltage threshold, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is controlled to be equal to the second voltage threshold, and the second voltage threshold is used to generate the (n+1)th pulse in the m-th pulse sequence.

When simulating calculations in incremental form, in order to prevent dramatic changes in energy, which may cause system to be unstable or damaged. Thus, a third voltage threshold and a fourth voltage threshold are set for the amount of change between pulses in the implementation. When the calculated ΔHV(m, n+1) is greater than the third voltage threshold, in the case of the first mathematical model, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is controlled to be equal to the discharge voltage value of the (n+1)th pulse in the (m−1)th pulse sequence plus the third voltage threshold. In the case of the second mathematical model, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is equal to the discharge voltage value of the n-th pulse in the m-th pulse sequence plus the third voltage threshold. When the calculated ΔHV(m, n+1) is less than the fourth voltage threshold, in the case of the first mathematical model, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is equal to the discharge voltage value of the (n+1)th pulse in the (m−1)th pulse sequence plus the fourth voltage threshold. In the case of the second mathematical model, the discharge voltage value of the (n+1)th pulse in the m-th pulse sequence is controlled to be equal to the discharge voltage value of the n-th pulse in the m-th pulse sequence plus the fourth voltage threshold.

In a specific embodiment according to the present disclosure, the light output frequency of the laser is 4 k Hz, each laser pulse sequence contains 375 pulses, and the preset energy value of the laser energy stability controller is 10 mJ. The energy distribution diagram obtained by the above-mentioned control method is illustrated inFIG.2. ComparingFIG.2againstFIG.1, it can be seen that the energy overshoot in the original pulse sequence is well suppressed, and the energy value of each pulse in each pulse sequence17stays at about 10 mJ, and the relative standard deviation of the pulse energy is 1.6% by calculation. Thus, the energy stability is well controlled.

Therefore, this segmented feedback control method based on the position of the pulse in the pulse sequence that is provided by the present disclosure can effectively control the serious energy overshoot of the first few pulse of each pulse sequence, while stabilizing the energy of all pulses in a pulse sequence. Thus, the energy of all laser pulses emitted by the laser is stabilized at a certain level thus meeting the precision requirements of semiconductor lithography. This control method is simple and the control effect is significant.

Another aspect of the present disclosure provides a control system based on the above control method. Now referring to the block diagram of the control system illustrated inFIG.3. The control system includes a high-voltage discharge module1, a laser cavity2, a laser parameter measurement module3, and an energy stabilization controller9. The high voltage discharge module1may generate a pulse high voltage5according to the set discharge voltage value4. The laser cavity2contains an operating gas, which when being shocked by the pulse high voltage5will trigger the laser cavity2to generate a laser pulse6. The laser parameter measurement module3in this system serves the main function of detecting the energy of individual pulses of the laser thus providing a reference for the operation of the energy controller9. Referring toFIG.4which illustrates a block diagram of the laser parameter measurement module3provided by an embodiment of the present disclosure. The laser parameter measurement module3includes a beam splitter16and an energy detector15. After passing through the beam splitter16, the laser pulse6is divided into a laser pulse7used for operation and a laser pulse14used for calculating the discharge voltage value of the next laser pulse6. The laser pulse14irradiates on the energy detector15and is converted into an electric signal8to be sent to the energy stabilization controller9. The energy stabilization controller9collects the energy value of the laser pulse14through the electric signal8, and obtains the energy of the laser pulse7through proportional conversion, and then compares the energy value with the preset energy value10, and estimate the discharge voltage value4of the next laser pulse6according to the control algorithm described above. Then the discharge voltage value4is transmitted to the high-voltage discharge module1to control the energy of the laser pulse7so that the energy value is consistent with the preset energy value10as much as possible. Therefore, the light output energy7of the laser is stabilized at the preset energy value10, and the phenomenon of energy overshoot13of the laser in the Burst mode is eliminated.

According to some embodiments of the present disclosure, the ratio of the energy of the laser pulse7to the energy of the laser pulse14is (90˜95)%:(10˜5)%, and only the separating (10˜5)% of the energy of the laser pulse6is able to satisfy the energy stability of the system.

The above-described control system provided by the present disclosure has a simple structure, and only a small amount of energy separated from the emitted laser pulse can stabilize the energy of the entire system.

Although the description of this disclosure refers to some exemplary illustrative embodiments, those having ordinary skill in the art will appreciate that the above illustrative embodiments are merely used to illustrate the present disclosure and rather than limit the scope of protection of the present disclosure. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle scope of the disclosure shall be included in the scope of protection of the present disclosure.